Brakes: ABS - Brickboard
Contents
1. set a code and put the system in default Another problem on wheel speed sensors is an internal break in a wire Be careful doing any wheel work because those sensors are delicate The second thing is rust where the sensor mounts or in the wheel speed sensor bore which makes the sensor hard to remove As time goes on we re going to see more and more corrosion in the electrical parts Another problem is moisture infiltration into speed sensor electrical connectors which causes intermittents Anti Lock Brake System Service Precautions from Raybestos Courtesy 1998 Brake Parts Inc amp Online Technologies Corporation Raybestos all rights reserved 1 Always refer to the appropriate anti lock service manual before attempting to service any portion of the brake system 2 Warning Some ABS systems store brake fluid in an accumulator under high pressure Failure to depressurize these types of anti lock systems before servicing can cause physical injury The majority of these ABS systems can be depressurized by simply turning off the ignition and firmly depressing the brake pedal between 20 and 40 times Check service manual for exact number Editor s Note Not true for Volvo cars see Abe Crombie tips on bleeding flushing below 3 Use the proper DOT 3 or DOT 4 brake fluid specified by the vehicle manufacturer This can be found on the master cylinder cap or reservoir body DOT 5 silicone brake fluid can not be used in any ABS system See2. Brake Fluid Comparison for more information 4 Do not hammer or pry on wheel speed sensors and or sensor rings to adjust the air gap These components are delicate and can easily be damaged 5 ABS and other on board computer can be easily damaged by high electrical system voltage Do not attempt to jump start an ABS equipped vehicle with a gasoline powered booster or 110 volt type battery charger on the fast charge boost setting Slow charge the battery first before attempting to start If this is impractical disconnect the negative battery cable before fast charging the battery 6 All four tires must be of the same size and type Failure to observe this rule can cause the ABS and or Traction Control system to disengage and the warning light to come on Follow the vehicle manufactures recommendations before installing any optional tire sizes 7 Never unplug or reconnect any electrical ABS component with the ignition on This can cause a current surge and damage one or more of the system components 8 When installing any add on electrical accessories CB s Telephones Stereos etc it is important that any antennas or other wiring be located away from the ABS computer and sensor wiring A magnetic field is generated as current flows through this additional wiring The magnetic field that is created produces electro magnetic interference EMI that can affect the signals from the wheel speed sensor to the ABS computer 9 When elec
3. a glowing anti lock warning light Just opening the bleeder as you may have done in the past isn t good enough anymore You ve got to choke off that hose Editor s note you can buy inexpensive plastic pliers with pounged jaws designed to clamp rubber brake hoses without damage just enough to keep most of the fluid from passing through Check your auto parts store And don t squash the hose just clamp it And note there is some controversy about this Ford and GM do not recommend clamping because of the potential for damage to the hose Wheel Sensor Damage Another thing to remember about ABS is that it s easy to damage those delicate wheel speed sensors while doing brake work or to cause metal particles to attach themselves to the magnet Either is likely to put the system into default or make it go permanently into anti lock mode called false modulation and turn on the warning lamp On ABS speed sensors are at the top of the list of failures As far as the ABS problems are concerned the first is speed sensors with metallic particles sticking to the sensor nose This may cause system default or make it constantly file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM Brakes ABS go into anti lock mode called false modulation Make sure the air gaps are correct and the sensors and tone wheels are clean with no iron filings or debris in the vanes which can give an erratic speed signal
4. at all Per the Volvo shop manual the required air gap is determined by an o ring inside the sensor housing make sure this is in place If you need to measure gap insert a non magnetic brass or plastic feeler gauge between the end of the sensor and ring Because wheel speed sensors are magnetic they attract metallic particles If debris cast off from semi metallic brake pads rotors or drums ends up in the vicinity of the sensors and sticks to the tip it will weaken the magnetic field and reduce the strength of the sensor signal Gap and signal can be seriously distorted by steel particles picked up by the magnetic sensor remove these at regular intervals A good wheel speed sensor will generally produce an AC voltage reading of 50 to 700 MV when the wheel is spun at a speed of about one revolution per second If the voltage reading is low or non existent check the sensor s resistance with the key off This can be done through a breakout box with a DVOM Checking resistance through the breakout box will tell you if the sensor s wiring harness is okay If you don t get the specified value between 900 and 2 200 ohms for Volvo RWD front sensors and 600 and 1600 ohms for the rear sensor disconnect the sensor from its wiring harness and check the sensor s resistance by attaching the DVOM test probes to the sensor leads A resistance reading that s now within range tells you the problem is in the wiring not the sensor If the sensor has to
5. than a pencil or with broken wires inside intact insulation at the waterproof connectors both at the differential and shock towers Most of the failures are very simple The system isn t rocket science which is probably why it works But it needs to be maintained just as any high tech system needs to be maintained Chris Herbst If you remove the sensors make sure the o rings are seated properly usually grease the O rings a little bit on reinstallation Cleaning Reluctor Wheels Tip from Chris Herbst Usually there is a big buildup of garbage in the sensor reluctor wheels that obscures the sensor from reading it The more junk that is on the wheel the less defined the impulses will be scrape them out usually with a small drift pin and then blow compressed air at them Some of the junk doesn t blow off of the wheels with compressed air and you will need to scrape it off You could also use an appropriately sized dowel or something with an edge that will scrape the debris from the wheels Try not to damage them by putting surface file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM Brakes ABS scratches in them If you use compressed air on brake parts use appropriate precautions so as not to inhale the dust ABS Light On Rear Sensor Inquiry Is there a ABS sensor in the rear somewhere have cleaned the 2 up front and sprayed them well with contact cleaner It seems to light up when b
6. 9 55 34 PM Brakes ABS Inquiry My 960 s ABS system makes a strange noise on stops Response The ABS does do a self test if you come to a complete stop for over a couple of seconds This is in addition to the initial test that occurs on the first roll over 4 6 MPH To verfiy that this is the noise pull out the fuse for the ABS and drive carefully so as to not need the now nonfunctioning ABS in the conditions to see if the noise is gone Putting in the fuse and switching ignition off and then restarting and driving again will turn off ABS warning lamp ABS Lamp Lights After Start up Inquiry 1 The Anti lock light illuminates on my 1990 760 a while after start driving before that ABS seems to work OK The light seems to become lit when exceed 25 mph but lately it seems like the speed has been higher yesterday had as much as 75 mph before it became lit Can a slight shiver in the front wheel bearing cause this Response 1 If the light comes on as you gain speed through 3 5 MPH then you might have a faulty pump motor relay The pump is tested as you gain speed the first time after starting car If it comes on after you have driven a distance and hit a bump then a problem with wheel speed sensor wiring is likely The ABS system on that year model doesn t have fault diagnosis code feature If it s a wagon with solid rear axle the wire from car to axle is subject to motion with every bump and will fatigue and break inside insul
7. Brakes ABS Brakes ABS FAQ Home Volvo Maintenance FAQ for 7xx 9xx 90 Cars Maintenance Pad and Rotor Identification and Replacement Brake Rotor Removal on 740 with ABS Brake Job Tips Hose Clamp ABS Sensors Anti Lock Brake System Service Precautions Brake Fluid Bleeding Flushing Procedures for Cars with ABS Troubleshooting ABS Diagnostic Code Retrieval Diagnostic Notes on ABS Systems Diagnostic Unit Malfunction Surge Protector Failure Intermittent Check ABS Lamp ABS Light On ABS Sensors Wheels Dirty ABS Light On Rear Sensor ABS Light On After Jump Start ABS Pulses at Low Speeds Noise from 960 ABS System ABS Sensor Replacement ABS Lamp Lights After Start up Replacing a Failed ABS Module Replacing a Toothed ABS Sensor Ring Maintenance Pad and Rotor Identification and Replacement See the detailed tables describing pads and rotors for ABS brakes at Brake Pad Identification and Brake Rotor Identification Brake Rotor Removal on 740 with ABS Inquiry have two new front rotors to install on my 1990 745T with ABS My Chilton manual isn t very descriptive From looking at the new rotors it doesn t appear that need to remove the wheel bearing Is that correct Are the ABS file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM Brakes ABS sensors difficult to remove The manual mentions an internal toothed ring which has to be removed from the old r
8. No error code set 1 2 1 Left side front wheel sensor faulty signal at speed less than 40km h 1 2 2 Right side front wheel sensor faulty signal at speed less than 40km h 1 2 3 Left side rear wheel sensor faulty signal at speed less than 40km h 1 2 4 Right side rear wheel sensor faulty signal at speed less than 40km h 1 2 5 Signal faulty from at least one wheel sensor for a long period 1 3 5 Control Module CM faulty 1 4 1 Faulty pedal sensor shorted to ground or supply 1 4 2 Faulty stop brake lamp switch open circuit 1 4 3 Control Module CM faulty 1 4 4 Brake discs overheated 1 5 1 Left front wheel sensor open circuit or short circuit to battery voltage 1 5 2 Right front wheel sensor open circuit or short circuit to battery voltage 1 5 5 Rear axle sensor open circuit or short circuit to battery voltage 2 1 1 Left front wheel sensor no signal on moving off 2 1 2 Right front wheel sensor no signal on moving off 2 1 3 Left rear wheel sensor no signal on moving off file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM Brakes ABS 4 1 3 4 1 4 Right front wheel inlet valve open circuit or short circuit Right front return valve open circuit or short circuit 4 1 5 4 2 1 Rear valve open circuit or short circuit Rear wheel circuit inlet valve open circuit or short circuit Rear wheel circuit retu
9. ation This usually is no sooner than 100 K miles unless the car is driven on poor roads a lot If it were the cable it would predictably do it on bumps If there is a severe build up of metallic particles on front sensors that can make light come on If your wheel bearing has expelled grease that may have built up on sensor and made it retain the metal pad wear particles That might be the place to look first A bent pulse wheel part of hub and runs next to sensor to create the signal looks like a gear can do the same The system is designed to be a little forgiving when the signals from wheels are not equal as this could be due to normal things like having the mini spare on car If the signal is too different then the ABS lamp will light The wheel bearing is a ball not tapered roller type that is not adjustable and must be replaced if it is loose Inquiry 2 The ABS light on my 90 740T Wagon came on as soon as started the car after lunch today very hot very humid And it stayed on after drove it and after restarting the engine a couple of times Response 2 If the light is on at start and doesn t go off then you likely have a problem with the surge relay next to ABS ecm or the solenoid valve relay on ABS modulator under hood The fuse on the surge relay can come lose or relay can fail The solenoid valve relay the smaller of the two on ABS modulator under black cover gets eroded contacts and will need replacement if that is in fac
10. d for the ABS modulator under hood Second there is a fuse on that year model 94 and later 900 s don t have this fuse or the relay on a relay under dash clipped to the bracket for ABS control unit This control unit is high up on the left on 940 and over under the kick panel in right passenger s foot well forward of door on 960 It is a 10A push in fuse The relay that the ABS fuse is located on blows the fuse if there is an over voltage condition to protect from excessive voltage level to control unit The usual reason that it blows is when you jump start vehicle ABS Sensor Replacement If suggestions of cleaning the front ABS senders sensor and pulse wheel and associated electrical connectors doesn t stop the intermittent ABS fault then you may need new sensors found the original old style sensors rectangular tipped on my 89 were over sensitive especially at low speeds on very smooth surfaces Your car may or may not have received the in warranty upgrade to the less sensitive new style sensor diamond shaped tip If not it would be a relatively easy DIY upgrade file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM Brakes ABS Tips on Removing ABS Sensors How do you safely remove the ABS sensors After removing the 5mm Allen screw mine seem to be seized in place don t want to break them so I m looking to some experienced people for some advice My ABS is pulsing inappropriately at very
11. e until traction is regained or the vehicle comes to a halt Electrically operated solenoid valves in the ABS modulator hold release and reapply hydraulic pressure to each brake circuit This produces noise and a pulsating effect that can be felt in the brake pedal All ABS systems keep track of wheel deceleration rates with wheel speed sensors Volvo RWD cars use Ja common sensor in the differential for both rear wheels three channel system The common sensor for the rear wheels reads the combined or average speed of both rear wheels Wheel speed sensors consist of a magnetic pickup and a toothed sensor ring sometimes called a tone ring Wheel speed sensors generate an alternating current AC signal that increases in frequency and amplitude as wheel speed increases Because the sensors are magnetic they can attract metallic debris from semi metallic brake linings and rotors that stick to the tip and interfere with the signal If the ABS module doesn t see a clean WSS signal it may think there s something wrong and set a wheel speed sensor code The distance or air gap between the end of a wheel speed sensor and its ring is also critical A close gap is necessary to produce a strong reliable signal You don t want metal to metal contact between the sensor and its ring since this would damage both But neither do you want too much clearance An air gap that s too wide may produce a weak or erratic signal or worse yet no signal
12. ed sensor wheel until the end of the pry bar is against the hub body Hold the pry bar with one hand Using the other hand take the hammer and strike flat of the prybar about four inches from the end closest to the hub Remove the hub and turn it 45 degrees Re set it in the vise and repeat the procedure Depending on the extent to which the toothed sensor wheel is corroded my test hub was pretty corroded it may take several cycles to loosen the toothed file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM Brakes ABS sensor wheel As the toothed sensor wheel rises along the hub core loosen the wire holding in place the bearing races and re snug the wire The process requires moving in one direction around the edge of the toothed sensor wheel lifting it a fraction of an inch with each cycle Do not shift to a point opposite your starting point as if you were tightening wheel lugs Once you have removed the toothed sensor wheel you can replace damaged wheel lugs Once you have done that you can replace the toothed sensor wheel on the hub Before you do that use a brass wire brush to remove rust dirt from the area of the hub onto which you will re seat the toothed wheel Do not remove your placement mark To re seat the toothed sensor wheel set it atop the hub core and align your placement marks Lay the block of wood across the center of the toothed sensor wheel With the hammer str
13. fference in rotational speeds between one or more tires eventually exceeding a threshold value and triggering a fault light It is good practice to carefully clean the reluctor wheel and ABS sensors whenever the brake rotors are removed usually during replacement of the rotors as these parts lie inside the rotor hat Unfortunately in this case Volvo brake rotors last quite a long time The result is considerable build up and occasional fault light activation in otherwise functional systems Three channel ABS systems employ a rear wheel sensor in the differential housing These are very reliable and need not be removed nor cleaned unless a specific problem is encountered A variety of Bosch and Teves systems are of the four channel type employing reluctors and sensors at each wheel these are not used on Volvo 200 700 or 940 cars Cleaning Sensors Response Paul Grimshaw Bob Dietz When cleaning the ABS sensors it is best to avoid the urge to remove them from the dust shield as their plastic bodies can become brittle with age and crack if forced A better cure would be to clean the wheel sensors by wiping them clean with a rag or soft brush Another problem is the possibility of signal loss through non waterproof connectors or cracks in the wiring or its insulation have seen cars where the rear wheel sensor anchor bolts were absent with metal shavings on sensors that looked like a dead mouse the sensor magnet is smaller
14. ght and correctly installed e Water intrusion The most common causes of excessive harness resistance are water intrusion and corrosion at the sensor to main wiring harness connector Water intrusion can occur at the sensor can or at the harness connector e Bad grounds Check body grounds especially on the modulator and ABS module Your Volvo reports diagnostic scan codes through the OBD diagnostic connector Once you have a code you have a place to start your diagnosis Refer to the diagnostic chart or procedure for the code and follow the steps to isolate the fault This will usually involve measuring voltage or resistance within various wiring circuits or components You ll need a multimeter and possibly a breakout box to make these checks Recommended Volvo OEM green manual for brakes ABS Diagnostic Unit Malfunction Surge Protector Failure Tip from 10333 This info pertains to Volvo 740 940 through 1993 model years If your ABS light is on and the DLC procedure for obtaining the codes from the ABS ECU produces no codes no blinks after holding the button for gt 1sec with the DLC probe attached to 3 in the A unit yet the DLC LED lights when the button is pushed the transient surge or over voltage protection relay may be defective In later cars this relay is located near the ABS control unit high under the kick panel you ll need to remove this behind the instrument panel to the left of the steering wheel It
15. h vise with 6 wide jaws or wider a wide blade minimum 1 5 pry bar e g a Stanley Wonder Bar a block of wood 0 75 20mm thick 1 5 wide and 8 long and a hammer NOTE DO NOT EVER hit the toothed sensor wheel with the hammer While the toothed sensor wheel is wide it is hollow underneath If hit directly with a hammer it will dent readily and so become useless You will then need to replace it It is part 3530983 It costs about US 50 Clamp the hub in vise with the toothed sensor wheel upwards Position the hub such that the top edge of the hub s flange is level with the top of the vise jaw Tighten the vise as much as you can Use a length of thin wire to hold in place the bearing retainers which are inside the hub opening Run the wire through the hub opening for the stub axle and twist the wire a few times until it snug You ll need to loosen it in steps as the toothed sensor wheel comes off Using a magic marker or white out make a mark on the toothed sensor wheel and on the hub body so you can re install the toothed sensor wheel exactly where it was Position the block of wood so that it within 1 8 3 mm of the edge of the toothed sensor wheel Set the block so most of it is on top of the vise jaw and thus over the edge of the hub flange So positioned the wood block will help to hold down the hub Rest the flat side of the pry bar on the block of wood Slide the edge of the pry bar under the tooth
16. ike the block of wood gently in the center of the block The toothed sensor wheel should start to slide down the hub core to its original place on the hub After each blow check to see that the toothed sensor wheel is not skewed i e that its top edge is the same distance from the hub base at all points on its circumference If one edge is higher move the block of wood so it is atop the high point and tap the block of wood until the top edge of the toothed sensor wheel is level Move the block of wood back to the center and continue tapping until the toothed sensor wheel s top edge is even with the top edge of the hub opening Re install the hub and all other removed parts If in a road test the ABS light does not come on you should not have any problems To test the ABS system go into a completely empty parking lot or find a road on which there s no traffic e g very early in the morning Attempt an emergency stop If you hear the growl of the ABS system then all s well If the ABS System does not work consult a Volvo Master Tech Volvo Maintenance FAQ for 7xx 9xx 90 Cars file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM
17. is on the other side of metal bracket that holds the ABS computer the large metal box with many wires going into it through a sturdy connector very close to the left fender In earlier cars it is located in the well in the rear trunk that contains the ABS unit The surge protector looks like a regular file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM Brakes ABS relay except that there is a 10amp fuse pushed into the top of it If you follow the wires coming from it you ll see they lead to the main ABS computer The relay can be removed from the bracket by pushing up and twisting the bottom towards the footpedals Now first remove the fuse and check that it is OK If it is put the fuse back in the relay and obtain a voltmeter Leave the wiring connector on the relay and switch the ignition on start the engine and leave it running The pins are labled 1 2 3 4 on the connector as well as inside the relay but the numbers are hard to see Pin 1 red wire should show 12V Pin 3 black wire should show continuity to ground Pin 2 yellow red wire is the switched circuit and should show 12V with the engine running and OV with the car switched completely off If you are not observing this behavior from Pin2 the relay is bad and needs to be replaced The relay is essentially impossible to dissasemble without destroying it so a replacement unit will be required My dealership charges 85 for the par
18. l Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM Brakes ABS instructions in the Volvo OEM technical manual A fault in the ABS unit will be indicated by an illuminated ABS lamp on the instrument panel and the ABS system but not the normal brakes will cease to function The internal fault code may be cleared by disconnecting the battery 1992 Cars With Underhood Diagnostic Units Tip from Tom Irwin To retrieve any ABS diagnostic codes which are stored and are noted by a lit ABS lamp turn the ignition switch on to KPII and leave the engine off Look under the hood to locate the Diagnostic Link Connector boxes Your car can have one box A or two A and B 1992 The A box has the diagnostic connector pigtail the test button and the LED readout lamp Put the Diagnostic Link Connector probe into Box A Position 3 Push button on DLC for gt lsec and capture codes flashing in the red lamp To clear codes place the DLC probe into position 3 Hold DLC button down for gt 5secs Release for 3 secs then press again for gt 5 secs This will clear the codes The codes are shown in the table below Note that these are generic your car may not have two rear wheel sensors and thus the code will not apply If you cannot pull any codes but the lamp still lights see Diagnostic Unit Malfunction Surge Protector Failure Code ABS System Diagnostic Information Mode 1 40 km h 25 mph 1 1 1
19. n the sensor wire itself is under a lot of tension I ve found many of these with only one or two strands of wire left unbroken even though the insulation looks fine This can cause lost ABS signals Volvo can supply replacement pigtails It s a piece of white wire with the identical end molded on line splice and solder the joint shrink tube over Try to maintain the twist All the signal cables for ABS are twisted pair There is also the possibility of signal loss through non waterproofed connectors Tip from JSW 1991 940 The symptoms started with just a groaning from the ABS under light braking at very low speeds This progressed to the ABS warning light coming on at about 15 mph and staying on I d cleaned the sensors and reluctor rings but this didn t cure it But discovered that it is possible to trick the system into not showing a fault drove the car until the ABS light showed then put it into neutral switched off the engine and restarted Continued driving the ABS light stayed off Then slowed the car with the brakes At about 5 mph the ABS cut in with the usual groaning noise and the car briefly pulled to the right This told me the left front sensor was detecting a lockup and therefore releasing brake pressure to that wheel This explains the car pulling to the right replaced the left front sensor and all is well Noise from 960 ABS System file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14
20. o much internal resistance opens or too little resistance shorts the sensor is defective file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM Brakes ABS and needs to be replaced Damage to the teeth on the tone ring can also upset the WSS signal If one or more teeth are damaged even slightly it may cause enough of a variation in signal strength to confuse the control module We re talking a difference of only 050 inches in tooth height in some cases Grounds or shorts in the wheel speed sensor cables can be found by checking continuity between the wiring connectors If a defect is found in the wires that run between the sensor and the chassis replacing the wires with new ones is a better repair choice than trying to fix or splice them These wires undergo a great deal of flexing every time the suspension encounters a bump so new wires will hold up better than ones that have been soldered spliced or taped The operation of the ABS system can often be affected by electrical problems in the vehicle as well as the ABS system itself Underlying conditions that may cause trouble include e Low battery charge A low charge can interfere with the operation of the anti lock control electronics e Blown fuses Check the brake control module fuse main relay fuse and pump motor fuse e Corroded loose connectors The main relay fluid level sensor and control module connectors all must be ti
21. of fluid instead of a mere bleed highly recommended both to remove air as well as to purge and renew the fluid required every two years Using Line Clamps When Flushing Bleeding Tip from Motor Magazine Add the step of line clamping the brake hose to the caliper when you retract caliper pistons If you don t you may be forcing brake fluid from the caliper bore where rust and other debris may have collected and sending this dirty fluid upstream It s long been standard practice to open the caliper bleeder screws to give brake fluid a place to go while retracting pistons However sometimes the bleeders are partially blocked In these cases they may not let out all of the fluid that you meant to let escape Instead the fluid may take the path of least resistance upstream Nowadays that s likely to send it to sensitive ABS components If the vehicle you re servicing has a blocked bleeder you may get around the problem by cracking open the fitting where the brakeline attaches to the caliper before retracting the piston For added assurance you may also want to line clamp the hose feeding the caliper with genuine clamps designed for this purpose Troubleshooting ABS Diagnostic Code Retrieval 1984 1991 Cars without ABS Self Diagnostic Capability Pre 1992 cars lack any self diagnostic capability for the ABS system Fault tracing is done with a multimeter circuit by circuit and sensor by sensor according to detailed file C
22. otors and installed in the new The manual suggest that there may be some problems removing these and that a puller may be necessary What size puller works best if necessary Response If yours is like my 90 740 GLE forget about Chilton The disc and hub are separate on your car not like the setup Chilton is describing Once the caliper is removed the disc simply lifts off after removing the locater pin The ABS sensor is on the hub not the disc so you don t touch it at all 2 bolts hold the caliper support it after removing the bolts so the weight will not strain the flex lines Cleaning Sensor Rings Tip from Tim While the rotor disc is removed take the time to use a soft bristle brush and clean the ABS sensor area and the toothed sensor ring that is part of the hub It is usually pretty dirty with an accumulation of brake dust and magnetized sacrificial brake material did this with a fan blowing the ever toxic dust away from me and out the garage door Most of the stuff fell onto a newspaper for quick clean up Brake J ob Tips Hose Clamp ABS Sensors Forcing Dirty Fluid Back into the ABS From Motor Service Magazine We believe the biggest brake bungle techs are making today is forcing caliper pistons back without clamping the hose and opening the bleeder Since the line comes in near the bottom of the cylinder where the sediment is this forces debris up into the ABS unit and that will likely result in a comeback for
23. raking and hitting a bump then it just stays lit The wires that lead to the sensors up front look in good condition Response Chris Herbst There s a single rear sensor single wire that goes to the differential The sensor sends the same signal to both speedometer and ABS units through one pair of wires leaves the sending unit The over bumps part is a good clue That thing acts up over bumps first and eventually breaks or becomes totally ineffective Some days your system will work until you hit a bump and then BINGO your ABS lite comes on and the speedometer fails Play around with the sensor and connector and see if it might be your problem It often is ABS Light On After J ump Start Inquiry On my 1993 940 had to change the battery and after the car was started the ABS light was on and now doesn t go off checked all fuses at the fuse box read there s another one up by the ABS ECU Please tell me there s another fuse to check fear this may be an expensive repair What else can check Response 1 Jim Bowers A possible source of system overvoltage is a loose battery cable If the cable disconnects even for a few thousands of a second while the alternator is charging the alternator output voltage will jump to a very high value usually killing the alternator diodes also Response Abe Crombie First make sure the leads that attach to the bolt on the positive battery terminal are tight as one of these is the fee
24. rn valve open circuit or short circuit Traction control system TRACS valve open circuit or short circuit 2 2 1 Left front wheel sensor ABS operation signal missing sid 2 2 3 Left rear wheel sensor ABS operation signal missing sid 2 3 2 Right front wheel sensor signal missing 3 1 2 Right front wheel sensor open circuit or short circuit 3 1 4 Right rear wheel sensor open circuit or short circuit 3 2 3 Left rear wheel sensor irregular interference at speeds over 40 km h Rear wheel circuit return valve open circuit or short circuit Pressure switch for TRACS faulty or short circuit Control Module CM faulty FER EEEEE Pump pressure low 4 4 3 Pump motor relay electrical or mechanical fault 4 4 4 No power supply to valves in hydraulic unit Diagnostic Notes on ABS Systems by Larry Carley Import Car Magazine March 2002 adapted for Volvo RWD All anti lock brake systems control tire slip by monitoring the relative deceleration file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM Brakes ABS rates of the wheels when the brakes are applied If one wheel starts to slow at a faster rate than the others it means the wheel is starting to slip and is in danger of losing its grip locking up and skidding The ABS system responds by momentarily isolating the brake circuit for that wheel releasing hydraulic pressure and then reapplying the brake in rapid sequenc
25. slow speeds have reseated both electrical connections on the inside of the shock towers Response Abe Crombie The sensors fit into the holes in the spindles with clearance to spare Twist them as you pull on them and they should come right out If they are the style constructed with two steel strips bonded to a magnet in the center to make the tip made by VDO then they will have a much greater tendency to have the low speed modulation as you have experienced The other style Bosch have a diamond shaped tip that is all steel ABS Pulses at Low Speeds Inquiry The ABS acts up when I m driving at low speeds and applying hardly any pressure to the brake pedal of my 1990 740 GLE The ABS lifts the brake pedal in the same way as if it was locking up Response You should remove and clean the front wheel speed sensors To do this turn wheel full lock to one side and then remove the sensor from wheel that is turned out by undoing a 5 mm Allen screw you will see adjacent to arm where tie rod for steering attaches Then turn wheel to full lock the other way and do the Same on other side Also unplug and then reconnect the sensor connectors that are in the engine compartment near top of struts This should effectively clean the connections This can also cause low speed inappropriate ABS activation Tip from Bob Dietz Apparently when the waterproof connectors to the ABS sensors at the differential rear and shock towers front are molded o
26. t an Intermittent Check ABS Lamp Inquiry have a 1989 740GLE that sometimes has the anti lock brake warning light on The brakes are new so know the re OK Is there a connector somewhere that should be cleaned Response Check ABS sensors at front wheels and clean them Check B wires at the positive battery cable Corroded positive terminal could be the problem Another Inquiry about ABS Lamp On Found a 1990 760T in great shape original owner with all the maintenance records BUT the ABS light stays permanently on Am facing a major repair or just an adjustment clean up of the ABS sensors Response 1 Roy Magne Mo had exactly the same problem with mine Clean the connector at the front wheels and check the wires going to the connectors The wires were the problem with mine Response 2 John B agree with Roy but you won t know until you have the system checked out Before bought it would pay the 80 or so to get the car and the system checked out by a Volvo dealer You DON T want to buy a new Response 3 RC I should tell you that had similar characteristic with my ABS Luckily a flush of the brake fluid did the trick can only hope that your fix is as simple ABS repairs sound scary ABS Light On ABS Sensors Wheels Dirty ABS Sensor Basics Inquiry During the snow here over last couple of weeks UK our 1992 Volvo 940 2litre turbo started flashing the ABS light up and even now the sno
27. t failed Turn on key start engine check to see that ABS light is on and then tap on AS modulator if light goes off then sol valve relay is at fault Otherwise check surge relay next to ABS ecm for loose fuse See also Speedometer Relay and ABS Lamp Replacing a Failed ABS Module Randy Duke After my 94 940 s BS modulator seized and would not allow brake fluid to pass on to the caliper got a used ABS file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM Brakes ABS hydraulic modulator for 125 was able to get most of the air out of the replacement modulator by placing the assembly flat on the ground with the open connections pointing straight up It turns out the old brake lines cut off the car fit those connections so used them and a syringe to force fluid into the part Lots of bubbles came out left the hoses on the bottom two connections so the fluid would not drain out when was installing it After that attached the top brake lines first so the fluid did not drain much when removed the temporary hoses that were in the bottom hole attached the bottom two lines last The brakes are working just fine now Removing and Replacing a Toothed ABS Sensor Ring J ay Simkin If your car has ABS brakes the toothed sensor wheel on the hub rotation of which wheel feeds the ABS sensor can be removed to ease stud removal installation To remove the toothed wheel you ll need a workbenc
28. trical welding on a vehicle it is recommended that all of the file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM Brakes ABS computers be disconnected from the wiring harness to prevent possible damage Care should be taken not to damage the connectors 10 When replacing unitized wheel bearings half shafts steering knuckles or any other component that could affect the air gap between the wheel speed sensor and sensor ring then the air gap must be checked 11 When servicing disc brakes open the bleeder screws and vent the brake fluid if it is necessary to push the caliper pistons in There is sediment that naturally collects in calipers over a period of time This sediment if allowed to flow back into the master cylinder along wi Brake Fluid Bleeding Flushing Procedures for Cars with ABS Depressurizing Fluid Tip from Abe Crombie The ABS doesn t need to be de pressurized on a Volvo There are a few cars out there that have the master cylinder hydro boost ABS pump as a unit These are the ones for that precaution You never get the fluid completely changed on ABS cars as some fluid is in a storage accumulator that is not accessible If you periodically change fluid this is never an issue as the fluid left in these areas will be circulated eventually through much of the system and it will be diluted by the clean fluid and will pose no problem Editor s Note Consider a pressure flush
29. w s gone it came on once but brakes are all fine and the light hasn t come back on for week or so does anyone reckon we have anything to worry about reckon it may have just been the cold weather and snow and surely if the ABS was faulty the light would have stayed on all the time Response Paul Grimshaw Bob Dietz A common problem with ABS is that the sensors and reluctor wheels used to measure and compare tire rotational speeds can become fouled by dirt and brake carbon This file Cl Users Steve Documents Volvo 20FAQ 20Updated BrakesABS html 01 13 14 9 55 34 PM Brakes ABS typically occurs on the front axle where the sensors and reluctor wheels attract a considerable quantity of brake dust shed by the front calipers As the wheel turns an analog signal is generated from each sensor and monitored by the control unit If the amplitude of the signal decreases due to iron particles accumulating on the sensor the control unit cannot accurately compute the wheel speed and begins to modulate the brake pressure to the wheel whose signal has been lost Since higher wheel speeds generate a higher voltage the problem does not occur at higher speeds since the differential value detected among the channels falls into an acceptable range As accumulation of brake dust grows heat fuses it into a carbon like coating that attenuates the minute changes in reluctance measured by the sensors This is interpreted by the car s ABS computer as a continuous di
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