Computers and Sensors— Operation, Diagnosis, and Service
Contents
1. engine and low with a warm engine E A ground wire a h s THERMOSTAT i HOUSING ENGINE COOLANT k TEMPERATURE SENSOR i Pa A a A UPPER BROIATBR Og Figure 25 14 A typical engine coolant temperature ECT sensor ECT sensors are located near the thermostat housing on most engines YEL 410 BLK 452 TPS 410 452 COOLANT SENSOR CONNECTOR BLACK CONNECTOR Computers and Sensors Operation Diagnosis and Service 599 If the coolant temperature sensor has a poor connec tion high resistance at the wiring connector the com puter will supply a richer than normal fuel mixture based on the resistance of the coolant sensor There fore poor fuel economy and a possible rich trouble code can be caused by a defective sensor or high re sistance in the sensor wiring If the sensor was shorted or defective and has too low a resistance a leaner than normal fuel mixture would be supplied to the en gine A too lean fuel mixture can cause driveability problems and a possible lean diagnostic trouble code Oxygen Sensors Most automotive computer systems use oxygen sen sors 02S in the exhaust system to measure the oxy gen content of the exhaust See Figure 25 17 If the Coolant Sensor Temperature to Resistance Values Approximate 185 450 1800 3400 7500 13 500 25 000 100 700 Figure 25 15 Engine coolant temperature resistance values Note the resistance decreases as the temperatu2. signal is available at the sensor carefully backprobe the 5 volt reference grey on this General Motors vehicle wire at the connector on the TP sensor Simply push the T pin alongside the wire until it touches the metal terminal inside the connector P19 2 Consult the factory service manual for the specifications and wire colors used for the TP sensor as well as the recommended testing procedure P19 4 A scan tool can be used to observe the output voltage and the calculated percentage of throttle opening P19 6 Connect the red lead from the digital multimeter to the T pin and attach the black meter lead to a good clean engine ground 617 P19 7 Select DC volts and turn the ignition key on engine off The meter reads slightly over 5 volts confirming the computer is supplying the reference voltage to the TP sensor P 9 8 Another important step when testing a TP sensor is to verify that the ground circuit is okay To check the ground of the TP sensor carefully backprobe the ground wire at the TP sensor connector black on this General Motors vehicle and connect the red meter lead to the I pin P19 9 Attach the black meter lead to a good clean engine ground P19 10 With the ignition on engine off and the digital meter still set to read DC volts read the voltage drop of the TP sensor ground The voltage
3. It is named for J M E Baudot 1845 1903 a French inventor and telegraphy expert Just as km h helps in estimating the length of time required to travel a certain dis tance the baud rate is useful in estimating how long a given computer will need to transmit a specified amount of data to another computer Storage of a sin gle character requires eight bits per byte plus an ad ditional two bits to indicate stop and start This means that transmission of one character or word requires 10 bits Dividing the baud rate by 10 tells us the maximum number of words per second that can be transmitted For example if the computer has a baud rate of 600 approximately 60 words can be re ceived or sent per minute Automotive computers have evolved from a baud rate of 160 used in the early 1980s to a baud rate as high as 60 500 The speed of data transmission is an important factor both in system operation and in system troubleshooting Control Module Locations The on board automotive computer has many names It may be called an electronic control unit module controller or assembly depending on the manufacturer and the computer application The Society of Automotive Engineers SAE bulletin J1930 standardizes the name as a powertrain control module PCM The computer hardware is Figure 25 7 This powertrain control module PCM is located under the hood on this pickup truck all mounted on one or more circuit boards and in
4. KS knock sensor TP could enter the engine without being measured This extra air is often called false air See Figure 25 38 Because this extra air is unmeasured the computer does not provide enough fuel delivery and the engine operates too lean especially at idle A small hole in the air inlet hose would represent a fairly large percentage of false air at idle but would represent a very small percentage of extra air at highway speeds To diagnose for false air hook up a scan tool and look at long term fuel trim numbers at idle and at 3000 rpm Tap Test With the engine running at idle speed gently tap the MAF sensor with the fingers of an open hand If the engine stumbles or stalls the MAF sensor is defec tive This test is commonly called the tap test Digital Meter Test of a MAF Sensor A digital multimeter can be used to measure the fre quency Hz output of the sensor and compare the reading with specifications The frequency output and engine speed in RPM can also be plotted on a graph to check to see if the 616 CHAPTER 25 frequency and RPM are proportional resulting in a straight line on the graph E SENSOR TESTING USING DIAGNOSTIC TROUBLE CODES Many vehicles display diagnostic trouble codes DTCs yet do not display scan data To check if the problem is the sensor itself or the electrical sensor circuit that is at fault follow these steps 1 Clear the DTC 2 Create the opposite sensor cond
5. at operating temperature even at idle speeds The three wires include the O2S signal the power and ground for the heater E Four wire oxygen sensor The four wire sensor is heated O2S HO2S that uses an O28 signal wire and signal ground The other two wires are the power and ground for the heater Zirconia Oxygen Sensors The most common type of oxygen sensor is made from zirconia zirconium dioxide It is usually con structed using powder that is pressed into a thimble shape and coated with porous platinum material that acts as electrodes See Figure 25 18 and 25 19 The oxygen sensor reacts with the exhaust gases to produce a voltage from 0 volts to 1 volt 0 mV to 1 000 Figure 25 18 Typical zirconia oxygen sensor OUT Sie Ai Ox YGEM PLATINUM CONGUCTOR LAYER ON BITH SIDES _ OF ZIRCONIA SENSOR EXHAUST EXHAUST GAS i 7 GAS OXYGEN OXYGEN Figure 25 19 A cross sectional view of a typical zirconia oxygen sensor mV by comparing the oxygen content of the exhaust to the oxygen content of the outside air 21 Zirconia oxygen sensors O2S are constructed so that oxygen ions flow through the sensor when there is a difference between the oxygen content inside and outside the sensor An ion is an electrically charged particle The greater the differences in the oxygen content between the inside and outside of the sensor the higher the voltage E Rich mixture A rich mixture results in little oxygen in the exhaus
6. drop is the difference in voltage between the leads of the meter General Motors specifies that this voltage drop should not exceed 35 mV 0 035 V This TP sensor ground shows 31 1 mV 0 0311 V P19 I To measure the signal voltage backprobe the signal wire dark blue on this General Motors vehicle P19 12 Select DC volts and manually range the meter This Fluke meter changes from the 4 volt scale to the 40 volt scale as the sensor voltage goes slightly higher than 4 volt For an instant OL appears on the display as it switches ranges This OL could also indicate a fault 618 i eg a zy alakom n 0 doinn o nN ANo a Jla ecnosis leo ja nnil Ka e ia L d bi W L _ a L y d S h d gt 4 d a t kp 7 a gt il ise h yo pi A Z b bo P19 13 Slowly move the throttle from idle speed to P19 14 The high reading for this sensor was wide open and back to idle speed position For best 4 063 volts results this test should be performed by depressing the accelerator pedal This puts the same forces on the sensor as occurs during normal driving a EES J h u as es 1 A P19 15 Pushing the MIN MAX button shows the P19 16 A Snap On Vantage graphing multimeter or minimum voltage the meter recorded during the test digital storage oscilloscope can also be used to test a 0 399 volts TP sensor To test the sensor using the Snap On Vantage select TP senso
7. o Mna oxygen Sensor TestGhat Below 200 mV Below 800 mV Below 400 mV System is operating lean Add propane to the intake air to see if the oxygen sensor reacts If not the sensor is defective Below 200mV 200 mV Above 800mV 800 mV Above 500mV 500 mV system is operating rich a 200 mV _ 800 mV _ 500 mV system is operating rich Remove a vacuum hose to see if the oxygen sensor reacts If not the sensor is defective Check for an exhaust leak upstream from the 02S or ignition misfire that can cause a false lean indication before further diagnosis Frequently Asked Question What Is the Difference Between a False Lean and a Real Lean Oxygen Sensor Reading A false lean signal is a result of oxygen flowing past the oxygen sensor that did not result from combustion inside the engine Two examples of a false lean oxygen sensor indication include l A cracked exhaust manifold or an exhaust leak upstream from the oxygen sensor between the exhaust valve and the oxygen sensor can cause a false lean As an exhaust pulse occurs an area of lower pressure develops behind the pulse of exhaust This lower pressure area draws outside air into the exhaust stream and flows past the oxygen sensor The oxygen sensor voltage drops as a result of this extra oxygen brought into the exhaust at the leak The drop in oxygen sensor voltage is interpreted by the engine computer as a message that the mixture supplied
8. problems and there were no di agnostic trouble codes stored A check was made on the oxygen sensor activity using a DMM The voltage stayed above 600 millivolts most of the time If a large vacuum hose was removed the oxygen sensor voltage would temporarily drop to below 450 millivolts and then return to a reading of over 600 millivolts Remember The O Sensor Is Lying to You e High O2S readings rich exhaust low O content in the exhaust Low O2S readings lean exhaust high O content in the exhaust As part of a thorough visual inspection the tech nician removed and inspected the spark plugs All the spark plugs were white indicating a lean mixture not the rich mixture the oxygen sensor was indicating The high O2S reading signalled the computer to reduce the amount of fuel resulting in an excessively lean operation After replacing the oxygen sensor the engine ran great But what killed the oxygen sensor The techni cian finally learned from the owner that the head gas ket had been replaced over a year ago The silicone silicate additives in the antifreeze coolant had coated the oxygen sensor Because the oxygen sensor was coated the oxygen content of the exhaust could not be detected the result a false rich signal from the oxygen sensor DIAGNOSTIC STORY The owner of a pickup truck complained that the engine ran terribly It would hesitate and surge yet there were no diagnostic trouble codes DTCs After ho
9. stalled in a metal case to help shield it from electro magnetic interference EMI The wiring harnesses that link the computer to sensors and actuators con nect to multipin connectors or edge connectors on the circuit boards On board computers range from single function units that control a single operation to multifunction units that manage all of the separate but linked electronic systems in the vehicle They vary in size from a small module to a notebook sized box Most early engine computers were installed in the pas senger compartment either under the instrument panel or in a side kick panel where they can be shielded from physical damage caused by tempera ture extremes dirt and vibration or interference by the high currents and voltages of various underhood systems See Figures 25 7 and 25 8 Later model PCMs are larger have increased memory and are usually located in the engine compartment where they are cooled by air from the radiator fan Shorter wiring harnesses with fewer connections are another advantage E FUEL CONTROL SYSTEM OPERATING MODES A computer controlled fuel metering system can be selective Depending on the computer program it may have different operating modes The on board computer does not have to respond to data from all of its sensors nor does it have to respond to the data in the same way each time Under specified conditions Computers and Sensors Operation Diagnosis and Service 595 F
10. 50 00 45 00 40 00 35 00 30 00 25 00 20 00 15 00 10 00 5 00 0 00 A KENEEN JA RERREEE BEERREE Ete TT KEE EKGRRE MAP kPa E aan PN ET HN RENAJESERN PLL IN TL AUNAT ERGERUAS ANT EESE Yo ANTT ONT BREAN ERREERNGEE ATN HERRAR ARICA EGREME FERETE HN ERGRESEE titty yy ETT ERRRREEE AT A AIA ci Volt gt ONTON qY rier yP N N MA Yt O O O C S ge b POSITION SENSOR x 3 a e 8 R e PCM SIGNAL NASNESN ACEC CNC COON TRET COOCCN COON TN N tT Ww Ft st s Figure 25 13 a Schematic of a typical manifold absolute pressure MAP sensor circuit b As manifold pressure vacuum changes the voltage signal to the computer also changes Courtesy General Motors puter to adjust for changes in atmospheric pressure due to weather but also is the primary sensor used to determine altitude A MAP sensor and a BARO sensor are usually the same sensor The MAP sensor is capable of read ing barometric pressure just as the ignition switch is turned to the on position before the engine starts Therefore altitude and weather changes are avail able to the computer During mountainous driving it may be an advantage to stop and then restart the en gine so that the engine computer can take another barometric pressure reading and recalibrate fuel de livery based on the new altitude The computer on some vehi
11. Computers and Sensors Operation Diagnosis and Service After studying Chapter 25 you should be able to Prepare for the interprovincial Red Seal certification examination in Appendix VIII Engine Performance on the topics covered in this chapter Explain the purpose function and operation of on board computers Discuss programming differences between a PROM and an EEPROM Discuss the operation and testing procedures for throttle position manifold absolute pressure and coolant temperature sensors Explain the operation of heated and non heated exhaust gas oxygen sensors Explain adaptive strategy COMPUTER CONTROL Modern automotive control systems consist of a net work of electronic sensors actuators and computer modules designed to regulate the powertrain and ve hicle support systems The powertrain control module PCM is the heart of this system It coordi nates engine and transmission operation processes data maintains communications and makes the con trol decisions needed to keep the vehicle operating Automotive computers use voltage to send and re ceive information Voltage is electrical pressure and does not flow through circuits but voltage can be used as a signal A computer converts input information or data into voltage signal combinations that represent number combinations The number combinations can represent a variety of information temperature speed or even words and letters A comp
12. NSOR The intake air temperature IAT sensor is a nega tive temperature coefficient NTC thermistor that decreases in resistance as the temperature of the sensor increases The IAT sensor can be located in one of the following locations E In the air cleaner housing E In the air duct between the air filler and the throttle body as shown in Figure 25 25 E Built into the mass air flow MAF or air flow sensor Figure 25 25 The IAT sensor on this General Motors 3800 V 6 engine is in the air passage duct between the air cleaner housing and the throttle plate Computers and Sensors Operation Diagnosis and Service 605 E Threaded into the intake manifold where it senses the temperature of the air entering the cylinders NOTE An IAT installed in the intake manifold is the most likely to suffer damage due to an engine backfire which can often destroy the sensor The purpose and function of the intake air tem perature sensor is to provide the engine computer PCM the temperature of the air entering the engine E Cold air is more dense and contains more oxygen and therefore requires a richer mixture to achieve the proper air fuel mixture Air at 0 C 32 F is 14 denser than air at 40 C 104 F Hot air is less dense and contains less oxygen and therefore requires a leaner mixture to achieve the proper air fuel mixture The IAT sensor is a low authority sensor and is used by the computer to modify the
13. P sensor 5 Describe how a zirconia oxygen sensor works and how best to determine if it is operating correctly E RED SEAL CERTIFICATION TYPE QUESTIONS 1 Which of the following is an input sensor to the vehicle computer a Fuel injector b Idle speed control motor c Combustion chamber temperature sensor d Engine coolant sensor 2 Which part of the computer does the actual calculations a PROM b RAM c CPU d KAM 3 Typical TP sensor voltage at idle is about a 2 50 to 2 80 volts b 0 5 volts or 10 of WOT TP sensor voltage c 1 5 to 2 8 volts d 13 5 to 15 0 volts 10 The voltage output of a zirconia oxygen sensor when the exhaust stream is lean excess oxygen is a Relatively high close to 1 volt b About in the middle of the voltage range c Relatively low close to 0 volt d Dependent on atmospheric pressure The sensor that most determines fuel delivery when a fuel injected engine is first started is the a Oxygen sensor 025S b Engine coolant temperature ECT sensor c Engine MAP sensor d BARO sensor The standardized name for the sensor that measures the temperature of the air being drawn into the engine is called a n a Intake air temperature sensor IAT b Air temperature sensor ATS c Air charge temperature ACT d Manifold air temperature MAT sensor e Which sensor is generally considered to be the elec tronic accelerator pump of a fuel injected engine a Oxygen se
14. PTER 25 Figure 25 36 A GM hot film mass air flow MAF sensor that has been taken apart The electronic circuit measures the cooling effect of the air entering the engine and generates a frequency output signal that is proportional to the amount of air passing through the sensor DIAGNOSTIC STORY The Dirty MAF Sensor Story The owner of a Buick Park Avenue complained that the engine would hesitate during acceleration showed lack of power and seemed to surge or miss at times A visual in spection found everything to be like new including a new air filter There were no stored diagnostic trouble codes DTCs A look at the scan data showed airflow to be within the recommended 3 to 7 grams per second A check of the frequency output showed the problem Idle frequency 2 177 kHz 2177 Hz Normal frequency at idle speed should be 2 37 to Figure 25 37 A typical hot wire MAF sensor located 2 52 kHz Cleaning the hot wire of the MAF sensor re between the air filter and the throttle plate stored proper operation The sensor wire was covered with what looked like fine fibres possibly from the re placement air filter Testing Mass Airflow Sensors NOTE Older AC MAF sensors operated at a lower Start the testing of a MAF sensor by performing a frequency of 32 to 150 Hz with 32 Hz being the aver thorough visual inspection Look at all the hoses that age reading at idle and 150 Hz for wide open throttle direct and send air esp
15. ad The typical MAP sensor consists of a ceramic or silicon wafer sealed on one side with a perfect vacuum and exposed to intake manifold vacuum on the other side See Figure 25 12 As the engine bulkhead with a vacuum hose attached that runs to the intake manifold Some MAP sensors are attached directly to the intake manifold vacuum changes the pressure difference on the The PCM uses information from the MAP sen wafer changes the output voltage or frequency of sor to control ignition advance timing and fuel de the MAP sensor livery A typical MAP sensor uses three wires see Figure 25 13 similar to a TPS E A 5 volt reference feed wire from the computer A ground wire Engine Manifold Manifold MAP Sensor E A voltage or frequency wire back to the Load Vacuum Pressure Voltage Signal computer as manifold vacuum changes the voltage signal back to the computer also changes Heavy Low High High WOT almost 0 mm Hg almost atmospheric 4 6 4 8 V Light High Low Low idle 430 530 mmHg lowerthan 0 8 1 6 V The barometric pressure BP or BARO sensor 17 21 in Hg atmospheric is used by the engine computer to sense the baro metric pressure This input not only allows the com Barometric Pressure Sensor CHAPTER 25 MAP SENSOR 598 TO THROTTLE INTAKE MANIFOLD ABSOLUTE PRESSURE VACUUM LT GRN 432 110 00 105 00 100 00 95 00 90 00 85 00 80 00 75 00 70 00 65 00 60 00 55 00
16. amount of fuel and ignition timing as determined by the engine coolant temperature sensor Engine temperature is most accurately deter mined by looking at the engine coolant temperature ECT sensor In certain conditions the IAT has an effect on performance and driveability One such con dition is a warm engine being stopped in very cold weather In this case when the engine is restarted the ECT may be near normal operating temperature such as 93 C 200 F yet the air temperature could be 30 C 20 F In this case the engine requires a richer mixture due to the cold air than the ECT would seem to indicate Testing the Intake Air Temperature Sensor If the intake air temperature sensor circuit is dam aged or faulty a diagnostic trouble code DTC is set and the malfunction indicator lamp MIL may or may not be on depending on the condition and the type and model of the vehicle To diagnose the IAT sensor follow these steps Step After the vehicle has been allowed to cool for several hours use a scan tool and observe the IAT and compare it to the engine coolant tem perature ECT The two temperatures should be within 3 C 5 F of each other Step 2 Perform a thorough visual inspection of the sensor and the wiring If the IAT is threaded into the intake manifold remove the sensor and check for damage Step 3 Check the voltage and compare to the follow ing chart 606 CHAPTER 25 Manifold Intake Air Temper
17. ature Sensor Temperature vs Resistance and Voltage Drop Approximate Voltage Drop Across Sensor V Poor Fuel Economy Black Exhaust Smoke Look at the IAT If the intake air temperature sensor is defective it may be signaling the computer that the intake air temperature is extremely cold when in fact it is warm In such a case the computer will supply a mixture that is much richer than normal If a sensor is physically damaged or electrically open the computer will often set a diagnostic trouble code DTC This DTC is based on the fact that the sensor temperature did not change for a certain amount of time usually about 8 minutes If however the wiring or the sen sor itself has excessive resistance a DTC will not be set and the result will be lower than normal fuel economy and in serious cases black exhaust smoke from the tailpipe during acceleration Testing the Manifold Absolute Pressure Sensor Most pressure sensors operate on 5 volts from the computer and return a signal voltage or frequency based on the pressure vacuum applied to the sen sor If a MAP sensor is being tested make certain that the vacuum hose and hose fittings are sound and making a good tight connection to a manifold vacuum source on the engine TECH TIP Check the Hose A defective vacuum hose to a MAP sensor can cause a va riety of driveability problems including poor fuel econ omy hesitation stalling and rough idle A small a
18. ch on engine off Clean transitions and the lack of any glitches in this waveform indicate a good sensor Courtesy of Fluke Corporation TECH TIP Check Power and Ground Before Condemning a Bad Sensor Most engine sensors use a 5 volt reference and a ground If the 5 volt to the sensor is too high shorted to voltage or too low high resistance then the sensor output will be skewed or out of range Before replacing the sensor that did not read correctly unplug the sensor and meas ure both the 5 volt reference and ground To measure the ground simply turn the ignition on engine off and touch one test lead of a DMM set to read DC volts to the sen sor ground and the other to the negative terminal of the battery Any reading higher than 0 6 volt 600 mV repre sents a poor ground See Figures 25 29 and 25 30 readings are obtained or if the correct setting at idle cannot be obtained Testing the Oxygen Sensor Zirconia oxygen sensors produce a voltage like a small battery when in the absence of oxygen when the sensor is hot over 315 C or 600 F The output Figure 25 29 Checking the 5 volt reference from the computer being applied to the TP sensor with the ignition switch on engine off voltage of a typical oxygen sensor varies depending on the oxygen content of the exhaust gases passing the sensor Typical oxygen sensor values are as follows Rich exhaust Oxygen sensor voltage above 800 mV E Lea
19. cles will monitor the TP sensor and use the MAP sensor reading at wide open throttle WOT to update the BARO sensor if it has changed during driving Engine Coolant Temperature Sensor When the engine is cold the fuel mixture must be richer to prevent stalling and engine stumble When the engine is warm the fuel mixture can be leaner to provide maximum fuel economy with the lowest possible exhaust emissions Because the computer controls spark timing and fuel mixture it will need to know the engine temperature An engine coolant temperature sensor ECT threaded into the engine coolant passage will provide the computer with this information See Figure 25 14 This will be the most important sensor while the engine is cold The igni tion timing can also be tailored to engine coolant temperature A hot engine cannot have the spark timing as far advanced as a cold engine Most coolant sensors have very high resistance when the coolant is cold and low resistance when the coolant is hot This is referred to as having a negative tem perature coefficient NTC which is opposite to the situation with most other electrical components See Figure 25 15 A typical CTS uses only two wires see Figure 25 16 E The computer sends a 5 volt signal through an internal resistor to the coolant temperature sensor and measures the voltage between the two resistors The changing resistance in the sensor causes the voltage to be high with a cold
20. drawn into the engine passes through the sensor this type of sensor can ac tually produce an output based on the mass of the airflow The output of this type of sensor is usually a frequency based on the amount of air entering the sensor The more air that enters the sensor the more the hot film is cooled The electronics inside the sen sor therefore increase the current flow through the hot film to maintain the 75 C 165 F temperature differential between the air temperature and the temperature of the hot film This change in current flow is converted to a frequency output that the com puter can use as a measurement of airflow Most of these types of sensors are referred to as mass air flow MAF sensors because unlike the air vane sensor the MAF sensor takes into account relative humidity altitude and temperature of the air The denser the air the greater the cooling effect on the hot film sensor and the greater the amount of fuel re quired for proper combustion E HOT WIRE SENSOR The hot wire sensor is similar to the hot film type but uses a hot wire to sense the mass airflow instead of the hot film Like the hot film sensor the hot wire sensor uses a temperature sensing resistor ther mistor to measure the temperature of the air enter ing the sensor See Figure 25 37 The electronic circuitry within the sensor keeps the temperature of the wire at 75 C 165 F above the temperature of the incoming air 614 CHA
21. e airflow moves the vane The vane is attached to a potentiometer similar to a throttle position TP sensor E AIRFLOW METHOD The airflow method measures the amount of air as part of the computer input information necessary for accurate fuel delivery control There are three basic types of airflow sensors used on port injected en gines the air vane sensor the hot film sensor and the hot wire sensor E AIR VANE SENSOR This air vane sensor uses a movable vane that translates the amount of movement of the vane into the amount of air being drawn into the engine An air vane sensor can be tested using a digital meter or an oscilloscope See Figures 25 34 and 25 35 E HOT FILM SENSOR The hot film sensor uses a temperature sensing re sistor thermistor to measure the temperature of the incoming air Through the electronics within the sensor a conductive film is kept at a temperature 75 C 165 F above the temperature of the incoming air See Figure 25 36 Because the amount and density of the air both tend to contribute to the cooling effect as the air Computers and Sensors Operation Diagnosis and Service 613 Figure 25 35 A typical air vane sensor with the cover removed The movable arm contacts a carbon resistance path as the vane opens Many air vane sensors also have contacts that close to supply voltage to the electric fuel pump as the air vane starts to open when the engine is being cranked and air is being
22. e engine manufac turer s exact specifications A defective or misad justed throttle position sensor can cause hesitation on acceleration and other driveability problems On some vehicles equipped with an automatic trans Computers and Sensors Operation Diagnosis and Service 597 THROTTLE POSITION SENSOR CONNECTOR PCM THROTTLE POSITION SENSOR A 416 GRY SV REF C 417 DKBLU TPS SIGNAL B T 452 BLK TO COOLANT SENSOR Figure 25 I Schematic of a typical throttle position TP sensor circuit Movement of the wiper on the potentiometer sends a voltage signal 0 5 to 4 5 volts to the computer Courtesy General Motors mission the throttle position sensor also affects the application of the torque converter clutch TCC The throttle position TP sensor used on fuel injected vehicles acts as an electronic accelerator pump If the TP sensor is unplugged or defective the engine may still operate satisfactorily but hesitate upon acceleration as though the carburetor were in need of a new accelerator pump Holding the throttle r to the floor while cranking usually causes fuel injec tion to stop or reduce This is called clear flood TETT mode and is used to clear a flooded engine iii Ch he oe T rh Manifold Absolute Pressure s Sensor The manifold absolute pressure MAP sensor Figure 25 12 This MAP sensor is installed on the is used by the engine computer to sense engine lo
23. e oxygen sensor check the manufacturers recommended procedures E Ifthe oxygen sensor reads high all the time above 550 millivolts the fuel system could be supplying too rich a fuel mixture or the oxygen sensor may be contaminated E Ifthe oxygen sensor voltage remains low below 350 millivolts the fuel system could be supplying too lean a fuel mixture Check for a vacuum leak or partially clogged fuel injector s Before replacing the oxygen sensor check the manufacturer s recommended procedures Testing the Oxygen Sensor Using the Min Max Method A digital meter set on DC volts can be used to record the minimum and maximum voltage with 610 CHAPTER 25 Figure 25 31 Testing an oxygen sensor using a digital multimeter set on DC volts With the engine operating in closed loop the oxygen voltage should read over 800 millivolts and lower than 200 millivolts and be constantly fluctuating Courtesy of Fluke Corporation Ground NEG the engine running A good oxygen sensor should Post Catalytic Converter be able to produce a value of less than 300 milli Oxygen Sensor Testing volts and a maximum voltage above 800 millivolts Replace any oxygen sensor that fails to go above 700 millivolts or lower than 300 millivolts The oxygen sensor located behind the catalytic con verter is used on OBD II On Board Diagnostics Generation II vehicles to monitor converter efficiency Min Max Oxygen Sensor Test Chart
24. ecially between the MAF sen sor and the throttle body Also check the electrical connector for E Corrosion False Air 5 Termi nals that are bent or pushed out of the Airflow sensors and mass airflow MAF sensors are plastic connector designed to measure all the air entering the engine E Frayed wiring If an air inlet hose was loose or had a hole extra air Computers and Sensors Operation Diagnosis and Service 615 CHECK THE SNORKEL TUBE HERE FOR Figure 25 38 Carefully check the hose between the MAF sensor and the throttle plate for cracks or splits that could create extra false air in the engine that is not measured by the MAF sensor Frequently Asked Question What Is Meant by a High Authority Sensor A high authority sensor is a sensor that has a major influ ence over the amount of fuel being delivered to the engine For example at engine start up the engine coolant temper ature ECT sensor is a high authority sensor and the oxy gen sensor O2S is a low authority sensor However as the engine reaches operating temperature the oxygen sensor becomes a high authority sensor and can greatly affect the amount of fuel being supplied to the engine See the chart ECT especially when IAT intake air temperature the engine starts and sensors modify and back is warming up O2S after the engine TFT transmission fluid reaches closed loop temperature operation MAP PRNDL shift position sensor
25. el into the engine The resulting air fuel mixture is monitored by an exhaust mounted oxygen sensor which sends data back to the computer When the oxygen sensor detects a lean or rich condition the computer increases or decreases fuel volume as a correction When a major shift is deter mined over a period of time the computer changes the original program to reflect different fuel require ments This is called adaptive strategy and is stored in KAM Many vehicles run well even with low fuel pressure or restricted fuel injectors If the battery is disconnected all adaptive infor mation stored in KAM is lost the computer now de faults to the original program and begins the relearning process Nonvolatile RAM memory can retain its infor mation even when the battery is disconnected One use for this type of RAM is the storage of odometer information in an electronic speedometer The mem ory chip retains the distance accumulated by the ve Ne 12 volts a OY Check Engine Light Canister Purge Solenoid E G R Solenoid If Used T C C Solenoid Air Divert Solenoid lf Used Air Switch Solenoid If Used Carb Solenoid Or injector Solenoid 2nd Injector Solenoid If Used DOQQOOE hicle When speedometer replacement is necessary the odometer chip is removed and installed in the new speedometer unit The computer processes the input voltage sig nals through a series of logic circuits maintained in its
26. ge at the ECT sensor check with the service manual for the proper voltage at each temperature Testing the ECT Sensor Using a Scan Tool Comparing the temperature of the engine coolant as displayed on a scan tool with the actual temperature of the engine is an excellent method to test an engine coolant temperature sensor 1 Record the scan tool temperature of the coolant ECT 2 Measure the actual temperature of the coolant using an infrared pyrometer or contact type temperature probe HINT Often the coolant temperature gauge in the dash of the vehicle can be used to compare with the scan tool temperature Although not necessarily ac curate it may help to diagnose a faulty sensor espe cially if the temperature shown on the scan tool varies greatly from the temperature indicated on the dash The maximum difference between the two readings should be 5 C 10 F If the actual tem perature varies by more than 5 C 10 F from the temperature indicated on the scan tool check the ECT sensor wiring and connector for damage or cor rosion If the connector and wiring are okay replace the ECT sensor If the connector and wiring are okay check the sensor with a DVOM for resistance and compare to the actual engine temperature chart If that checks out okay check the computer NOTE Many manufacturers use two coolant sen sors one for the dash gauge and another one for the computer E INTAKE AIR TEMPERATURE SE
27. gnition Switch ap POMI Coolant S C Nose Switch Where Used Neutral Park Switch High Gear Switch Oxygen Sensor Reference Figure 25 9 Basic computer inputs Courtesy General Motors E THROTTLE POSITION SENSOR Most computer equipped engines use a throttle po sition TP sensor to signal the position of the throt tle See Figure 25 10 and 25 11 The TP sensor consists of a potentiometer variable resistor A typ ical sensor uses three wires E A 5 volt reference feed wire from the computer A ground wire E A voltage signal wire back to the computer as the throttle is opened the voltage to the computer changes Normal throttle position voltage on most vehicles is about 0 5 volts at idle closed throttle and 4 5 volts at wide open throttle WOT The TP sensor voltage at idle is usually about 10 of the TP sensor voltage when the throttle is wide open The computer senses this change in throttle position and changes the fuel mixture and ignition timing The actual change in fuel mixture and ignition timing is also partly determined by other sensors such as the manifold pressure engine vacuum engine RPM THROTTLE POSITION THROTTLE SENSOR lt a Figure 25 10 A typical throttle position TP sensor mounted on the throttle plate of this port injected engine the engine coolant temperature and oxygen sen sor s Some throttle position sensors are adjustable and should be set according to th
28. grated circuit IC devices store the computer operating pro gram system sensor input data and system actu ator output data information necessary for CPU operation Computer Programs By operating a vehicle on a dynamometer and man ually adjusting the variable factors such as speed load and spark timing it is possible to determine the optimum output settings for the best driveability economy and emission control This is called engine mapping See Figure 25 4 Engine mapping creates a three dimensional performance graph that applies to a given vehicle Computers and Sensors Operation Diagnosis and Service 593 Figure 25 4 Typical ignition timing map developed from testing and used by the vehicle computer to provide the optimum ignition timing for all engine speeds and load combinations Figure 25 5 A replaceable PROM used in a General Motors computer Notice that the sealed access panel has been removed to gain access and powertrain combination Each combination is permanently mapped digitally onto an IC chip called a programmable read only memory PROM This allows an automaker to use one basic computer for all models a unique PROM individualizes the computer for a particular model Also if a driveabil ity problem can be resolved by a change in the pro eram the manufacturers can release a revised PROM to supersede the earlier part Some manufacturers use a single PROM that plugs into the computer See F
29. high This is interpreted by the engine computer PCM as being a lean exhaust Titania Oxygen Sensor The titania titanium dioxide oxygen sensor does not produce a voltage but rather modifies one as it samples the presence of oxygen in the exhaust All ti tania oxygen sensors use a four terminal variable re sistance unit with a heating element A titania sensor samples exhaust air only and uses a reference voltage from the PCM Titania oxide oxygen sensors use a 14 mm thread and are not interchangeable with zirconia oxygen sensors One volt is applied to the sensor and the changing resistance of the titania oxygen sensor changes the voltage of the sensor cir cuit As with a zirconia oxygen sensor the voltage signal is about 450 mV when the exhaust is rich and low below 450 mV when the exhaust is lean E PROCESSING AND MEMORY The microprocessor is the decision making part of the computer It takes data from the various input sensors and compares it with information stored in memory See Figure 25 20 Computers have two types of memory permanent and temporary Permanent memory is called read only memory ROM because the computer can only read the contents it cannot change the data stored in it This data is retained even when power to the com puter is shut off Part of the ROM is built into the com puter and the rest is located in an IC chip called a programmable read only memory PROM or calibration assembly Temporar
30. igure 25 5 Other com puters use a non replaceable calibration module that 594 CHAPTER 25 contains the system PROM If the on board computer needs to be changed the replaceable type of PROM or calibration module must be removed from the defec tive unit and installed in the replacement computer The original PROM was programmed to re duce emissions improve fuel economy and provide acceptable power Replacing the factory PROM with an aftermarket hot PROM to increase en gine performance often increases engine emis sions as well In order to reduce tampering and the use of af termarket PROMs the Environmental Protection Agency EPA mandated that the on board com puter be tamper resistant As a result beginning in 1994 PROMs are soldered into place and are not replaceable Some PROMs are made in a way that they can be erased by exposure to ultraviolet light and repro grammed These are called EEPROMs electroni cally erasable or KPROMs erasable PROMs The new EEPROM chips allow technicians to re program them with special electronic service tools Replacement computers must be programmed ei ther in the car or on the bench before the vehicle will run further updating can be done any time This type of service is usually done by dealership techni cians although aftermarket reprogramming tools are becoming common Clock Rates and Timing The microprocessor receives sensor input voltage signals processes the
31. igure 25 8 This PCM ona Chrysler vehicle can only be seen by hoisting the vehicle because it is located next to the radiator in the airflow to help keep it cool it may ignore sensor input Or it may respond in dif ferent ways to the same input signal based on inputs from other sensors Most current control systems have two operating modes open and closed loop The most common application of these modes is in fuel metering feedback control where the computer re sponds to a signal from the oxygen sensor and if needed changes the amount of fuel delivered this is closed loop mode During periods of prolonged idle cold engine op eration wide open throttle or no oxygen sensor sig nal the computer only looks at ROM read only memory permanent memory stored in the com puter This is open loop mode The latest PCMs have increased memory and op erate in closed loop mode under many conditions that were not monitored on older systems E BASIC COMPUTER OPERATION Input Battery power is supplied to the computer when the ignition switch is closed Because most input sen sors must operate with a fixed voltage in order to generate a reliable signal battery voltage is reduced to 5 volts by an internal regulator before being sent to the major input sensors See Figure 25 9 In our example these are the throttle position manifold absolute pressure and the engine coolant tempera ture sensors 596 CHAPTER 25 12 volts I
32. ing to the pressure sensor Most pressure sensors use three wires 1 A 5 volt wire from the computer 2 A variable signal wire back to the computer 3 A ground or reference low wire The procedure for testing the sensor is as follows 1 Turn the ignition on engine off 2 Measure the voltage or frequency of the sensor output 3 Using a hand operated vacuum pump or other variable vacuum source apply vacuum to the sensor HARNESS CONNECTOR JUMPER Wis TO VACUUM PUMP BLACK RED Computers and Sensors Operation Diagnosis and Service 607 Figure 25 26 A digital multimeter set to test a MAP sensor 1 Connect the red meter lead to the V meter terminal and the black meter lead to the COM meter terminal 2 Select DC volts 3 Connect the test leads to the sensor signal wire and the ground wire 4 Select hertz Hz if testing a MAP sensor whose output is a varying frequency otherwise keep it on DC volts 5 Read the change of frequency as the vacuum is applied to the sensor Compare the vacuum reading and the frequency or voltage reading to the specifications A good pressure sensor should change voltage or frequency in relation to the applied vacuum If the signal does not change or the values are out of range according to the manufacturers specifications the sensor must be replaced Testing the Throttle Position Sensor A TP sensor can be tested using one or more of the following too
33. ir leak vacuum leak around the hose can cause these symp toms and often set a trouble code in the vehicle com puter When working on a vehicle that uses a MAP sensor make certain that the vacuum hose travels con sistently downward on its route from the sensor to the source of manifold vacuum Inspect the hose especially if another technician has previously replaced the factory original hose It should not be so long that it sags down at any point Condensed fuel and or moisture can be come trapped in this low spot in the hose and cause all types of driveability problems and MAP sensor codes Four different types of test instruments can be used to test a pressure sensor 1 A digital voltmeter with three test leads connected in series between the sensor and the wiring harness connector see Figure 25 26 2 A scope connected to the sensor output power and ground 3 A scan tool or a specific tool recommended by the vehicle manufacturer 4 A breakout box connected in series between the computer and the wiring harness connection s A typical breakout box includes test points at which pressure sensor values can be measured with a digital voltmeter or frequency counter if a frequency type MAP sensor is being tested NOTE Always check service literature for the exact testing procedures and specifications for the vehicle be ing tested Use jumper wires T pins or a breakout box to gain electrical access to the wir
34. ition For example if the DTC indicates an open engine coolant temperature ECT circuit unplug the sensor and using a jumper wire short the two terminals of the harness not the sensor together NOTE If the ECT sensor wires are shorted together the scan tool will display about 150 C 300 F and about 40 C 40 F if the sensor wires are open disconnected When checking three wire sensors such as the throttle position TP sensor MAP or MAF use a jumper wire to jump the 5 volt reference back into the signal return after disconnecting the connector from the sensor Shorting the 5 volt reference to the signal should cause the vehicle computer to set a shorted sensor DTC If a shorted sensor DTC is stored simply clear the DTC and unplug the sensor If the wiring is okay the opposite open sensor DTC should be set TECH TIP The Unplug It Test If a sensor is defective yet still produces a signal to the computer the computer will often accept the reading and make the required changes in fuel delivery and spark ad vance If however the sensor is not reading correctly the computer will process this wrong information and per form an action assuming that the information being sup plied is accurate For example if a mass airflow MAF sensor is telling the computer that 2 grams of air per sec ond is going into the engine the computer will then pulse the injector for 6 4 ms or whatever figure it is program
35. iver transistor in the computer E SENSOR TESTING The correct operation of computerized engines de pends on accurate and dependable sensors Proper testing of sensors is an important part of computer problem diagnosis and troubleshooting Testing the Engine Coolant Temperature by Visual Inspection The correct functioning of the engine coolant tem perature ECT sensor depends on the following items that should be checked or inspected E Properly filled cooling system Check that the radiator reservoir bottle is full and that the radiator itself is filled to the top The ECT sensor must be submerged in coolant to be able to indicate the proper coolant temperature E Proper pressure maintained by the radiator cap If the radiator cap is defective and cannot allow the cooling system to become pressurized air pockets could develop These air pockets could cause the engine to operate at a hotter than normal temperature and prevent proper temperature measurement especially if the air pockets occur around the sensor E Proper antifreeze water mixture Most vehicle manufacturers recommend a 50 50 mixture of antifreeze and water as the best compromise between freezing protection and heat transfer ability Proper operation of the cooling fan If the cooling fan does not operate correctly the engine may overheat Testing the ECT Using a Multimeter Both the resistance in ohms and the voltage drop across the sensor ca
36. load TP sensor The position of the throttle plate and its rate of change are used as part of the equation to calculate the proper amount of fuel to inject E Temperature sensors Both engine coolant temperature ECT and intake air temperature VOLT 5 00 SEC DIV VOLT 5 00 SEC DIV CATALYTIC CONVERTER POSI CATALYST OXYGEN SENSOR Figure 25 32 Most 1996 and newer vehicles use an oxygen sensor behind the catalytic converter The purpose of the oxygen sensor is to sense the percentage of oxygen in the exhaust to check the efficiency of the catalytic converter IAT are used to calculate the density of the air and the need of the engine for fuel A cold engine low coolant temperature requires a richer air fuel mixture than a warm engine OXYGEN SENSOR BEFORE THE CONVERTER OXYGEN SENSOR AFTER THE CONVERTER GOOD EFFICIENT CONVERTER VOLT 5 00 SEC DIV OXYGEN SENSOR AFTER THE CONVERTER BAD INEFFICIENT CONVERTER Figure 25 33 The post catalytic converter oxygen sensor should display very little activity if the catalytic converter is efficient AIR VANE a AIRFLOW TO INTAKE INTAKE MANIFOLD BYPASS AIR a SCREW TO INTAKE MANIFOLD AIRFLOW INTAKE BYPASS AIR b SCREW Figure 25 34 A typical air vane sensor a At idle the air flows through a bypass passage The bypass air screw is adjusted at the factory and should not require adjustment b As the throttle is opened th
37. ls A digital voltmeter with three test leads connected in series between the sensor and the wiring harness connector or backprobing using T pins E A scan tool or a specific tool recommended by the vehicle manufacturer A breakout box that is connected in series between the computer and the wiring harness connector s A typical breakout box includes test points at which TP voltages can be measured with a digital voltmeter E An oscilloscope Use jumper wires T pins or a breakout box to gain electrical access to the wiring to the TP sensor See Figure 25 27 NOTE The procedure that follows is the usual method used by many manufacturers Always refer to service literature for the exact recommended procedure and specifications for the vehicle being tested The procedure for testing the sensor using a dig ital multimeter is as follows 1 Turn the ignition switch on engine off 2 Measure the voltage between the signal wire and ground reference low wire The voltage should be about 0 5 volt 608 CHAPTER 25 Figure 25 27 A meter lead connected to a T pin that was gently pushed along the signal wire of the TP sensor until the point of the pin touched the metal terminal inside the plastic connector NOTE Consult the service literature for exact wire colours or locations 3 With the engine still not running but with the ignition still on slowly increase the throttle opening The voltage signal from
38. m by using information from other memory units and then sends voltage signals to the appropriate actuators The microprocessor communicates by transmitting long strings of 0s and ls in a language called binary code But the micro processor must have some way of knowing when one signal ends and another begins That is the job of a crystal oscillator called a clock generator See Fig ure 25 6 The computer s crystal oscillator generates a steady stream of one bit long voltage pulses Both the microprocessor and the memories monitor the clock pulses while they are communicating Because they know how long each voltage pulse should be they can distinguish between a 01 and a 0011 To complete the process the input and output circuits also watch the clock pulses Computer Speeds Not all computers operate at the same speed some are faster than others The speed at which a com puter operates is specified by the cycle time or clock speed required to perform certain measurements Cycle time or clock speed is measured in megahertz 4 7 MHz 8 0 MHz 15 MHz 18 MHz etc CRYSTAPOSCILLATOR _ CLOCK GENERATOR Figure 25 6 The clock generator produces a series of pulses that are used by the microprocessor and other components to stay in step with each other at a steady rate Baud Rate The computer transmits bits of a serial data stream at precise intervals The computer s speed is called the baud rate or bits per second
39. mbina tion of binary Os and 1s for the digital computer A digital computer changes the analog input sig nals voltage to digital bits binary digits of informa tion through an analog to digital AD converter circuit The binary digital number is used by the com puter in its calculations or logic networks Output sig nals usually are digital signals that turn system actuators on and off The digital computer can process thousands of digital signals per second because its circuits are m E s RESISTORS amp 1 SK DOA INEIN RND s FNPACIT ORS a Ji a a i il i ul 3 9 eRe a i T Figure 25 3 Many electronic components are used to construct a typical vehicle computer Notice all of the chips resistors and capacitors that are used in this computer able to switch voltage signals on and off in billionths of a second See Figure 25 3 Parts of a Computer The software consists of the programs and logic func tions stored in the computer s circuitry The hard ware is the mechanical and electronic parts of a computer Central Processing Unit CPU The micro processor is the central processing unit CPU of a computer Since it performs the essential mathe matical operations and logic decisions that make up its processing function the CPU can be considered the heart of a computer Some computers use more than one microprocessor called a coprocessor Computer Memory Other inte
40. med to provide However if the air going into the engine is ac tually 14 grams per second the amount of fuel supplied by the injectors will not be enough to provide proper engine operation If the MAF sensor is unplugged the computer knows that the sensor is not capable of supplying airflow information so it defaults to a fixed amount of fuel based on the values of other sensors such as the TP and MAP sensors If the engine operates better with a sensor un plugged then suspect that the sensor is defective A sen sor that is not supplying the correct information is said to be skewed The computer will not see a diagnostic trouble code for this condition because the computer can often not detect that the sensor is supplying wrong information 7 k t oF E y ii i S T E L hr I i E E F 4 ngl J YT ft x E P19 1 Besides a scan tool other equipment that can be used to check a throttle position TP sensor includes a scope or graphing multimeter a digital multimeter equipped with MIN MAX function and T pins to safely backprobe the sensor wires P19 3 A scan tool display showing no diagnostic trouble codes DTCs A fault could still exist even though a diagnostic trouble code is not set it depends on what type of fault and when it occurs P19 5 Most throttle position sensors use a 5 volt reference voltage from the computer To test that this
41. n be measured and compared Computers and Sensors Operation Diagnosis and Service 603 Figure 25 22 Measuring the resistance of the ECT sensor The resistance measurement can then be compared with specifications Courtesy of Fluke Corporation with specifications See Figure 25 22 See the fol lowing chart showing examples of typical engine coolant temperature sensor specifications Some ve hicles use a second resistor in the ECT circuit to pro vide a more accurate measure of the engine temperature See Figure 25 23 General Motors ECT Sensor without Pull Up Resistor Voltage Drop Across C F Ohms Sensor 4 0 177 100 212 0 75 4 104 1459 2 01 5 i 604 CHAPTER 25 Figure 25 23 When the voltage drop reaches approximately 20 volts the PCM turns on a transistor The transistor connects a kQ resistor in parallel with the 10 kQ resistor Total circuit resistance now drops to around 909 ohms This function allows the PCM to have full binary control at cold temperatures up to approximately 50 C 122 F and a second full binary control at temperatures greater than 50 C 122 F Figure 25 24 A typical ECT sensor voltage of the sensor after the engine was stopped As the resistance of the sensor increased the voltage decreased Courtesy of Fluke Corporation lempershure sensory ane recetrially Negative Temoersture Cosfcent hecrustons HTE Tes pa rmiun HIT being tested using a digital s
42. n exhaust Oxygen sensor voltage below 200 mV Testing an Oxygen Sensor Using a Digital Voltmeter The oxygen sensor can be checked for proper opera tion using a digital high impedance voltmeter 1 With the engine off connect the red lead of the meter to the oxygen sensor signal wire See Figure 25 81 2 Start the engine and allow it to reach closed loop operation To achieve closed loop operation the engine computer must have achieved three criteria including a The engine coolant temperature must be above a certain temperature usually above 40 C 104 F Computers and Sensors Operation Diagnosis and Service 609 Figure 25 30 Checking the voltage drop between the TP sensor ground and a good engine ground with the ignition on engine off A reading of greater than 0 6 V 600 mV represents a bad computer ground b The oxygen sensor s must be producing a usable variable voltage signal c A certain amount of time must elapse after engine start for closed loop to be achieved This time could vary from a few seconds to several minutes depending on the vehicle and the temperature 3 In closed loop operation the oxygen sensor voltage should be constantly changing as the fuel mixture is being controlled The results should be interpreted as follows E If the oxygen sensor fails to respond and its voltage remains at about 450 millivolts the sensor may be defective and require replacement Before replacing th
43. nsor b Coolant temperature sensor c Throttle position sensor d Engine manifold absolute pressure sensor The sensor that must be warmed up and functioning before the engine management computer will go to the closed loop is the a Oxygen sensor 025 b Engine coolant temperature ECT sensor c Engine MAP sensor d BARO sensor Which of the following describes an acceptable oxygen sensor voltage range a 0 5 to 0 7 volt b 200 mV to 800 mV c 300 mV to 500 mV d 400 mV to 800 mV A pull up resistor inside the computer ECT circuit is used to a Expand the scale of the ECT sensor b Dampen voltage fluctuation c Prevent amperage from back feeding into other circuits d Compare resistance with the intake air temperature IAT sensor
44. programmed instructions The logic circuits change the input data into output voltage signals or commands that control output transistors E OUTPUT Actuators are electrical or mechanical devices that convert electrical energy into mechanical action The computer sends a voltage signal to the base circuit of an output driver transistor which activates the de vice usually a relay or solenoid See Figure 25 21 When the ignition switch is closed battery voltage is supplied to the actuators no current flows until ground is supplied by turning the transistor on This is known as ground side control and is the most common circuit Technicians often check output circuits by grounding the wire between the actuator and the mi croprocessor Any actuator that runs at 100 duty cy cle can be tested by grounding Actuators such as fuel injector solenoids never run at 100 duty cycle and can be damaged by grounding Follow the service instruc tions exactly when working with computer circuits ge e See l Figure 25 21 Basic computer outputs Courtesy General Motors Our basic computer shows only a single driver transistor for each device most computers bank the transistors into a group of four called quad drivers but the function is the same Output devices are usually tested for resistance with an ohmmeter or checked for current flow a shorted solenoid coil as an example will allow ex cessive current to flow which could damage the dr
45. r from the menu P 9 17 The Vantage has a built in database that can be P 9 18 After attaching the meter leads to the signal accessed to show connector position and wire colour wire and ground ignition key on engine off the graphing information multimeter shows the waveform of the voltage signal as the throttle is depressed released and depressed again These are normal for a TP sensor A fault would show as a vertical line or dip in the waveform 619 620 CHAPTER 25 E SUMMARY 1 The vehicle computer is called the powertrain control module PCM because it controls the engine and the transmission on most vehicles 2 The four basic computer functions include input pro cessing storage and output 3 Permanent memory is called ROM PROM EPROM or EEPROM 4 Temporary memory is called RAM or KAM 5 The central processing unit CPU is the brains of the computer and does all the calculations 6 As the temperature of the engine coolant increases the resistance of the ECT sensor decreases 7 A throttle position sensor can best be checked with a voltmeter set on MIN MAX or with a scope 8 An oxygen sensor should switch rapidly from high to low on a fuel injected engine operating in closed loop E REVIEW QUESTIONS 1 List the four functions of a computer 2 What is meant by the term Baud rate 3 Explain how to test an engine coolant temperature sensor 4 Describe the best method to test a MA
46. re rises These are typical values only always refer to the vehicle specifications Courtesy General Motors Figure 25 16 Schematic of a typical engine coolant temperature sensor circuit The computer measures the voltage signal between the PCM resistor and the ECT resistance Courtesy General Motors PCM SV SIGNAL 600 CHAPTER 25 rw EXHAUST MANIFOLD a SENSOR p fae Figure 25 17 Many fuel control oxygen sensors are located in the exhaust manifold near its outlet so that the sensor can detect the presence or absence of oxygen in the exhaust stream for all cylinders that feed into the manifold exhaust contains very little oxygen Oz the com puter assumes that the intake charge is rich too much fuel and reduces fuel delivery On the other hand when the oxygen level is high the computer assumes that the intake charge is lean not enough fuel and increases fuel delivery There are several different designs of oxygen sensors including E One wire oxygen sensor This one wire of the one wire oxygen sensor is the O28 signal wire The ground for the O28 is through the shell and threads of the sensor and through the exhaust manifold Two wire oxygen sensor The two wire sensor has a signal wire and a ground wire for the O28 E Three wire oxygen sensor The three wire sensor design uses an electric resistance heater to help get the O25 up to temperature more quickly and to help keep the sensor
47. s a movable contact to vary resistance and send an analog signal These basic functions are not unique to comput ers they can be found in many noncomputer sys tems However we need to know how the computer handles these functions Input First the computer receives a voltage signal input from an input device The device can be as simple as a button or a switch on an instrument panel or a sensor on an automotive engine See Figure 25 2 for a typical type of automotive sensor Vehicles use various mechanical electrical and magnetic sensors to measure factors such as vehicle speed engine RPM air pressure oxygen content of exhaust gas airflow and engine coolant tempera ture Each sensor transmits its information in the form of voltage signals The computer receives these voltage signals but before it can use them the sig nals must undergo a process called input condi tioning This process includes amplifying voltage signals that are too small for the computer circuitry to handle Input conditioners generally are located inside the computer but a few sensors have their own input conditioning circuitry Processing Input voltage signals received by a computer are processed through a series of electronic logic circuits maintained in its programmed instructions These logic circuits change the input voltage signals or data into output voltage signals or commands Storage The program instructions for a computer are
48. stored in electronic memory Some programs may require that certain input data be stored for later reference or future processing In others output commands may be delayed or stored before they are transmitted to devices elsewhere in the system Output After the computer has processed the input signals it sends voltage signals or commands to other de vices in the system such as system actuators An ac tuator is an electrical or mechanical device that converts electrical energy into a mechanical action such as adjusting engine idle speed altering suspen sion height or regulating fuel metering Computers also can communicate with and con trol each other through their output and input func tions This means that the output signal from one computer system can be the input signal for another computer system E DIGITAL COMPUTERS In a digital computer the voltage signal or process ing function is a simple high low yes no on off sig nal The digital signal voltage is limited to two voltage levels high voltage and low voltage Since there is no stepped range of voltage or current in be tween a digital binary signal is a square wave The signal is called digital because the on and off signals are processed by the computer as the digits or numbers 0 and 1 The number system containing only these two digits is called the binary system Any number or letter from any number system or language alphabet can be translated into a co
49. t stream Compared to the outside air this represents a large difference and the sensors create a relatively high voltage of about 1 0 volt 1000 mV E Lean mixture A lean mixture leaves some oxygen in the exhaust stream that did not combine with the fuel This leftover oxygen reduces the difference between the oxygen content of the exhaust compared to the oxygen content of the outside air As a result the sensor voltage is low or almost zero volts E QO2S voltage above 450 mV is produced by the sensor when the oxygen content in the exhaust is Computers and Sensors Operation Diagnosis and Service 601 REFERENCE VOLTAGE 5 VOLTS FROM VOLTAGE REGULATOR MICROCOMPUTER SENDS SIGNAL TO SIGNAL LINES INPUT CONDITIONERS WAI rats RESISTOR pa A M rron VOLTAGE GENERATOR SIGNAL RETURN PROVIDED THROUGH PROCESSOR QUTPUT DRIVERS OUTPUT DRIVER OPENS AND CLOSES THE GROUND CIRCUIT OF THE OUTPUT DEVICE BATTERY D SOLENOID i RELAY DISPLAY REFERENCE VOLTAGE REGULATOR OUTPUT MICROCOMPUTER MICROPROCESSOR PROCESSOR Figure 25 20 Microprocessor schematic showing input processing and output Note that input sensors operate from a 5 volt reference signal and the output devices operate from battery voltage Courtesy Ford Motor Co low This is interpreted by the engine computer PCM as being a rich exhaust E O2S voltage below 450 mV is produced by the sensor when the oxygen content is
50. the TP sensor should also increase Look for any dead spots or open circuit readings as the throttle is increased to the wide open position See Figure 25 28 for an example of how a good TP sensor would look when tested with a digital storage oscilloscope DSO HINT If TP sensor specifications are not available re member that the TP sensor voltage at idle should be about 10 of the voltage at the wide open throttle WOT position Therefore if the WOT voltage is 4 5 volts then TP sensor voltage at idle should be about 0 45 volts 4 With the voltmeter or scan tool still connected slowly return the throttle down to the idle position The voltage from the TP sensor should also decrease evenly on the return to idle The TP sensor voltage at idle should be within the acceptable range as specified by the manufac turer Some TP sensors can be adjusted by loosening their retaining screws and moving the sensor in re lation to the throttle opening This movement changes the output voltage of the sensor All TP sensors should also provide a smooth transition voltage reading from idle to WOT and back to idle Replace the TP sensor if erratic voltage A 1VDC 1 1PROBE 8 200m OFF 111 PROBE 200ms 01V SINGLE Trigtaf 3D1 SCOPE SEN FREE CAPTURE MIN MAX TRIGGER RUN IOBMDIV on A at 50 Figure 25 28 A typical waveform of a TP sensor signal as recorded on a DSO when the accelerator pedal was depressed with the ignition swit
51. to the engine is too lean and it increases the amount of fuel supplied As a result the mixture now being supplied to the cylinder is too rich because the oxygen sensor was fooled and provided a false lean signal to the computer An ignition misfire as a result of a defective spark plug wire or fouled spark plug can cause a false lean When a spark plug does not fire the unburned gas and air inside the cylinder are pushed into the exhaust manifold by the piston s on the exhaust stroke The unburned gas and air contain oxygen that is detected by the oxygen sensor as too lean a mixture NOTE Remember the oxygen sensor is a sensor to detect oxygen not unburned fuel hydrocarbons or HC As a result of this oxygen being detected the voltage produced by the oxygen sensor is lower This lower voltage signal is interpreted by the computer as a sign that the mixture being supplied is too lean The computer then increases the amount of fuel delivered This extra fuel can often cause more spark plug fouling and even more unburned oxygen passing the oxygen sensor Because a lean condition can be false the wise ser vice technician checks the exhaust system and the igni tion system before trying to correct a lean indication DIAGNOSTIC STORY A technician was trying to solve a driveability problem with a V 6 passenger car The car idled roughly hesitated and accelerated poorly A thorough visual inspection did not indicate any possible
52. torage Pd oscilloscope The illustration shows the if ECT Sensor ECT Signal circuit Engine Block Ground Connection 5V VREF over 50 C 5V VREF under 50 C A D Converter ees Temperature i3 decree eng oo Haana io nee 4 ie femperniues GOLD Temparaiure sensor meing one Typically made over a hong period of theres If resistance values match the approximate coolant temperature and there is still a coolant sen sor trouble code the problem is generally in the wiring between the sensor and the computer Always consult the manufacturer s recommended procedures for checking this wiring If the resistance values do not match the sensor may need to be replaced Normal operating temperature varies with vehi cle make and model Some vehicles are equipped with a thermostat with an opening temperature of 82 C 176 F whereas other vehicles use a thermo stat that is 90 C 195 F or higher Before replacing the ECT sensor be sure that the engine is operating at the temperature specified by the manufacturer Most manufacturers recommend checking the ECT sensor after the cooling fan has cycled twice indicat ing a fully warmed engine See Figure 25 24 NOTE Many manufacturers install a pull up resistor inside the computer to change the voltage drop across the ECT sensor This is done to expand the scale of the ECT sensor and to make the sensor more sensitive Therefore if measuring volta
53. urs of troubleshooting the technician discovered while talking to the owner that the problem started after the trans mission had been repaired Before the transmission was repaired the problem started yet the transmission shop said that the problem was an engine problem and not related to the transmission A thorough visual inspection revealed that the front and rear oxygen sensor connectors had been switched The computer was trying to compensate for an air fuel mixture condition that did not exist Reversing the OSS connectors restored proper operation of the truck The Pickup Truck Story 611 612 CHAPTER 25 A changing air fuel mixture is required for the most efficient operation of the converter If the converter is working correctly the oxygen content after the con verter should be fairly constant See Figures 25 32 and 25 33 E SPEED DENSITY Fuel injection computer systems require a method for measuring the amount of air the engine is taking in to be able to match the correct fuel delivery There are two basic methods used 1 Speed density method 2 Airflow method The speed density method does not require an air quantity sensor but rather calculates the amount of fuel required by the engine The computer uses in formation from sensors such as the MAP and TP to calculate the needed amount of fuel E MAP sensor The value of the intake inlet manifold pressure vacuum is a direct indication of engine
54. uter processes the input voltage signals it receives by com puting what they represent and then delivering the data in computed or processed form Standardized Emissions Terminology In the early 1990s the Society of Automotive Engineers developed a common list of terms SAE J1930 for emis sion related parts i e ignition fuel delivery and emis sion control components These terms by law have been used in all Canadian and U S automotive service and training publications since January 1 1995 Many automobile manufacturers began using the new terms in 1993 when California adopted J1930 As an example the on board computer had been known as a Micro computer a Processor an Engine Control Assembly ECA or an Engine Control Unit ECU depending on the manufacturer The new term standard in the industry is Powertrain Control Mod ule PCM It is important to note that older publications be fore the mid 1990s may use different terms than cur rent texts THE FOUR BASIC COMPUTER FUNCTIONS The operation of every computer can be divided into four basic functions See Figure 25 1 E Input E Processing E Storage Output 591 592 CHAPTER 25 Figure 25 All computer systems perform four basic functions input processing storage and output E AEPERe Re Fn a o CALLY s voLTs 1 i BOCVWABLE i rr P CONTACT a i i ai H M _ i e BLE A i L WOLTAGE Figure 25 2 A potentiometer use
55. y memory is called random access memory RAM because the microprocessor can write or store new data into it as directed by the computer program as well as read the data already in it Automotive computers use two types of RAM memory volatile and nonvolatile Volatile RAM memory is lost whenever the ignition is turned off However a type of volatile RAM called keep alive memory KAM can be wired directly to battery power This prevents its data from being erased when the ignition is turned off Both RAM and KAM have the disadvantage of losing their memory when disconnected from their power source One example of RAM and KAM is the loss of station settings in a programmable radio when the battery is discon nected Since all the settings are stored in RAM they have to be reset when the battery is recon nected System diagnostic trouble codes DTC are commonly stored in RAM and can be erased by dis connecting the battery Adaptive strategies that compensate for wear and aging are another function of KAM The original 602 CHAPTER 25 computer program in ROM is written for the average engine operating under average conditions but this is often not the case Fuel delivery calculations are based on informa tion from the engine sensors throttle position air and coolant temperatures engine speed and load are a few of these inputs Based on these data the computer refers to a look up table stored in ROM and injects a given amount of fu
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