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Chapter 15 - Goodheart

1. Replacing a PROM To replace a PROM make sure the ignition key is in the off position Remove the ECM fuse and remove the access cover over the PROM See Figure 15 53 Special tools are available to help remove and install the PROM safely Remove the old PROM from the ECM Then install the new PROM carefully pushing it into position Finally replace the access cover and reinstall the ECM fuse Replacing a Knock Sensor Module In some vehicles the ECM contains a knock sensor KS module This module can be replaced if it is defective Additionally it can be removed from a defective ECM and rein stalled in a new unit ensuring that the proper timing retard is maintained for engine knock protection To replace the knock sensor module make sure the igni tion key is in the off position Remove the ECM fuse and then i Figure 15 53 The PROM or mem cal can be replaced after removing the cover that protects it from water and dirt remove the access cover over the KS module Pinch the module locking tabs to release them and pull the module from the ECM Install the new module carefully making sure the mounting pins are not bent When pushing the KS module into position make sure the module s locking tabs snap into place Finally replace the access cover and reinstall the ECM fuse Flash Programming a Computer As mentioned the ECMs in late model vehicles can be updated by erasing old information from the computer s memory
2. Therefore even a minor computer control system problem can have a dramatic effect on one or more vehicle systems Be sure to obtain the vehicle s service manual or equivalent service information before you begin any diagnostic operation This chapter discusses the diagnosis and repair of the computer control system This includes the three main sections of any computer control system the input sensors the computer and the actuators Studying this chapter will help you understand the basic computer system testing procedures computers that control engine and vehicle operation For simplicity the term electronic control module ECM will be used when referring to these computers in this chapter There are also many names for the dashboard light that illuminates to indicate computer system problems In this chapter however this light will be called the maintenance indicator light MIL ie Note There are many names for the on board Diagnosing Computer Control System Problems Often a vehicle is brought in for service when the MIL is illuminated when fuel economy drops or when the vehicle fails an emissions test or has a driveability problem As a tech nician your first impulse may be to blame the computer control system for these problems The computer controls however should be suspected only after you have checked for more obvious and more common problems Remember that many noncomputer problems can cause the
3. A throttle position sensor can go out of adjust ment due to linkage wear or changes in the sensor s electrical material The sensor may also require adjustment when the throttle body is replaced On most engines the throttle position sensor is simply mounted on the throttle body with little or no provision for adjustment However some throttle position sensors have slotted mounting holes The slotted holes allow the sensor to be moved for adjustment See Figure 15 42 With the throttle plate completely closed rotate the sensor until a specified voltage output is obtained Specific adjustment procedures are required for some throttle position sensors requires special tools or adjustment procedures Check the vehicle s service manual for the correct procedure Slotted mounting holes Throttle position sensor TPS Retainer 2 Screw 2 Figure 15 42 In most cases the throttle position sensor mounts on the throttle shaft and is simply bolted on the throttle body Note the slotted mounting holes on this TPS The slotted holes allow the sensor to be turned for adjustment General Motors ae Note Adjusting some throttle position sensors To adjust the TPS with a multimeter obtain the correct voltage specifications and set the meter to a range that will measure these voltages Using jumper wires to the correct terminals measure the input and output voltages If the input voltage is incorrect check the wiring and the
4. ECM and make repairs as necessary If the output voltage is incorrect make sure the throttle plate is in the fully closed position Then loosen the throttle position sensor attaching screws and rotate the sensor until the proper value is shown on the voltmeter A scan tool can also be used to adjust the TPS Plug the tool into the DLC and select the TPS reading from the tool menu Check the TPS input with the throttle plate in the closed position and compare the input reading to specifications If necessary loosen the throttle position sensor attaching screws and rotate the sensor until the proper value is shown on the scan tool Adjusting Crankshaft and Camshaft Position Sensors Most crankshaft and camshaft position sensors do not require adjustment However some of these sensors must be adjusted to provide the proper signal to the ECM and prevent damaging contact with rotating engine parts The adjustment process usually involves the use of a special alignment gauge as shown in Figure 15 43 In this illustration a special tool is used to adjust a single pickup assembly in relation to the shutter assembly This positions the shutter precisely between the two parts of the pickup assembly If the alignment gauge is not used when specified damage to the sensor may occur Consult the vehicle s service manual to ensure that the sensor does not require the use of an alignment gauge Some crankshaft sensors use a small piece of cardboard
5. ECM that is suspected of being defective To check a solenoid energize it and listen for a Define the process of forcing an actuator The service manual gives a solenoid winding resis tance specification of 0 5 0 7 ohms Which of the following readings indicates that the winding has the correct resistance A Oohms B 0 5 ohms C 7 ohms D Infinity 18 19 20 21 22 23 24 25 Computer System Diagnosis and Repair 309 Removing and installing an oxygen sensor usually requires aspecial_ Which of the following can cause a false oxygen sensor reading A A crack in the exhaust manifold B Excessive anti seize compound on the sensor threads C Tight electrical connectors D A discolored sensor shell To install a coolant temperature sensor the cooling system must be The cooling system bleed valve is used to remove from the cooling system A coolant B air C sealant D rust Knock sensors are threaded directly into the A engine block B cylinder head C intake manifold D All of the above When changing an ECM the first thing to remove is the _ _ ma A negative battery cable B ECM retaining bolts C ECM electrical harness D ROM or PROM When an ECM must be flash reprogrammed which of the following should be removed A PROM B ECM C ECM wiring harness D All of the above Relays are usually installed in one of the vehic
6. distributor However their location on or in the engine can make testing difficult Note A problem that appears to be caused by a position sensor can also be caused by problems in the ignition module ignition coils fuel injectors harness wiring or ECM The following generalized tests should only be used to narrow down a possible crankshaft or camshaft position sensor problem If a problem in the position sensor circuits is suspected it should be verified using tests outlined in the vehicle s service manual One test that can be used to check position sensor oper ation is to crank the engine while monitoring engine rpm using a scan tool A low rpm reading or no rpm reading when the engine is cranking indicates a possible problem in the crank shaft or camshaft sensor circuit Figure 15 35 To test a position sensor for ac voltage output discon nect the sensor connector Obtain a multimeter and connect it to the speed sensor Set the multimeter to the ac volts range Crank the engine The multimeter should read ac voltage as the engine is cranked Engine speed 125 RPM Sensor good Engine speed 0 RPM Sensor defective Figure 15 35 A scan tool can be used to quickly check the crankshaft position sensor for proper operation If the engine has a camshaft sensor a scan tool reading of engine speed is not reliable as most ECMs take readings from both sensors to determine engine speed Warning Some engines may star
7. sure the engine is warm enough for the computer to operate in closed loop before monitoring O sensor output If the engine is not in closed loop mode run it at fast idle for a few minutes to raise engine and exhaust temperature Then allow the engine to stabilize After the engine stabilizes monitor the O sensor readings Figure 15 14 Oxygen sensors use either Zirconia or Titania elements The signal from either type of sensor will be displayed as a voltage by the scan tool The sensor s output voltage should oscillate between 100 and 900 millivolts when the ECM is in the closed loop mode The voltage readout should change very quickly as the amount of oxygen in the exhaust changes The meter should show a minimum voltage of less than 300 mV a maximum of more than 600 mV and should average about 500 mV sensor to be bypassed by grounding the sensor input to the ECM Grounding the oxygen sensor input wire should cause the ECM to produce a set voltage reading If grounding the wire does not produce this reading the ECM is defective The technician should only ground the O sensor wire when specifically instructed by the service manual ie Note Some manufacturers allow the oxygen Checking Oxygen Sensors using a Multimeter Remember that a Zirconia type sensor will produce a voltage reading while a Titania type sensor will produce a resistance reading Most oxygen sensors are Zirconia types To measure the voltage output of a Zircon
8. while observing the voltage reading If the voltage reading increases with increases in vacuum the MAP sensor is probably good If the MAP sensor fails any of these tests replace it A similar test can be made to the barometric pressure BARO sensor on some vehicles Check the sensor s output voltage at the proper ECM terminals ignition on engine not running and compare it to the specifications for the altitude in your area A typical altitude compensation chart is shown in Figure 15 23 If the voltage is not within specifications replace the sensor Hand vacuum pump Figure 15 22 Setup for measuring dc voltage from the MAP sensor Apply vacuum to the sensor to simulate engine vacuum The readings should be similar to normal readings while the engine is at idle Compare all readings to service manual specifications Fluke 292 Auto Diagnosis Service and Repair Altitude Voltage range Meters Feet Below 305 Below 1000 3 8 5 5v 305 610 1000 2000 3 6 5 3v 610 914 2000 3000 3 5 5 1v 914 1219 3000 4000 3 3 5 0v 1219 1524 4000 5000 3 2 4 8v 1524 1829 5000 6000 3 0 4 6v 1829 2133 6000 7000 2 9 4 5v 2133 2438 7000 8000 2 8 4 3v 2438 2743 8000 9000 2 6 4 2v 2743 3048 9000 10 000 2 5 4 0v Low altitude High pressure High voltage Figure 15 23 MAP sensor output voltage varies with altitude The lower the altitude the higher the voltage General Motors A scan tool can be used to check MAP sensors Att
9. Diagnosis Service and Repair temperature sensors can be removed and replaced without special precautions To replace a temperature sensor installed in the intake manifold or plenum disconnect the electrical connector Then use the proper socket to remove the sensor from the manifold Coat the new sensor with sealing compound if necessary install it in the manifold and reat tach the electrical connector Check for vacuum leaks and sensor operation To replace a temperature sensor installed in the air cleaner housing remove the electrical connector pull the attaching clip from the sensor and remove the sensor from the housing Position the new sensor in the housing install the clip and reconnect the electrical connector To replace an exhaust gas temperature sensor allow the exhaust system to cool then disconnect the electrical connector Use a special tool or a six point socket to remove the sensor without damaging its shell Place anti seize compound on the threads of the new sensor Install the sensor and reconnect the electrical connector Start the engine and check for exhaust leaks and proper sensor operation Knock Sensor Replacement Knock sensors are threaded directly into the engine block cylinder head or intake manifold Figure 15 51 This allows them to easily detect and transmit the sound of engine detonation Knock sensors can be removed and replaced without special precautions Since the knock sensor shell is usu
10. Figure 15 36 shows some typical crankshaft and camshaft position wave forms Due to the many variations always check the manu facturers service literature for the standard waveform Any waveform variation indicates that the position sensor is either defective or out of adjustment Testing Knock Sensor The knock sensor allows the ECM to retard ignition timing when spark knock occurs The knock sensor produces a voltage that is sent to the ECM or modifies a reference voltage from the ECM Check the service manual for the type of knock sensor you are testing make sure that an internal engine problem is not the cause of the knocking condition co Note Before performing the knock sensor test Testing a knock sensor is very easy All that is needed is a scan tool voltmeter or waveform meter and a wrench or other metal object To test the sensor using a scan tool connect the tool to the DLC and set it to monitor knock sensor output Turn the ignition switch to the on position but do not start the engine Lightly tap on the engine block with the wrench itself Do not tap on aluminum plastic or sheet V Caution Tap only on cast iron parts or the block metal parts Chapter 15 Computer System Diagnosis and Repair 297 If the sensor is working properly the scan tool reading will change indicating that the ECM is detecting the artificial engine knock Figure 15 37 If the scan tool reading does not change disconn
11. Using a Ohmmeter Most temperature sensors can be checked for a speci fied resistance at a certain temperature Unlike most resistors temperature sensor resistance decreases as temperature rises To test temperature sensors you must measure the Intake air temperature sensor 77 F 25 C Normal reading Intake air temperature sensor 30 F 34 C Low temperature Intake air temperature sensor 260 F 127 C High temperature Figure 15 27 Scan tool readings for the intake air temperature sensor The high temperature reading may be lower than shown here but it will be much higher than ambient air temperature output resistance in relation to the temperature at the sensor s tip This will determine whether the sensor is correctly converting the temperature into resistance which varies the voltage signal during engine operation A temperature sensor can be checked with a digital multimeter or an ohmmeter If a multimeter is used set the meter to measure resistance in ohms Next disconnect the sensor from the wiring harness and measure the resistance across the sensor leads Compare the meter readings with the manufacturers specifications The readings can be correlated to specific temperatures Figure 15 28 If necessary a heat gun can be used to raise the temperature to check for resis tance variations with temperature If the resistance measure ments are incorrect the sensor is defective If the resista
12. a system that uses an anolog meter the needle on the meter will move 1 needle pulse sweep of 1 needle pulse sweep twice pause and then move six times Some systems repeat the same code up to three times before moving on to the next code Some systems repeat the first code as an indication that there are no more codes stored Write down the numbers of all codes displayed Retrieving Trouble Codes with a Scan Tool On OBD Il equipped vehicles the proper scan tool must be used to retrieve trouble codes Figure 15 7 shows a commonly used scan tool Never attempt to retrieve trouble codes from an OBD II system by grounding a DLC terminal as this will damage the ECM On OBD systems it is far easier to use a scan tool to retrieve codes than to perform the procedures explained earlier Scan tools display the code numbers on a screen and 2 needle pulses sweeps for 1st digit 2 SECOND PAUSE BETWEEN DIGITS 1 2 g second pause 1 needle pulse sweep for 1 2 second 1 needle pulse sweep of for 1 2 second BO Service a s g amp code fa al 1 2 Q second pause 3 needle pulses afa 1 needle pulse sweep sweeps for for 1 2 second 2nd digit 4 SECOND PAUSE BETWEEN SERVICE CODES WHEN MORE THAN ONE CODE IS INDICATED Figure 15 6 An analog meter can be used to read the codes on most OBD systems Some digital meters can read codes as digital pulses Ford
13. airflow through the MAF sensor will cause an increase in the frequency reading Figure 15 21B If the frequency reading does not change or if it decreases the sensor is probably defective FEAK FEAE FREGQUENMC DUTY CYCLE T PULSE WIOTH T 1Gms DI A FEAK FEAE FREQUENCY DUTY CYCLE T PULSE WIOTH T zms DIV B Figure 15 21 A The frequency of a digital MAF sensor signal can be measured along with the voltage output B Note the increased frequency and pulse width as the engine is started Fluke Testing Manifold Absolute Pressure MAP Sensors Manifold absolute pressure and similar vacuum and barometric pressure sensors allow the ECM to compensate for manifold vacuum and high altitudes This sensor is very important in vehicles that use speed density to calculate airflow Before testing a MAP sensor check the vacuum hoses for splits or obstructions Also make sure the engine is providing sufficient manifold vacuum Consult the service manual to determine whether this test can be made and for the exact procedures Begin testing by turning the ignition key on without starting the engine Measure the dc voltage at the ECM and compare it to service manual specifications Tap the sensor with a small screwdriver while watching for voltage jumps that could signal intermittent problems Repeat the tapping proce dure while warming the sensor with a heat lamp Next apply vacuum to the MAP sensor Figure 15 22
14. any actuator Replacing a Solenoid Several solenoids are used on modern vehicles including fuel injectors idle controls EGR solenoids and transmission solenoids To replace a solenoid make sure the ignition switch is in the off position Then remove any parts that block access to the solenoid Disconnect the electrical connector remove the fasteners holding the solenoid and remove the solenoid Inspect the solenoid Sometimes sole noid problems are the result of carbon or sludge buildup in the valve and passages It may be possible to clean the solenoid and restore it to service If the solenoid is definitely bad it must be replaced Compare the new solenoid to the old one to ensure that the replacement part is correct Install any needed gaskets or seals on the new solenoid and place it in position Install the fasteners and the electrical connector Finally install any other parts that were removed start the engine and check solenoid operation Replacing a Relay Relays are usually installed in one of the vehicle fuse boxes Figure 15 55 To replace a relay make sure the igni tion switch is in the off position Then locate the relay in the fuse box and pull it from its socket Compare the old and new relays then install the new relay Start the engine and check relay operation Replacing a Motor Some vehicles make use of motors as actuators For example a motor is sometimes used to control idle speed on fuel injected e
15. as an adjusting shim between the sensor tip and the toothed wheel The shim is placed over the sensor s tip before the sensor is installed in the opening The sensor is tightened into place with the shim holding it in the proper position When the engine is started the shim slides from between the sensor and wheel manual for exact procedures before replacing or adjusting any position sensor or pickup Severe damage can result if the pickup contacts rotating engine parts vV Caution Always check the appropriate service Replacing Computer System Components This section outlines the general procedures for replacing common computer system components Many computer system components can be replaced by simply unplugging the electrical connectors and removing the attaching screws Some devices such as oxygen sensors engine temperature sensors and knock sensors are Chapter 15 Computer System Diagnosis and Repair 301 Crankshaft position sensor tool Sensor Crankshaft Pinch bolt Pedestal Crankshaft position B sensor Crankshaft position sensor tool Interrupter rings S a Harmonic balancer C Figure 15 43 Some crankshaft position sensors require adjustment after installation A and B Using a position sensor tool to adjust the crankshaft position sensor C Using the tool to check the interrupter rings on the crankshaft pulley General Motors threaded directly into the related
16. cal D Knock sensor All the following are methods of reprogramming the ECM except A direct programming B indirect programming C remote programming D mechanical programming An engine runs rough during the first few minutes of the ECM relearn procedure Technician A says this indi cates another system problem Technician B says this indicates that the ECM is adjusting to new engine oper ating conditions Who is right A A only B B only C Both A and B D Neither A nor B 15 Su 1 Chapter 15 Computer System Diagnosis and Repair 311 Technician A says that a drive cycle test is performed before the emissions are checked Technician B says that a drive cycle test is performed after the ECM or battery has been disconnected Who is right A A only B B only C Both A and B D Neither A nor B ggested Activities Using factory service information locate the computer diagnostic test connectors on several different makes and years of vehicles Make a chart comparing the connector locations by manufacturer and the type of equipment needed to access the trouble codes Obtain a vehicle with an on board computer and retrieve trouble codes Use the procedure outlined in the service manual If your instructor okays it create trouble codes by disconnecting a computer sensor or an output device and briefly running the engine What codes were found What do the codes indicate
17. codes on the other hand will generally not reset immediately Hard codes are usually caused by a defective compo nent an open or shorted circuit or a faulty connector In many Figure 15 10 The fuse that supplies power to the ECM is usually located in the main fuse block Pull this fuse to clear stored codes in OBD systems Remember that this will not work with most OBD II systems cases an intermittent code does not indicate that a sensor or output device is defective only that it is responding to another problem An example is an intermittent trouble code for a rich oxygen sensor reading Although the oxygen sensor may be defective it is more common for another component such as a leaking fuel injector or a defective airflow sensor to be causing the rich mixture problem Remember that many sensors in older computer systems can go slightly out of range without setting a trouble code The ECM in an older system often will respond to the incorrect input in the same way it responds to the correct input OBD II systems are designed to monitor sensor perfor mance and will usually set a diagnostic trouble code if a sensor is slightly out of range even if there is no apparent driveability problem However the absence of a trouble code in any computer control system is not absolute proof that a sensor is good Other Scan Tool Information In addition to retrieving diagnostic trouble codes some late model scan tools can display d
18. component They must be removed carefully to avoid stripping the threads Before replacing any computer control system component always turn the ignition switch to the off position and remove the negative battery cable This will prevent damage to the ECM the sensors and other electronic devices from stray electrical charges Input Sensor Replacement The following sections deal with replacing input sensors There are many different types of input sensors This makes it necessary to consult the vehicle s service manual for the proper replacement procedures for specific sensors Figure 15 44 Remove oxygen sensors with the appropriate tool Be sure you are removing the correct sensor on vehicles Note The procedures presented in this chapter are with multiple sensors general in nature and are meant to describe typical procedures Always refer to the vehicle s service manual for the exact replacement procedure sensor will be reused always use a special tool to remove it If the sensor will be replaced and does not require a special Oxygen Sensor Replacement tool for removal loosen it from the manifold using a socket or The oxygen sensor is probably the most delicate of all box end wrench that will contact all sides of the hex the input sensors and should always be handled carefully Since it is installed in the exhaust manifold or the exhaust _ _ _ enc ee pipe it is subject to high temperatures and corros
19. dc volts and attach it to the MAF sensor as shown in Figure 15 18 With the ignition key in the on position and the engine off voltage will be about 1 volt Start the engine and observe the scan tool or voltmeter Voltage should rise to about 2 5 volts Figure 15 19 Tap on the sensor with a small 290 Auto Diagnosis Service and Repair S 1 oO a 4 Frequency Hz Frequency Hz AC Delco MAF sensor AC Delco MAF sensor good bad Multimeter Ground Signal 12v supply Jumper wires Figure 15 18 A digital meter can be used to check mass air flow sensor operation Do not use an analog meter to perform this test Fluke screwdriver handle There should be no voltage fluctuation or engine misfire If needed repeat the test while warming the sensor with a heat lamp If the voltage fluctuates or engine operation changes dramatically stalls or idles rough as the sensor is heated the sensor is defective and should be replaced In a variation of this procedure the technician can blow through the MAF sensor with the ignition in the on position Voltage should rise indicating that the sensor is responding to air movement On some vane type MAF sensors the sensor output voltage is determined by inserting an unsharpened pencil into the sensor The pencil holds the vane in a certain position to produce a specified voltage reading The sensor voltage output is measured with the ignition on and the engine off If th
20. is covered later in this chapter Diagnostic Trouble Code Format The diagnostic trouble codes for most computer controlled vehicles are generated as two or five character codes that correspond to specific problem areas Older scan tools displayed trouble codes as numbers and letters The technician then matched the code to specific problems by looking up the code in the service literature Newer scan tools are able to process the code letters and numbers and produce readouts that describe the actual problem Some scan tool software can also suggest possible solutions and tell the technician if the problem is common on the vehicle being tested necessarily mean that a computer system part or circuit is defective For example a constant rich reading from an oxygen sensor may mean the sensor itself is defective However it may also mean that a fuel system defect is causing the engine to run rich at all times triggering the rich sensor reading ie Note Remember that trouble code does not OBD and OBD II Code Formats OBD systems normally display two digit trouble codes These codes are correlated to a specific sensor or circuit Ina few cases the code will specify a certain problem such as low or high voltage readings In most cases however the code will simply indicate that there is a problem in the system Figure 15 8 The best way to read codes on any computer system is to use a scan tool A Connecting a scan tool to a
21. should not contact the sensor signal terminal If the resistance is within specifications the heater circuit is usually good Set the meter to the voltage scale and test the heater power terminals on the wiring harness If 12 volts are present the circuit is good Creating an Artificial Lean and Rich Mixture Condition To test oxygen sensor response time it is necessary to create artificial lean and rich conditions To create an artificial lean mixture disconnect the PCV hose or another large vacuum hose Cover the hose opening with your thumb and start the engine As you remove your thumb from the hose a lean condition is created due to the vacuum leak Partially cover the vacuum hose opening if the engine begins to stall Then observe the meter or scan tool for several minutes It should show a lower than average voltage or a high resistance as the oxygen sensor reacts to the lean mixture react too fast for this test to be effective the ie Note The ECM on some fuel injected engines will engine speed will just rise and fall After making this test reconnect the vacuum hose and create an artificial rich mixture with a propane enrichment device such as that used to adjust carburetors Do not create a rich mixture with carburetor cleaner or gasoline as this will simply overload the oxygen sensor On some vehicles it is possible to create an artificial rich mixture by blocking the air inlet ahead of the throttle plate On newer vehic
22. the device operates The counts can be measured to determine whether the solenoid is operating properly If readings are within the normal range the solenoid or relay is working properly If the counts are not as specified either the device or ECM is defec tive or the ECM is receiving an incorrect input from a sensor Some scan tools allow the technician to diagnose actua tors by bypassing the ECM and directly operating the devices This is sometimes called forcing the actuator The scan tool then monitors operating voltage during the forcing procedure to determine whether the device is operating properly Some scan tools can simulate engine and vehicle conditions to check the interaction among various system actuators Follow the manufacturers instructions to test solenoids and relays using this method Ohmmeter Tests Solenoid and relay windings can be checked with an ohmmeter as shown in Figure 15 40 Solenoid and relay windings have a small range of acceptable resistance Any reading outside this range means that the actuator should be replaced Always check the service manual for exact resis tance specifications A reading of zero ohms means the winding is shorted while an infinite reading indicates an open winding Either reading means the winding is defective and must be replaced The circuit in a relay that passes through Chapter 15 Computer System Diagnosis and Repair 299 the internal relay contacts should read zer
23. 0 mV or greater with an average of 450 mV Ss Minimum peak voltages B should reach 200 mV or less Figure 15 16 A Waveform meter hookup for testing an oxygen sensor B This waveform shows the oscillations of a properly operating oxygen sensor Lack of oscillations or a pattern that is not consistent indicates a problem Fluke engine must be completely warmed up before the road test If a scan tool is used connect the tool to the DLC Use an exten sion lead if the DLC is under the hood If equipped use the scan tool s snapshot feature to take a picture of sensor and vehicle operation when the malfunction occurs If a multimeter or waveform meter is used start by connecting the meter to the oxygen sensor through leads that allow the meter to be placed inside the passenger compart ment If the meter you are using has memory and averaging capabilities follow the manufacturer s instructions to set the meter to record the average reading Have someone drive the vehicle while you observe oxygen sensor operation A high average voltage or low resistance reading means that the engine is running too rich such as when the ECM is leaning out the mixture but cannot compensate for a leaking injector A low average reading or high resistance indicates too little fuel such as when the ECM is richening the mixture but cannot compensate for a vacuum leak After the road test is t 1 2 A B Figure 15 17 Comparing the
24. 3 Based on the preceding activities consult the appro priate factory service manual to determine the next steps to be taken to isolate a computer problem Determine the effect on the computer system of discon necting a non computer device For instance what happens to the fuel mixture when the air injector smog pump belt is removed What happens to the timing advance if a vacuum leak develops on an engine with MAP sensor What happens to the computer open and closed loop cycle if the cooling system thermostat is removed Discuss you findings with the other members of the class and try to figure out how one system affects another 312 Auto Diagnosis Service and Repair A coil pack such as the one shown here is commonly used on vehicles with distributorless igniton This particular coil pack is for a 6 cylinder engine Each coil fires two spark plugs
25. All of the above Technician A says that testing a BARO sensor requires a vacuum tester Technician B says that testing a MAP sensor requires a vacuum tester Who is right A A only B B only C Both A and B D Neither A nor B Technician A says that most pressure sensors are on off switches Technician B says that most pressure sensors can be tested with a voltmeter Who is right A A only B B only C Both A and B D Neither A nor B 10 11 12 13 14 Technician A says that tapping an engine block with a metal tool will cause the ignition timing to retard Technician B says that tapping the engine block with a metal tool will cause the knock sensor circuit to operate Who is right A A only B B only C Both A and B D Neither A nor B All the following statements about speed sensors are true except A the output of some vehicle speed sensors is measured in ac volts B some vehicle speed sensors are checked by measuring resistance C the output of some speed sensors is measured as a pulsed dc signal D some speed sensors can be checked with a scan tool Which of the following computer control system compo nents can be adjusted A MAF sensor B MAP sensor C TPS D IAC Some ECMs have internal memory devices that can be replaced to update the ECM Which of the following internal parts cannot be replaced A PROM B ROM C Mem
26. Chapter 15 Computer System Diagnosis and Repair 283 FPI GG et I INTEGRATE 125 i Tota a BLOCK LEE A Rite Gat F ia F LEED ES f ree uke oh HEE CUD 1 6 E Figure 15 7 This scan tool is widely used It retrieves trouble codes and displays other engine operating data This tool also provides information about possible causes of the particular trouble code Snap On store them for future reference They also perform the grounding step placing the ECM in the diagnostic mode Typical scan tool connectors are shown being attached to vehicle DLCs in Figure 15 8 Always follow the manufacturer s instructions when installing the scan tool and retrieving the trouble codes In addition to retrieving trouble codes scan tools often display sensor and output data and test certain output devices Some scan tools have the capability to identify the type of ECM and or PROM in use Others can be used for reprogramming the ECM Any trouble code number s present should be written down for later comparison with the corresponding numbers in the service manual If you are unable to retrieve trouble codes B or access sensor operating information make sure that the DLC wiring is properly connected to the ECM and that the scan tool is properly connected to the DLC If all the connec tions are good and you still cannot retrieve codes or access operating information the ECM is most likely defective Checking for a defective ECM
27. ECM to set trouble codes illuminate the MIL or cause malfunctions that might be blamed on the computer control system Typical noncomputer related problems that might be blamed on the computer control system include an engine miss caused by a burned valve or a fouled spark plug a rough idle caused by a vacuum leak stalling caused by a plugged fuel filter and overheating caused by a defective fan clutch Similarly the ECM can be fooled into sending improper commands to the actuators by such conditions as a dirty cooling system that results in incorrect temperature sensor readings or a leaking injector that causes the oxygen sensor to read a rich air fuel mixture It is a good idea to check overall engine condition before proceeding to the computer control system Typical computer control problems and some of the systems they can affect are shown in Figure 15 1 The first step in diagnosing computer control system problems is to check for obvious system defects Always 279 280 Auto Diagnosis Service and Repair gt tampering w Tl M Figure 15 1 The computer monitors and controls a variety of vehicle systems The problems listed here are only of few of the possi ble malfunctions that can occur in computer controlled systems Oldsmobile obtain the correct system schematic and determine which circuits and devices directly affect the ECM Also check the ECM fuses The ECM may be powered through more than one fuse fus
28. Figure 15 3 However it is important to check for trouble codes any time a computer system problem is suspected since trouble codes may be stored even if the MIL is not on To retrieve trouble codes from the ECM follow the procedures outlined in the factory service manual Basic procedures for code retrieval are discussed later in this chapter On most older vehicles trouble codes can be retrieved without special equipment although using a scan tool makes the process easier For many years each manufacturer used a different style of diagnostic connector which is sometimes called a data link connector or data terminal connector This made accessing diagnostic trouble codes difficult These connectors could be located almost anywhere depending on the manufacturer and vehicle model However the OBD II diagnostic protocol required manufacturers to use a standardized 16 pin MILES TO Empy Figure 15 3 The check engine or malfunction indicator light MIL is the first place to look when diagnosing any problem Check the status of this light while test driving the vehicle Toyota connector located inside the vehicle s passenger compart ment A 16 pin OBD II connector is shown in Figure 15 4 In this chapter we will refer to the diagnostic connector as a data link connector DLC Note The standard 16 pin DLC is used on some non OBbD II vehicles Some older computer systems do not have a DLC and other methods must be used to
29. GM diag nostic connector B Some diagnostic connectors require a separate power lead such as this Ford connector Snap On Tools 284 Auto Diagnosis Service and Repair OBD Il systems display five character alphanumeric codes consisting of one letter and four numbers The letter identifies the general function of the vehicle system causing the problem The first number indicates whether the code is a standardized code code assigned by SAE or a manufac turer specific code The second number indicates the specific system causing the problem The last two numbers which are called the fault designators identify the specific component sensor output device or ECM that might be at fault as well as the type of fault detected OBD and OBD II trouble code formats for a coolant temperature sensor problem are shown in Figure 15 9 In OBD systems the two digit format limits the number of trouble codes to 100 In the five character OBD II format there are over 8000 potential codes OBD II trouble codes do not correspond with OBD codes Never try to correlate the older trouble codes with OBD II fault designators For example there are only two or three codes used by most older computer control systems to indicate a possible oxygen sensor problem OBD Il systems have thirty or more stan dardized codes for the oxygen sensor and may have addi tional manufacturer specific codes depending on the system A complete list of the SAE design
30. OM or accessed through a connection to the manufacturer s database is entered into the ECM through the DLC The shop recalibration device is not a scan tool and a scan tool is not needed for this programming procedure Indirect Programming To perform indirect programming the proper scan tool is used to transfer information from a separate programming computer to the ECM See Figure 15 54B The scan tool can also be used to reset some computer controlled vehicle systems after programming is complete The programming computer may resemble a personal computer used in the home or it may be a computerized analyzer that is similar to the one used for direct programming In indirect programming the scan tool is connected to the programming computer and programming information is downloaded from the computer to the scan tool Most scan tools use a high capacity memory cartridge to store the programming information Some newer scan tools have enough fixed memory to hold the programming information and do not use a separate memory cartridge Once the programming information has been downloaded the scan tool can be disconnected from the programming computer taken to the vehicle and connected to the DLC Programming infor mation is then downloaded from the scan tool to the ECM through the data link connector Remote Programming Remote programming is done with the ECM removed from the vehicle See Figure 15 54C This procedure is used when ch
31. This sample chapter is for review purposes only Copyright The Goodheart Willcox Co Inc All rights reserved 15 Computer System Diagnosis and Repair After studying this chapter you will be able to e Retrieve trouble codes e Identify trouble code formats e Interpret trouble codes e Use a scan tool to check components and systems e Use a multimeter to check computer system components Identify computer system waveforms e Explain how to adjust throttle and crankshaft position sensors e Explain how to replace computer control system input sensors e Explain how to replace a computer control system ECM e Explain how to replace an ECM memory chip e Explain how to program an ECM with an erasable PROM e Explain how to replace computer control system actuators Technical Terms Electronic control module Titania type sensor ECM Artificial lean mixture Maintenance indicator light Artificial rich mixture MIL Forcing the actuator Flex test Anti seize compound Data link connector DLC EPROM Fault designators FEPROM Hard codes Flash programming Intermittent codes Direct programming Datastream values Indirect programming Snapshot Remote programming Zirconia type sensor Drive cycle test The on board computer controls many vehicle systems The level of control ranges from simply monitoring system operation to aggressively adjusting components or systems to compensate for changes in vehicle operating conditions
32. access the diagnostic infor mation in the ECM The Trouble Code Retrieval Process Trouble codes are retrieved from the ECM using a two step process The first step is to ground a DLC terminal to place the ECM in the diagnostic mode The second step involves reading the codes that the ECM displays All methods of trouble code retrieval are variations of these two steps regardless of whether they are performed manually by the technician or automatically by a scan tool Retrieving Trouble Codes without a Scan Tool In OBD systems trouble codes can be retrieved without using a scan tool In these systems the most common retrieval method is to ground one terminal of the DLC and then observe a series of flashes from a diagnostic light The light used may be the MIL or it may be an LED light emitting diode located on the ECM s case The sequence of light flashes represents a numerical code The factory service manual contains instructions for correctly interpreting the light flashes See Figure 15 5 Another common method of retrieving trouble codes involves connecting an analog voltmeter to specified DLC terminals Another DLC terminal is grounded and the trouble code output is displayed as a series of deflections of the volt meters needle Figure 15 6 Always follow the manufac turer s procedure exactly since the wrong sequence of steps can erase the trouble codes or in some cases cause false codes Some digital multimeters can dis
33. ach the scan tool to the DLC and set the tool to read MAP sensor output Turn the ignition switch to the on position without starting the engine and note the reading Some scan tools will display the actual vacuum reading which should be O in hg when the engine is off Other scan tools will display voltage which will be near the reference voltage with the engine off This reading will vary with altitude and must be compensated for Next attach a vacuum pump to the MAP sensor fitting Apply vacuum to the sensor with the ignition on and the engine off Scan tool readings should vary smoothly with changes in vacuum Voltage readings should drop or vacuum readings should increase as vacuum is applied If the MAP sensor fails these tests replace it MAP sensor signals can also be checked with a wave form meter Begin testing by determining whether the MAF sensor signal is digital or analog Then make the proper waveform meter connections and start the engine See Figure 15 24 Check the waveform against a known good MAP sensor Figure 15 24 Connections for checking a MAP sensor with a waveform meter Fluke waveform Figure 15 25 illustrates a good analog MAP sensor waveform Opening the throttle plate should cause the sensor voltage signal to rise smoothly Figure 15 26 is an example of a good digital MAP sensor waveform The sensor s output frequency should increase as the throttle plate is opened Always check the waveform fo
34. adapt to new components and updated programming information The relearn procedure can often be accomplished by driving the vehicle at various speeds for about ten minutes Some vehicles require a specific relearn procedure which may include idling in drive for a specified amount of time or until the engine reaches its normal oper ating temperature Always check the manufacturer s service literature for specific relearn procedures Ignore any unusual engine and transmission conditions until the relearn proce dure is complete OBD Il Drive Cycle Test OBD ll equipped vehicles must pass a drive cycle test when the ECM or the battery has been replaced or after trouble codes have been erased The drive cycle test is also part of some states I M emissions testing procedures The test is performed before the actual emission testing begins The drive cycle test involves attaching a scan tool to the vehicle and driving the vehicle for a set time The drive cycle consists of specific acceleration cruising and deceleration steps The drive cycle test is designed to tell the technician whether the OBD II system is operating and whether the vehicle is operating efficiently enough to have a reasonable chance of passing an emissions test Figure 15 57 is a chart showing a typical drive cycle and the OBD II systems moni tored Note that the entire test takes 12 15 minutes starting with a cold engine If the vehicle fails the drive cycle test the
35. age the replacement ECM A shorted actuator for instance could destroy the driver in the new ECM corrected by updating the ECM with new information This is called reprogramming and is covered later in this chapter 4 Note An ECM related problem can sometimes be Checking Actuators Actuators are the output devices of the computer control system The following section provides a general overview of common actuator tests Testing Solenoids and Relays Most actuators operated by the computer control system are solenoids or relays Solenoids are simply wire coils that are energized to move a plunger Relays are wire coils that are energized to open or close a set of electrical contacts Some relays are electronic and use transistors in place of the wire coil and contacts Many solenoids and relays will make a clicking noise when they are energized It is often possible to energize the winding and listen for the click Most solenoids and relays are operated by battery voltage and can be energized with jumper FRQ 416 7H P P 2 83v 250mv 2ms Figure 15 39 Speed sensor waveforms will be a series of oscillations similar to the ones shown here Ferret Instruments wires from the battery terminals Be sure to disconnect the actuator s electrical connector before energizing the winding If the connector is attached current flow may damage the ECM Listen for a click as you make and break the circuit with the jumper wires It
36. ally made of brass never try to remove this type of sensor with an open end wrench Install and tighten the new sensor carefully Use a socket or box end wrench to avoid distorting the shell After tightening reinstall the electrical connector and check the operation of the knock sensor as explained earlier Crankshaft Camshaft Position Sensors Most crankshaft and camshaft position sensors are installed on the engine block timing cover camshaft housing or transmission bell housing These sensors are secured by a clamp and a single bolt or attached to a bracket held by one or more bolts After removing the electrical connector and bolt s pull the position sensor from the engine Check the end of the sensor to ensure that it has not contacted the crankshaft or camshaft sensor ring Figure 15 52 If the end of the sensor has been damaged by contact with the sensor ring or other moving parts determine the cause and make the necessary corrections before installing a new sensor To install the new sensor make sure replacement O rings or gaskets are in place Then lightly lubricate the sensor tip with engine oil and push the sensor into position Reinstall the clamp and bolt then install the electrical connector ECM Service Before replacing an ECM make sure the ignition switch is turned to the off position Disconnect the negative battery cable if recommended by the manufacturer After locating the ECM disconnect the wiring connect
37. and Repair 293 hot or cold The following tests will work with engine coolant intake air exhaust gas and automatic transmission transaxle fluid temperature sensors Note Before performing tests on an engine ie coolant temperature sensor make sure the coolant level in the radiator is correct The sensor must be in contact with the coolant to operate properly Also make sure that the sensor electrical connector is attached and that the wiring shows no obvious damage Tests Using a Scan Tool Temperature sensor operation can also be checked using a scan tool To measure temperature sensor output using this procedure you must have a scan tool that can convert sensor voltage inputs to temperature readings This is possible on many scan tools Follow the manufacturer s instructions Begin the test procedure by setting the scan tool to read the temperature as measured by the sensor Remove the sensor s connector and note the temperature indicated on the scan tool The scan tool readings should indicate the sensor s lowest reading usually between 30 and 40 F 34 and 40 C Figure 15 27 Then jumper the connector leads to simulate the sensor s highest possible reading which can be 260 F 127 C and higher If the scan tool data shows the output to be correct the sensor is the likely cause of the problem If not there is high resistance in a sensor connec tion a shorted or grounded wire or a problem in the ECM Tests
38. and regulations Just keeping up from year to year takes considerable reading and study So what does this mean to you the future technician For starters trial and error learning is out One error on a computer controlled system will destroy several hundred dollars worth of computer equipment The only way to troubleshoot and repair late model vehi cles properly and efficiently is to get your hands on every bit of new information you can and study it thoroughly Now is the time to begin collecting information about modern vehicles and taking time to read about the latest developments Summary The first step in diagnosing computer control system prob lems is to check for obvious defects Look for problems in the wiring damaged parts and other vehicle problems that may be confused with the computer control system malfunctions Before performing any other diagnostic steps retrieve trouble codes from the ECM memory Trouble codes may be stored even if the MIL is not on To retrieve trouble codes follow the procedure outlined in the factory service manual On OBD vehicles trouble codes can be obtained without a scan tool On OBD II vehicles a scan tool must be used to retrieve the codes OBD and OBD II code formats are different The next step is to separate the hard codes from the intermittent codes Then interpret the codes to arrive at a defective system Or part After the trouble codes have been retrieved and inter preted chec
39. and reprogramming the unit with new information The new information often cures engine driveability and trans mission shifting problems The computer memory section that can be updated is generally called the EPROM electronically erasable programmable read only memory or the FEPROM flash erasable programmable read only memory Providing the existing memory with updated information is usually referred to as flash programming Actual reprogramming details vary between manufac turers but the basic procedures are the same for all vehicles One of three methods can be used to reprogram the ECM e Direct programming e Indirect programming e Remote programming Direct Programming Direct programming is the fastest and simplest method of reprogramming an ECM The new information is down loaded by attaching a shop recalibration device usually a computerized analyzer or a programming computer directly to the vehicle s DLC See Figure 15 54A The erasure and reprogramming is done by accessing the proper menu and Programming computer Programming computer Scan tool attached to ECM ECM removed from vehicle C Figure 15 54 Flash programming methods A Direct program ming B Indirect programming C Remote programming 306 Auto Diagnosis Service and Repair following the instructions as prompted by the recalibration device Then the information often stored in the device s memory contained on a CD R
40. anges must be made through a direct connection to a manufacturer s database Remote programming can also be done in cases where normal direct or indirect programming is not practical or possible Special connectors and tools are required for remote programming In most cases this proce dure is done only at new vehicle dealerships To perform remote programming remove the ECM from the vehicle as described earlier in this chapter Once the ECM has been removed take it to the programming device The programming device is generally a computer located in the shop This device may contain the new ECM information or it may be used with a modem to connect to a remote database Attach the programming device s electrical connectors to the ECM Access the device s programming menu and follow instructions given in the menu to program the ECM Programming normally takes only a few minutes When programming is complete as indicated on the menu remove the programming device s electrical connectors from the ECM and reinstall the ECM in the vehicle Actuator Replacement Replacing an actuator is often easier than diagnosing the original problem As you learned in earlier sections the ECM operates a wide variety of actuators The following procedures are general examples of actuator replacement The replace ment of specific actuators is covered in the applicable chap ters Always consult the proper service literature when removing and replacing
41. atastream values For example the scan tool can access the ECM and monitor sensors inputs to provide the technician with readings of engine idle RPM as well as the desired factory set idle speed From this information the technician can instantly determine whether idle speed is correct Another scan tool feature will give the technician a series of numbers or percent ages indicating whether the fuel trim air fuel ratio is correct for both short and long term engine operation Figure 15 11 Other inputs provide exact engine timing in degrees actual temperature reading from the engine temperature sensors and voltage or duty cycle readings that indicate manifold vacuum throttle position EGR position and many other oper ating conditions See Figure 15 12 Some scan tools can interface with the ECM to provide snapshots of engine operating parameters The snapshot is a method of recording the engine operating conditions present when a malfunction occurs This allows the technician to determine exactly what is happening to cause the problem To record the snapshot information the technician must drive the vehicle with the scan tool attached until the malfunction occurs The technician can then retrieve the snapshot read ings from the scan tool Some vehicle ECMs save snapshot information when a malfunction occurs and the technician can access it once the scan tool is attached Figure 15 13 which shows information obtained while looking fo
42. ated OBD II trouble codes is located in Appendix B of this text Interpreting Trouble Codes Once trouble codes have been retrieved they can be compared to the trouble code information in the service manual if this information is not provided by the scan tool Sometimes the same trouble code can be caused by more than one problem Therefore it is vital to correctly interpret the trouble codes properly Trouble codes usually indicate one of three things e An engine drive train or other vehicle problem is causing one of the sensors to transmit a voltage signal that is out of range too high or too low e A sensor s ability to perform is starting to deteriorate e Another part or circuit in the computer control system is defective Multiple codes should be addressed starting with the lowest code number and working sequentially toward the highest number In cases where multiple codes are stored one or more of the codes can often be eliminated using this process For example a fuel injector defect on the 4 cylinder P0204 could be the cause of a misfire on cylinder 4 P0304 If the technician determines the cause of code P0204 it is likely that the cause of code P0304 will also be determined Once the problem area has been identified concentrate your troubleshooting efforts on that particular area If the trouble code indicates that the problem is a defective output device or the ECM you will be able to go to the specific c
43. d show a smooth rise in voltage as the throttle is opened Figure 15 33 illustrates the waveform pattern produced by one TPS as the throttle is slowly opened and closed If there are spikes or breaks in the waveform the TPS is defective See Figure 15 34 Testing Crankshaft and Camshaft Position and Speed Sensors On most vehicles the crankshaft and camshaft position and speed sensors provide signals that influence the control of both the fuel and ignition systems A faulty crankshaft or Throttle position sensor uy ON Figure 15 32 Testing a throttle position sensor with a waveform meter Fluke 472 v maximum 680 we minimum wiv A 100ms 014 POTENTIOMETER REFEAT TEST RANGE Figure 15 33 This waveform indicates a properly operating throttle position sensor Note how the pattern rises and falls as the throttle is opened and closed Fluke 296 Auto Diagnosis Service and Repair i Spikes in a downward direction indicate a short to ground or an intermittent open in the resistive carbon strips Figure 15 34 This pattern was produced by a defective throttle position sensor The downward spikes indicate a short to ground or an intermittent open in the sensor Fluke camshaft sensor may produce either a no start condition or intermittent stalling The test procedures for crankshaft and camshaft sensors are similar to those for pickup coils Hall effect sensors and magnetic pickup switches mounted in the
44. de format D hard and intermittent trouble codes 4 Trouble codes in most OBD Il systems should be cleared by A disconnecting the negative battery cable B using a scan tool C removing a fuse to disconnect battery power from the ECM D All of the above 10 11 12 13 14 15 16 17 Chapter 15 Which of the following sensors can be checked with a scan tool A Oxygen sensor B MAP sensor C MAF sensor D All of the above The waveform meter provides the technician with a of what is happening in the circuit The two types of MAF sensors are the ______type and the__ _ type A MAP sensor measures engine ____ A temperature sensor can be checked with a n A scan tool B ohmmeter C waveform meter D All of the above A pressure sensor can be checked with a n A voltmeter B ohmmeter C ammeter D All of the above A noid light is used to check which of the following sensors A Crankshaft position sensor B Throttle position sensor C Temperature sensor D Knock sensor Throttle position sensors can go out of adjustment because of A linkage wear B changes in the ECM C changes in the sensor material D Both A and C Throttle position sensors are adjusted by attaching what type of meter to the sensor A Voltmeter B Ammeter C Ohmmeter D All of the above What two methods are often the only ways to check an
45. e output voltage is not as specified with the pencil inserted the sensor is defective A waveform meter can be used to check the operation of an analog MAF sensor Attach the waveform meter as instructed by the manufacturer Start the engine and compare the waveform with a known good waveform for proper shape frequency and amplitude height See Figure 15 20 Testing Digital MAF Sensors The output of a digital MAF sensor can be measured by most scan tools waveform meters or multimeters capable of reading RPM or duty cycles Consult the multimeter manual to determine the exact meter capabilities The test procedure is similar to that for analog MAF sensors The frequency will be at a set value with the ignition on and the engine off Mass airflow sensor 0 9 volts Key on engine off Mass airflow sensor 2 5 volts Engine running Mass airflow sensor 0 3 volts Reading from malfunctioning sensor Figure 15 19 Scan tool reading for mass airflow sensor signal Scan tool readings should be verified using a digital multimeter Fluke PEAK PEAK FREQUENCY DUTY CYCLE V PULSE WIDTH v S00ms D1 RA GENERAL SENSORS Pa m OHM OPEN LOSE B Figure 15 20 A Connections for testing an analog MAF sensor B This waveform indicates correct analog mass airflow MAF sensor operation Fluke Chapter 15 Computer System Diagnosis and Repair 291 Figure 15 21A When the engine is started increased
46. ect the knock sensor connector connect the voltmeter to the sensor and repeat the tap test If the sensor produces a voltage signal the problem is in the wiring or the ECM A similar test can be made using a waveform meter Connect the meter as recommended by the manufacturer When the engine is tapped the waveform produced should resemble the pattern in Figure 15 38 A timing light can be used to check the knock sensor circuit on a running engine Direct the light on the timing marks and tap on the engine Tapping on the engine should make the timing marks move in the retard direction Testing Vehicle Speed Sensors Most speed sensors produce an alternating current ac as they rotate Therefore the operation of many vehicle speed sensors can be checked by measuring their output in ac volts The simplest way to check a speed sensor is to use the proper scan tool Raise the drive wheels off the ground and shift the vehicle into drive Modern scan tools can convert the speed sensor output into a speed miles per hour reading If there is no reading check the speedometer If the speedometer is indicating speed a defect exists in the wiring to the ECM or in The upper horizontal limes should reach Voltage transitions should be straight and vertical reference voltage E Peak to peak voltages should equal reference we voltage The lower horizontal lines A should almost reach ground Maximum peak levels should be equal to each o
47. et by problems that occur occasionally or in some cases only once These problems are sometimes difficult to isolate A hard code will illuminate or flash the MIL whenever the engine is running Intermittent codes will illuminate the MIL only when the problem is occurring after which time the light will go out To differentiate between hard and intermittent codes record all the trouble codes stored in the ECM s memory Then erase the stored codes Most scan tools have the capability to erase trouble codes On some vehicles removing electrical power to the ECM for the period of time specified in the service manual will erase the codes This can be accomplished by disconnecting the negative battery cable or removing the fuse that supplies current to the ECM Figure 15 10 However the ECM in some OBD Il equipped vehicles can retain trouble codes for several days without battery power Removing battery power from the ECM on these vehicles may not erase stored trouble codes Therefore you must use a scan tool to clear the trouble codes in OBD II systems After erasing the codes restart the engine and allow it to run for the period of time specified by the manufacturer After the engine has run long enough stop it and re enter the diag nostic mode Hard codes will usually reset almost immediately after the engine is started when the ECM requests informa tion from a defective sensor or tries to activate a faulty output device Intermittent
48. he sensor from the vehicle Figure 15 46 Install the new sensor in the reverse order of removal When installing a MAF sensor be sure to face the sensor s inlet in the correct direction See Figure 15 47 Tighten all the retaining bolts or clamps Then reconnect all wiring and hoses Finally recon nect the negative battery cable Throttle Position Sensor Replacement To replace a throttle position sensor make sure the igni tion switch is in the off position Then remove the TPS elec trical connector Remove the fasteners holding the throttle position sensor to the throttle body and slide the TPS from the throttle shaft Slide the new TPS into position then reinstall the fasteners and electrical connector Figure 15 48 shows the sequence used to replace a typical throttle position sensor After the new sensor is installed it may be necessary to adjust it using the procedure explained earlier in this chapter High temperature sealant Figure 15 45 If recommended apply a thin coat of specified sealant to the oxygen sensor threads Temperature Sensor Replacement Temperature sensors are used in many places on the engine and other vehicle components With the exception of the coolant temperature sensor they are relatively easy to replace Engine Coolant Temperature Sensor Engine coolant temperature sensors are threaded into a coolant passage in the engine block or the radiator They should not be removed until the engine coo
49. ia type sensor you Oxygen sensor Oxygen sensor Oxygen sensor bank 1 sensor 1 bank 1 sensor 1 bank 1 sensor 1 21 volts 52 volts 96 volts Figure 15 14 Oxygen sensor voltage or resistance varies with the content of the oxygen in the exhaust gas Be sure that you are monitoring the correct sensor on vehicles with multiple oxygen sensors must have a voltmeter or multimeter that can read very low voltages in the 100 900 millivolt mV range To measure the resistance of a Titania type sensor you will need an ohmmeter or a multimeter that can measure resistance For this test to be accurate the engine must be warm enough for the ECM to be in closed loop mode Unless the oxygen sensor is a heated type the exhaust system temperature must be at least 600 F 350 C before starting the test When tested on the engine the sensor must be heated either by its heating element when used or by exhaust heat Heated oxygen sensors can be tested within seconds of start up if the engine is warm The service manual will tell you whether the sensor is the heated type In all cases the manufacturer s instructions must be followed closely current flow They can be destroyed by improper grounding or testing with test lights or low impedance multimeters Some manufacturers do not recommend meter tests of the oxygen sensor Always consult the manufacturer s instructions before testing an oxygen sensor vV Caution Oxygen sen
50. ible link or relay If one of the ECM fuses is blown the MIL may not illuminate at any time retrieve trouble codes Removing fuses can erase the trouble codes from the ECM s memory If the MIL is not on when the ignition switch is on and the engine is not running check the bulb before checking any fuses After checking the ECM fuses check all other vehicle fuses A blown fuse in an unrelated circuit may cause current to attempt to ground through the ECM affecting vehicle oper ation or illuminating the MIL Remember that computer input sensors operate on very low voltages and slight wiring problems can cause inaccurate readings Any arcing or sparking caused by loose electrical connections will affect computer system operation Carefully check all electrical connections Look for dis connected or corroded ground wire connections Figure 15 2 Connector problems can also be caused by water entering the connector and by overheating due to high resistance connec tions One way to check for problems in the connectors and wiring is to perform a flex test To make a flex test pull on the various system electrical connectors and wiggle the related ae Note Never remove fuses before attempting to Figure 15 2 Bad grounds are a common source of trouble Always check grounds by visual inspection and by pulling on the connections Also try to tighten the attaching bolt when applicable wiring as the engine operates If the engi
51. ion that can Caution Late model vehicles have multiple oxygen cause the threads to seize Many oxygen sensors require a Shoe Papaa banan eS removing the correct special socket or tool for removal Figure 15 44 If the oxygen 302 Auto Diagnosis Service and Repair Once the sensor is out check the manifold or exhaust pipe around the sensor fitting for cracks or pinholes These will draw in outside air during engine operation causing a false sensor reading Replace the manifold or exhaust pipe if any damage is found Inspect the oxygen sensor for indications of carbon loading contamination or poisoning Before installing the new oxygen sensor check the service manual to determine whether the sensor threads should be coated with anti seize compound Some manu facturers recommend a special high temperature sealant Use only a light coating of sealant to avoid plugging the external air vents Figure 15 45 Do not overtighten the oxygen sensor since this may damage the brass shell If anti seize compound or sealant is used wipe off any excess from the exhaust mani fold or exhaust pipe and the sensor after tightening MAF and MAP Sensor Replacement The mass airflow and manifold absolute pressure sensors on most vehicles are retained by one or two bolts or clamps and are relatively easy to replace Start by discon necting the negative battery cable Remove any wiring and hoses from the sensor Loosen the bolts or clamps and remove t
52. is often possible to determine whether a solenoid or relay clicks without using jumper wires Park the vehicle in a quiet place and turn the ignition switch to the off position Turn off the radio air conditioner and any other noise producing equipment Open the hood and locate the actuator On some vehicles the actuator may be located under the wheel well Place your ear close to the actuator and have someone turn the ignition switch to the on position without starting the engine Then listen for a click If a solenoid or relay does not click when energized it may be defective Make sure the device is being supplied with electricity usually a reference voltage from the ECM before deciding that it is defective Some solenoid plungers such as those used in the idle speed control or the transmission may become clogged with carbon or sludge and fail to operate even when the solenoid is good Cleaning can sometimes restore these solenoids to proper operation Electronic relays have no moving parts and will not click If the solenoid or relay appears to be defective always follow up by performing the tests outlined in the following sections Scan Tool Tests Most scan tools can be used to check solenoid and relay operation Examples of these types of solenoids are the idle air control IAC solenoid some transmission solenoids and mixture solenoids on older vehicles with carburetors Check the output voltage pulses usually called counts as
53. k indi ak Note What appears to be a sensor problem may be vidual computer system components Many tests can be Palised by A elect in tne engine Or anomer vehicte p y p y system Always make sure that the vehicle has no made using jumper wires multimeters or waveform meters mechanical or electrical defects that could cause out of The following sections will quickly review sensor and output range sensor readings before condemning a sensor device operation and discuss typical testing procedures Testing Sensors Sensors are the inputs to the computer system If a sensor is defective the ECM cannot correctly control the Testing Oxygen Sensors Oxygen sensors measure the amount of oxygen in the exhaust gases and send a signal that represents this amount Snapshot Data Captured from Vehicle Coolant temperature Intake air temperature Mass airflow sensor sensor sensor Throttle position sensor 212 F 100 C 79 F 26 C 3 4 volts 1 99 volts Engine speed Oxygen sensor Vehicle speed sensor Battery voltage 2476 rpm 31 volts 40 mph 13 2 volts Idle air control Evaporative emission Cooling fan EGR pintle position valve canister solenoid 5 percent Off On 11 percent Malfunction indicator Diagnostic trouble Open closed loop Fuel pump relay lamp codes P0272 P0304 Open Transmission gear Cruise control AC compressor clutch Knock signal Drive Ignition timing BTDC Base timing Actual timing Figure 15 13 Data captured
54. k the computer control system devices Sensors to be tested include the oxygen sensor mass airflow MAF sensor manifold absolute pressure MAP sensor tempera ture sensor throttle position sensor crankshaft and camshaft position and speed sensors knock sensor and vehicle speed sensor Testing can be done with a multimeter a scan tool or a waveform meter A faulty ECM can be identified by substituting a known good ECM for the questionable unit or through the process of elimination If every sensor actuator and system fuse as well as all wiring and connectors have been checked and are okay the problem is most likely in the ECM The ECM will occasionally produce a trouble code that cannot be confirmed by testing or a trouble code that does not exist This indicates that the ECM is defective Actuators include solenoids and relays and motors Actuators can be tested with scan tools ohmmeters or wave form meters Sometimes the actuator can be tested by observing whether or not it operates Some sensors such as throttle position and crank shaft camshaft position sensors can be adjusted Check the vehicle s service manual for correct procedures Replacing sensors is relatively easy Some are threaded into the engine Others are installed on brackets near the engine Most vehicles have multiple oxygen temperature knock and pressure sensors Be sure you are removing the correct sensor Once the sensor has been replaced it wil
55. l be necessary to operate the engine to allow the ECM to recali brate itself ECM service involves replacing defective parts or repro gramming the ECM itself PROMS must be replaced carefully to avoid damaging the ECM Reprogramming installs new information to cure driveability and other engine problems Solenoids relays and motors can be replaced without major disassembly Once the new parts have been installed start the engine and allow the ECM go through its relearn procedure Always recheck system operation after performing computer control system service If possible road test the vehicle to ensure that it operates properly After completing the road test recheck for trouble codes Review Questions Chapter 15 Please do not write in this text Write your answers on a separate sheet of paper 1 A vehicle is brought to your shop with a suspected computer system problem Which of the following should be done first A Obtain the vehicle s service manual B Check for obvious problems C Retrieve trouble codes D Road test the vehicle 2 A vehicle has a driveability problem and the charging system voltage is low Which should be diagnosed and corrected first the driveability problem or the low charging system voltage 3 An OBD II computer system will have all of the following except A a 16 pin diagnostic connector B an oxygen sensor mounted behind the catalytic converter C a two digit trouble co
56. le _____ ASE Type Questions 1 Trouble codes on most modern vehicles can be read only by using which of the following A The MIL B A voltmeter C A scan tool B A waveform meter 310 Auto Diagnosis Service and Repair Technician A says that grounding one of the diagnostic terminals on an OBD II system will cause the engine to go into closed loop operation Technician B says that a scan tool is needed to retrieve trouble codes from an OBD II system Who is right A A only B B only C Both A and B D Neither A nor B Hard trouble codes are being discussed Technician A says that hard codes will cause the MIL to illuminate only when the key is on and the engine is off Technician B says that hard codes will cause the MIL to be on when ever the engine is running Who is right A A only B B only C Both A and B D Neither A nor B An excessively rich reading from the oxygen sensor can mean all of the following except A the oxygen sensor is defective B a leaking fuel injector C the gasoline is the wrong octane rating D a defective airflow sensor The heater circuit of an oxygen sensor can be checked with which of the following A Ohmmeter B Voltmeter C Ammeter D Jumper wires The output of a frequency type MAF sensor can be measured by using a A scan tool B multimeter that can read RPM C multimeter that can read duty cycles D
57. les with idle air control IAC solenoids the IAC will simply open to allow more air into the engine Blocking the air intake on these engines may not produce definite results After the enrichment device is in place and operating check the meter or scan tool Voltage should be higher than average or resistance should be low indicating that the rich condition is being read by the oxygen sensor If the oxygen sensor does not react takes longer than two seconds to react or indicates an out of range reading the sensor is defective and should be replaced Checking Oxygen Sensor Waveforms A waveform meter can be used to detect problems that a voltmeter or scan tool will not reveal The waveform meter displays a picture of what is happening in the circuit Some waveform meters require that the wires leading to a particular component be pierced to access the component waveform Other waveform meters can access needed waveforms through the diagnostic connector To check the waveform of an oxygen sensor attach the waveform meter leads to the proper sensor lead and ground or to the diagnostic connector Set the meter to the proper voltage range Then start the engine and allow the sensor readings to stabilize Some waveform meter makers recom mend running the engine at 2500 RPM for 2 minutes to heat the sensor Compare the actual waveform with a known good waveform to determine whether the sensor is operating prop erly See Figure 15 16 Mode
58. ling system is depressurized and drained below the level of the sensor Always leave the radiator cap loose while changing any cooling system part to prevent pressure buildup Figure 15 49 Coolant temperature sensors have pipe threads which are similar to those in household plumbing systems and are usually installed in the block Most coolant temperature sensors can be removed by loosening them from the engine using the proper deep well socket and a ratchet handle In some cases the sensor can be removed with a box end Figure 15 46 When replacing a MAP sensor disconnect the vacuum line and the electrical connector Then remove the fasteners and remove the sensor from its mounting Arrow denotes direction Of airflow Figure 15 47 When reinstalling an airflow sensor make sure it is installed in the correct direction Note the arrow on this sensor which indicates the direction of airflow Chapter 15 Computer System Diagnosis and Repair 303 wrench A few sensors can only be removed with a special socket Always disconnect the electrical connector from the wiring harness before installing the socket over the sensor should not be removed until the engine has cooled for several hours This cooling period is necessary to prevent damage to the threads in the aluminum part vV Caution Sensors installed in aluminum parts c g Figure 15 48 This figure shows the sequence of steps necessary to remove a thr
59. nce measurements are correct the defect is in either the ECM or the wiring To check the wiring reinstall the sensor connector unplug the ECM and measure the sensor s resistance at the ECM harness If the resistance is now incorrect check for a wiring problem Remember that corroded or loose terminals will cause higher than normal resistance readings while shorts to ground will cause lower than normal readings If the resistance is correct at the harness the problem is in the ECM Temperature vs Resistance Valve Approximate o N nin olpalaAla ola da a alolasto ala N olm al are olo N ale o je N w N ALO 527 000 100 700 Figure 15 28 Most temperature sensors are thermistors The higher the temperature the lower the resistance General Motors 294 Auto Diagnosis Service and Repair Tests Using a Waveform Meter Coolant and air temperature sensors can be tested with a waveform meter To check a coolant temperature sensor connect the meter start the engine and then observe the waveform Figure 15 29 Coolant temperature sensors should produce a smooth voltage transition as the coolant tempera ture increases There should be no sharp spikes in the wave form Upward spikes indicate an open in the circuit Downward spikes indicate a short To test an air temperature sensor remove the sensor from the intake duct Connect the waveform meter and place the ignition switch in the on p
60. ne begins running erratically or the MIL illuminates when the connectors and wires are moved the connector or the wiring is defective Also look for problems in the charging system Low voltage caused by a defect in the charging system will confuse the ECM and cause erratic operation Check charging voltage with a voltmeter as the engine idles Normal charging voltage Chapter 15 Computer System Diagnosis and Repair 281 should be between 12 and 14 5 volts If charging voltage is low check the alternator voltage regulator battery and related connections Also check for a loose or glazed alter nator drive belt If the voltage at idle is within specifications increase engine speed to about 2500 RPM and recheck the voltage If the voltage is above 14 5 volts suspect a defective voltage regulator Correct charging system problems before attempting further diagnosis Check for any sign of modification or tampering This includes sensors and output devices that have been removed unplugged or disabled in some manner If a visual inspection does not reveal problems check for and retrieve any diag nostic trouble codes present in the ECM as outlined in the following section Retrieving Trouble Codes Before performing involved diagnostic routines on computer controlled vehicles always check for and retrieve trouble codes from the ECM If the MIL is on when the engine is running trouble codes are present in the ECM s memory
61. ngines Figure 15 56 To replace a motor make sure the ignition switch is in the off position Then disconnect Figure 15 55 Relays are installed in one of several fuse boxes Be sure that you are replacing the correct relay Chapter 15 the electrical connector and remove the fasteners holding the motor in position Transfer any brackets or other hardware from the old motor to the new one then install the new motor Install and tighten the fasteners and replace the electrical connector Start the engine and check motor operation Follow up for Computer Control System Repairs After replacing computer system parts or reprogramming the ECM recheck system operation to ensure that the repairs have been successful After making computer system repairs erase any trouble codes from the ECM s memory Then road test the vehicle to ensure that the MIL does not come on Finally make sure that none of the trouble codes has been reset If the repair involved correcting a lean or rich condition verify that the exhaust emissions output is within specifications Figure 15 56 The IAC is a commonly replaced motor located on the throttle body near the throttle plate It can usually be removed and replaced easily General Motors Computer System Diagnosis and Repair 307 ECM Relearn Procedures After service the computer system may require a relearn procedure which is a period of vehicle operation that allows the system to
62. o ohms or infinity depending on whether the contacts are normally open or normally closed Energizing the relay winding should cause the reading to change to the opposite value Figure 15 40 Use an ohmmeter to test solenoid and relay windings Any reading other than the one specified indicates a defective winding Operation of solenoids and relays can also be checked with a waveform meter Connect the meter and start the engine Operate the engine as needed to energize the actu ator Observe the waveform Figure 15 41 shows some possible waveforms Always consult the manufacturer s spec ifications to determine what the waveform should look like for the exact component being tested If the pattern does not look like it should or shows spikes or breaks the component is defective Testing Motors A few vehicles use stepper motors to control idle speed operate the EGR valve or provide pressure for the anti lock brake system These motors operate in small increments or steps to move the throttle plate or provide braking pressure Some anti lock brake and automatic level control systems have electric motors that operate pumps In addition the radi ator fan motors in most late model vehicles are controlled by the ECM whether the ECM operates it through a relay If so check the relay as explained previously ie Note Before checking any motor determine 7 40v 50ms Figure 15 41 Waveforms for various types of actuator
63. omponent and determine whether the problem is a discon nected or defective part a mechanical problem with the part or a wiring problem In the case of an out of range sensor Two digit diagnostic trouble code indicating a coolant temperature sensor problem 15 Equivalent OBD II three digit trouble code with alphanumeric designator P0115 Specific fault designator B Body C Chassis P Power train U Network O SAE assigned 1 Manufacturer specific 1 Fuel and air metering 2 Fuel and air metering 3 Ignition system or misfire 4 Auxiliary emissions control 5 Vehicle speed idle control and auxiliary inputs 6 Computer and auxiliary output 7 Transmission 8 Transmission 9 Category to be determined 0 Category to be determined Fuel system related malfunctions only Figure 15 9 OBD and OBD II systems use different diagnostic code systems Both systems are distinct and trouble codes from one system should not be correlated with those from the other system Chapter 15 Computer System Diagnosis and Repair 285 reading you must determine whether the problem is a defec tive sensor or if an abnormal operating condition is causing the out of range reading Hard and Intermittent Codes Two types of codes are stored in the ECM memory hard codes permanent and intermittent codes temporary Hard codes indicate an ongoing problem These problems are generally easy to track down Intermittent codes are s
64. or s and remove the attaching fasteners Most ECMs are attached to the vehicle Sensor tip Threads Connector B Figure 15 51 A Knock sensors are threaded into the engine This one is located under the intake manifold B External parts of a knock sensor General Motors and Ford Check tip for damage Figure 15 52 After removing a crankshaft or camshaft position sensor check its tip for signs of damage If damage is found be sure to determine and correct the cause before installing a new sensor Chapter 15 Computer System Diagnosis and Repair 305 with bolts or screws but a few are held in place with clips or plastic rivets To replace the ECM set the unit in place and reinstall the fasteners being sure to reattach any ground straps Then reconnect the electrical connectors On some vehicles it may be necessary to reconnect the electrical connectors before placing the ECM in position on the vehicle the new ECM follow the manufacturer s instructions concerning static discharge and unit grounding vV Caution To prevent static discharge from damaging Updating ECM Information Many ECMs can be restored to proper performance by providing them with updated information On some older vehi cles the PROM chip sometimes called a mem cal chip can be replaced On many late model vehicles the ECM can be reprogrammed from an outside source These processes will be outlined in the following sections
65. osition Spray a small amount of water on the sensor and observe the waveform Figure 15 30 A temperature drop should register in an upward movement of voltage as resistance increases As with the coolant tempera ture sensor spikes indicate an open or a short Testing Pressure Sensors Most pressure sensors are simple on off switches They can be tested with a scan tool or an ohmmeter If using a scan tool connect it to the DLC as specified by the tool Temperature 7 hot Temperature is decreasing o the resistance to increase Temperature cold Temperature sensor readings are typically made over a long period of time Temperature sensors are normally negative temperature B coefficient thermistors NTC Figure 15 29 A Setup for testing a temperature sensor with a waveform meter B Voltage changes in a temperature sensor as temperature increases The waveform should show a smooth change with no spikes Ferret Instruments manufacturer Before checking a pressure sensor with an ohmmeter remove the electrical connector Figure 15 31 shows a pressure sensor being tested with an ohmmeter Note that the placement of the test leads varies depending on the number of sensor terminals If the sensor is normally closed pressurizing it should cause it to open The scan tool will indicate that it is open or the ohmmeter will read infinite resistance The sensor should revert to the closed p
66. osition or zero ohms when the pressure is removed If the sensor is normally open pressurizing it should cause it to close The scan tool should indicate that it is closed or the ohmmeter should read zero resistance The sensor readings should go to the open position or infinite ohms when the pressure is removed Testing Throttle Position Sensors The throttle position sensor TPS monitors throttle opening and closing so the ECM can adjust fuel and ignition spark timing to match driver demand Typical TPS voltage can range from 0 2 volts at fully closed throttle to 5 0 volts at wide open throttle However this sensor is adjustable on most vehi cles so depending on the manufacturer and the engine anything between 0 2 and 1 25 volts might be considered acceptable at closed throttle Throttle position sensors modify a reference voltage therefore an ohmmeter can be used to test these sensors TPS resistance varies by manufacturer and application To test a throttle position sensor the internal resistance of the sensor must be measured at different throttle openings Start the test procedure by disconnecting the throttle position sensor s electrical connector Connect an ohmmeter to the sensor s input and output leads Next slowly open the throttle while watching the ohmmeter The ohmmeter reading should increase smoothly without jumping or skipping If it does jump or skip the sensor is defective MAX 3 30v MIN 1 90v 1 00v 5
67. ottle position sensor A Remove the electrical connector B Loosen and remove the attaching bolts C Remove the sensor from the shaft Note the position of the throttle shaft for reinstallation General Motors Before installing the new sensor coat the threads with the proper sealant When installing the sensor do not over tighten it as this may damage the threads or distort the sensor shell If the cooling system was drained refill it before restarting the engine Leave the radiator cap loose and recheck coolant level after allowing the engine to run for about 10 minutes A few front wheel drive engines require a special fill procedure Follow the manufacturers recommendations The bleed valve shown in Figure 15 50 can be loosened to remove air from the cooling system Note When bleeding air from a cooling system turn on the heater with the blower fan on high This will provide additional cooling and fluid circulation during the bleeding process Intake Air and Exhaust Gas Temperature Sensors Intake air and exhaust gas temperature sensors are not under pressure when the engine is not running Intake air Figure 15 49 Loosen the radiator cap before servicing any part of the cooling system Figure 15 50 After the coolant temperature sensor has been replaced it may be necessary to bleed air from the cooling sys tem Some engines are equipped with bleed valves This one is located on the thermostat housing 304 Auto
68. play codes as elec tronic pulses The trouble codes on some vehicles can be accessed by turning the ignition switch on and off three times within five OBD II standard connector Figure 15 4 This figure shows the standardized data link connector used in OBD II systems All vehicles built within the last several years use this connector 282 Auto Diagnosis Service and Repair N l On Flashing warning light Off eo f i 0 5 1 5 sec 0 5 sec sec 2 5 sec Figure 15 5 Older diagnostic systems made use of the MIL to display codes The series of flashes is interpreted as numbers The technician then looks up the numbers in the manufacturer s service literature to determine the problem area seconds The codes will then be displayed by the MIL Some trouble codes can be accessed by pressing and holding down a certain sequence of buttons on the radio and or air condi tioner control head for a specified period of time The codes will appear as numbers on the control head display This feature is found on only a few vehicles with electronic enter tainment and climate control systems Reading Trouble Codes To read the MIL flashes or meter pulses access the codes as described above and observe the light or the needle If a code 26 is present in a system that displays codes through the MIL the light will flash two times pause momentarily and then flash six times If a code of 26 is present in
69. r proper shape frequency and amplitude It is also possible to test MAP sensors with a waveform meter and a vacuum pump Make the hookups in the same manner as was done for the voltmeter test The ignition switch must be in the on position While slowly applying vacuum make sure the waveform changes smoothly with no jumping or spikes Testing Temperature Sensors Temperature sensors allow the ECM to compensate for changes in external air temperatures and internal engine temperatures If a temperature sensor malfunctions it will usually cause problems that occur when the engine is either A high voltage level indicates high intake manifold pressure low vacuum High engine load bN As the throttle plate opens 3 manifold pressure rises Mea manifold vacuum lowers N A low voltage level indicates low g intake manifold pressure high vacuum Figure 15 25 Waveform produced by an analog MAP sensor Fluke The uppper horizontal lines should reach reference voltage Voltage transitions should be straight and vertical lt gt Peak to peak voltage should equal reference voltage a _ 7 Voltage drop to ground should not exceed 400 mV The lower horizontal lines should almost reach ground If the voltage drop is greater than 400 mV look for a bad ground at the sensor or ECU Figure 15 26 Waveform produced by a digital MAP sensor Fluke Chapter 15 Computer System Diagnosis
70. r the cause Short term fuel trim Long term fuel trim 126 130 Figure 15 11 The ECM uses sensor inputs to determine fuel trim for short and long term operation The fuel trim is broken down into numbers or counts that can be read by a scan tool 286 Auto Diagnosis Service and Repair Data Scanned from Vehicle Coolant temperature Intake air temperature Mass airflow sensor sensor sensor Throttle position sensor 198 F 92 C 77 F 25 C 2 7 volts 78 volts Engine speed sensor Oxygen sensor Vehicle speed sensor Battery voltage 930 rpm 52 volts O mph 13 6 volts Idle air control Evaporative emission Short term fuel trim Long term fuel trim valve canister solenoid 38 percent Off 125 131 Malfunction indicator Diagnostic trouble Open closed loop Fuel pump relay lamp codes Closed PROM ID Cruise control AC compressor clutch Knock signal 5248C Ignition timing BTDC Base timing Actual timing Figure 15 12 Typical datastream values The PROM carries an identification code in its programming A scan tool can be used to access this code The programming in OBD II systems can be identified by the date code in the program of a rough idle is an example of the type of data obtained related vehicle system s The following section covers general using the snapshot feature test procedures for the most common types of sensors Perform Additional Tests as Needed Oo vwmn A variety of test equipment can be used to chec
71. rn waveform meters can record the operation of two oxygen sensors at the same time This allows the techni cian to check the operation of both oxygen sensors and the catalytic converter on OBD Il systems To make this test attach the meter leads to the upstream oxygen sensor sensor located ahead of the catalytic converter and to the down stream oxygen sensor sensor located after the converter If the waveform meter uses the vehicle diagnostic connector attach it now Set the meter to record the readings of both oxygen sensors Once the engine is started and allowed to stabilize compare the two readings See Figure 15 17 The flattened shape of the downstream sensor waveform in Figure 15 17A shows that the converter is operating to clean up the exhaust In Figure 15 17B both waveforms have a similar shape This indicates that the converter is faulty Road Testing Oxygen Sensors If the oxygen sensor tests at idle are inconclusive perform a road test while monitoring oxygen sensor operation with a scan tool voltmeter or waveform meter A road test will usually uncover a problem that is causing the oxygen sensor to read too lean or too rich rather than a defective sensor The Chapter 15 Computer System Diagnosis and Repair 289 Maximum peak voltages should reach at least 800 mV Rise and fall times increase by aging and poisoning of the sensor Peak to peak voltages should be at least 60
72. s A Ignition coil Although the coil is not a solenoid it is an output device and creates a common pattern B EGR valve solenoid EGR valve solenoids are pulsed on and off rapidly to maintain the proper EGR valve opening C Fuel injector There are many variations of the pattern shown here D ldle air control solenoid Ferret Instruments 300 Auto Diagnosis Service and Repair If the motor is visible it can be visually checked to deter mine whether it operates Some motors can be heard or felt when operating In some cases the ECM can be bypassed by using jumper wires to apply voltage directly to the motor Check the service manual to determine whether the ECM can be bypassed without damaging the motor or the ECM itself Motors can also be checked with an ohmmeter Winding resistance is usually about 0 5 7 ohms for most 12 volt auto motive motors A zero ohms reading indicates that the winding is shorted If the winding has infinite resistance it is open If the winding is defective the motor must be replaced Adjusting Sensors Some sensors can be adjusted Typical adjustment procedures for throttle and crankshaft camshaft position sensors are covered in the following sections The procedures for adjusting other sensors such as distributor pickups and ABS wheel speed sensors are covered in the appropriate chapters Throttle Position Sensor Adjustment A misadjusted throttle position sensor TPS can affect ECM operation
73. sec Figure 15 30 Note how the voltage changes in an air temperature sensor The waveform should rise and fall with changes in temperature with no spikes or loss of the pattern Ferret Instruments Chapter 15 Computer System Diagnosis and Repair 295 Using a Scan Tool to Check Operation The throttle position sensor is another sensor that can be checked with a scan tool Attach the tool and select the proper settings With the ignition on and the engine off open and close the throttle while observing the TPS voltage Voltage should rise smoothly as the throttle is opened The general range of voltage is from 0 5 5 volts Always check the service literature for exact specifications Some scan tools read the TPS input as a percentage of throttle opening If TPS readings are not correct the sensor should be adjusted or replaced If B Figure 15 31 Methods of checking a pressure sensor A If the sensor has two terminals check the resistance between the ter minals B lf the sensor has one terminal check the resistance between that terminal and ground the voltage reading fluctuates as the throttle is opened the TPS is defective and should be replaced Testing with a Waveform Meter The TPS can also be checked with a waveform meter Connect the meter as specified and place the ignition switch in the on position See Figure 15 32 Then observe the wave form while slowly opening the throttle The meter shoul
74. sors are sensitive to excess After ensuring that the engine is warm enough connect the meter to the oxygen sensor output wire as shown in Figure 15 15 Make sure the meter is connected to the output wire and not to the sensor input or ground wires One and three wire sensors are grounded through the sensor housing On two and four wire sensors one of the wires is a ground wire Be very careful to make the proper connections as slight voltage surges can ruin an oxygen sensor Observe the meter for several minutes after the ECM enters the closed loop mode Zirconia oxygen sensors should show a minimum voltage of less than 300 mV and a maximum of more than 600 mV They should average about 500 mV Titania oxygen sensors should show low resistance when the air fuel mixture is rich and high resistance when the mixture is lean Testing the Oxygen Sensor Heater Circuit The heater circuit of a heated oxygen sensor must be checked for proper resistance to ensure that the heater resistor is not burned out or shorted To test the heater set the multimeter to the ohms range Then connect the multimeter 288 Auto Diagnosis Service and Repair Multimeter Chassis ground Figure 15 15 A multimeter set to the voltage scale or resistance can check an oxygen sensor for proper operation Be careful when working around hot exhaust parts Fluke leads to the heater terminals of the oxygen sensor Polarity is not important but the leads
75. t even though the position sensor is disconnected Keep all tester wires and your hands away from moving parts while making this test Check the service literature for the correct specifications Replace the sensor if it does not produce the proper ac voltage A second test involves using a spark tester to check for spark at two or three adjacent spark plug wires If there is no spark at any of the plug wires a position sensor circuit problem is possible If some of the plug wires produce a spark the problem is most likely in the ignition module or coils Another test uses an injector harness noid light and a test light Remove the injector harness connector from one injector install the light crank the engine and note whether the noid light flashes Repeat this test on two or three adjacent injectors If injector pulse is present at all the injectors tested the position sensors are most likely working and the problem is in another circuit If there is no injector pulse at any of the injectors either the position sensors or the ECM is defective or there is a wiring problem If one or two injectors are not working use the test light at each injector harness terminal to check for power and ground as instructed by the vehicle s service manual Injector service will be covered in more detail in Chapter 18 Fuel Injection System Service A final test requires a waveform meter Connect the meter as recommended by the manufacturer
76. technician can use the data gathered by the scan tool to quickly isolate the defective system or component Once the system has been repaired the drive cycle test can be repeated to check OBD II system operation Emissions testing procedures are covered in more detail in Chapter 20 Emission System Testing and Service Typical OBD II Drive Cycle Diagnostic Time Schedule for I M Readiness Go san cosanionporauetessnaneoc Clutch engages man no braking decelerate to 32 km h 20 mph Figure 15 57 The OBD II drive cycle test should be performed to prepare the vehicle for emissions inspection and to reset systems after the battery or ECM has been disconnected General Motors 308 Auto Diagnosis Service and Repair Tech Talk About 35 years ago the only electronic part on an auto mobile was the radio The first electronic ignition systems and electronic voltage regulators were introduced in the mid 1960s and did not become common until almost ten years later Technicians who understood point type ignition systems and five or six circuits in the carburetor were considered current Technicians in those days could keep up with things by keeping their eyes open and gaining some trial and error experience as newer cars came into the shop Of course all of this has changed The modern techni cian must understand electronic ignitions fuel injection emis sion controls on board computers and all sorts of government rules
77. ther If one is shorter than another look for a chipped or bent tooth on the trigger wheel Maximum peak levels ee should be equal to each other If one is shorter than another look for a chipped or bent tooth on the trigger wheel These two pulses identify B cylinder 1 TDC Figure 15 36 Position and speed sensor waveforms vary between types of sensors and manufacturers Always look up the proper waveform to avoid an incorrect diagnosis A Waveform produced by a Hall effect sensor B Waveform produced by a magnetic position sensor Fluke the ECM itself If the speedometer is not working proceed to test the sensor directly with a multimeter To test the sensor using a multimeter stop the engine and disconnect the speed sensor connector Then set the multimeter to the ac volts range and connect it to the speed sensor terminals Restart the engine and with the drive wheels off the ground shift the vehicle into drive and accel erate The multimeter should start to read ac voltage as the engine begins turning This test can also be made using a waveform meter Look for a pattern similar to the one in Figure 15 39 The frequency of the oscillations should increase with vehicle speed Some manufacturers call for checking the resistance of the speed sensor winding but this is done only after other tests have indicated a sensor problem Checking the ECM Diagnosing a defective ECM is sometimes difficult since it may be
78. too damaged to assist in the diagnostic process If a scan tool cannot access the ECM s self diagnostic mode the ECM is probably faulty A damaged ECM will sometimes produce false trouble codes If the ECM produces a trouble code that cannot be confirmed by testing the suspected part Knock signal Yes Knock detected Knock signal No Defective sensor or no knock signal Figure 15 37 The knock sensor signal will show up on most scan tool readouts as a yes knock present or no knock not present P P 1 25v ze T 280mv 500us 9 Figure 15 38 Most knock sensor waveforms will resemble this one Ferret Instruments 298 Auto Diagnosis Service and Repair or if the ECM produces a trouble code number that does exist it is usually defective Often the only way to pinpoint a faulty ECM is through the process of elimination or through substitution The process of elimination involves checking every other possible cause of a problem and eliminating all of them If all the sensors actu ators fuses wiring and connectors have been checked and are okay the problem is most likely in the ECM Before you determine that the ECM is defective be sure to thoroughly check every other component and its related wiring Substitution involves replacing the suspect ECM with a unit that is known to be good Before checking by substitution make sure every related part has been checked If not a faulty component could dam
79. waveforms of upstream and downstream oxygen sensors allows the technician to determine the condition of the sensors and the converter 1 Signal from upstream oxygen sensor 2 Signal from downstream oxygen sensor Note that the that the signal amplitude from the down stream sensor increases when the efficiency of the catalytic converter declines Fluke complete enter the meter s averaging function This will allow you to get an average reading of the voltage levels running and the oxygen sensor disconnected false trouble codes will generally be set After all tests are complete be sure to clear the ECM s memory ie Note If the above tests are made with the engine Testing Mass Airflow MAF Sensors Mass airflow or MAF sensors produce either analog or digital signals Older MAF sensors are usually analog types while many newer designs are digital types type of MAF sensor you are dealing with since analog and digital sensors closely resemble each other If you are not sure which type of sensor you are testing check the service manual ae Note Always check carefully to determine which Testing Analog MAF Sensors A quick test using either a scan tool or a multimeter can be performed on analog MAF sensors Check the proper service manual to determine if the sensor will respond to this test If a scan tool is used connect it to the DLC and set it to monitor MAF sensor input If a multimeter is used set it to measure
80. when a malfunction occurs can help you find the cause of a driveability problem Some ECMs will capture this data automatically when a problem occurs What type of problem does the data in this chart indicate Chapter 15 Computer System Diagnosis and Repair 287 to the ECM The ECM determines the air fuel ratio based on the oxygen sensor input Oxygen sensors are sometimes called O sensors There are two basic types of oxygen sensors the elec trically heated and the nonheated Most vehicles made within the last several years are equipped with heated oxygen sensors Oxygen sensors with three or more lead wires are heated types Oxygen sensors with one or two lead wires are nonheated types If the MIL is on and a trouble code indicates an oxygen sensor problem check the sensors wiring harness for a proper connection Then make a quick check of the oxygen sensor by tapping on the exhaust manifold or exhaust pipe near the sensor with the engine running If the MIL goes out the sensor is defective Warning Exercise caution when working around exhaust manifolds Exhaust systems can reach extremely high temperatures very quickly If the MIL remains on after tapping on the manifold use a scan tool to access the data from the oxygen sensor in ques tion Late model vehicles use more than one oxygen sensor so make sure you are testing the right one Connect the scan tool to the DLC and enter the necessary vehicle information Make

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