Brakes: ABS                                                                                                  FAQ Home                                                                                                                           Volvo Maintenance FAQ for 7xx/9xx/90 Cars                                                                                                                     Version 5.0
Brake Job Tips: Hose Clamp; ABS Sensors

Anti-Lock Brake System Service Precautions

Brake Fluid Bleeding/Flushing Procedures for Cars with ABS

ABS Diagnostic Code Retrieval

Intermittent "Check ABS" Lamp

ABS Light "On": ABS Sensors Dirty

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

Pad and Rotor Identification and Replacement

Brake Rotor Removal on 740 with ABS

Brake Job 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 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 rounded jaws designed to clamp rubber brake hoses without damage. Check your auto parts store.]
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 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, 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. & 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.  [See 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 electrical welding on a vehicle, it is recommended that all of the 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 with the brake fluid, can possibly damage the ABS hydraulic unit. [Editor's Note: see above notes on clamping the rubber brake lines.]

Brake Fluid Bleeding/Flushing Procedures for Cars with ABS.  [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 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.]

ABS Diagnostic Code Retrieval. [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.

Put the Diagnostic Link Connector (driver's side strut tower, same as ignition and fuel injection diagnostic unit) probe into DLC, Position 3.  Push button on DLC for >1sec. and capture codes flashing in the red lamp.

To clear codes,  place the DLC probe into position 3. Hold DLC button down for >5secs.  Release for 3 secs,  then press again for >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.
ABS System Diagnostic Information
Mode 1:
(40 km/h=25 mph)
No error code set
Left side front wheel sensor: faulty signal at speed less than 40km/h
Right side front wheel sensor: faulty signal at speed less than 40km/h
Left side rear wheel sensor: faulty signal at speed less than 40km/h
Right side rear wheel sensor: faulty signal at speed less than 40km/h
Signal faulty from at least one wheel sensor for a long period
Control Module (CM) faulty
Faulty pedal sensor: shorted to ground or supply
Faulty stop (brake) lamp switch: open circuit
Control Module (CM) faulty
Brake discs overheated
Left front wheel sensor: open circuit or short-circuit to battery voltage
Right front wheel sensor: open circuit or short-circuit to battery voltage
Rear axle sensor: open circuit or short-circuit to battery voltage
Left front wheel sensor: no signal on moving off
Right front wheel sensor: no signal on moving off
Left rear wheel sensor: no signal on moving off
Right rear wheel sensor: no signal on moving off
Valve relay: open circuit or short-circuit
Left front wheel sensor: ABS operation signal missing
Right front wheel sensor: ABS operation signal missing
Left rear wheel sensor: ABS operation signal missing
Right rear wheel sensor: ABS operation signal missing
Left front wheel sensor: signal missing
Right front wheel sensor: signal missing
Rear axle sensor: signal missing
Left front wheel sensor: open circuit or short-circuit
Right front wheel sensor: open circuit or short-circuit
Left rear wheel sensor: open circuit or short-circuit
Right rear wheel sensor: open circuit or short-circuit
Left front wheel sensor: irregular interference at speeds over 40 km/h
Right front wheel sensor: irregular interference at speeds over 40 km/h
Left rear wheel sensor: irregular interference at speeds over 40 km/h
Right rear wheel sensor: irregular interference at speeds over 40 km/h
Left front wheel inlet valve: open circuit or short circuit
Left front return valve: open circuit or short circuit
Right front wheel inlet valve: open circuit or short circuit
Right front return valve: open circuit or short circuit
Rear valve: open circuit or short circuit
Rear wheel circuit inlet valve: open circuit or short circuit
Rear wheel circuit return valve: open circuit or short circuit
Traction control system (TRACS) valve: open circuit or short circuit
Pressure switch for TRACS: faulty or short circuit
Control Module (CM) faulty
Pump pressure low
Pump motor/relay: electrical or mechanical fault
No power supply to valves in hydraulic unit

Intermittent "Check ABS" Lamp. [Query:] I have a 1989 740GLE that sometimes has the anti-lock brake warning light on. The brakes are new so I 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 Query 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 I facing a major repair, or just an adjustment / clean-up  of the ABS sensors ?   [Response 1: Roy-Magne Mo] I 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] I agree with Roy.....but you won't know until you have the system checked out.  Before I bought it I 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 ABS head for that car
[Response 3: RC]  I should tell you that I had similar characteristic with my ABS. Luckily, a flush of the brake fluid did the trick. I can only hope that your fix is as simple; ABS repairs sound scary.

ABS Light "On": ABS Sensors Dirty.  [Query:] 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 snow'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, I 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] 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 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 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 difference 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.  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.
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.

ABS Light "On" After Jump Start.  [Query:] On my 1993 940, I had to change the battery and after the car was started the ABS light was on and now doesn’t go off. I checked all fuses at the fuse box, I read there's another one up by the ABS ECU? Please tell me there's another fuse to check, I fear this may be an expensive repair. What else can I check?  [Response 1: Jim Bowers] A possible source of system overvoltage is a loose battery cable. If the cable disconnects, even for a few tousands 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 feed 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. I 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.

[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. I don't want to break them so I'm looking to some experienced people for some advice. My ABS is pulsing inappropriately at very slow speeds. I 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.  [Query:] 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.

Noise from 960 ABS System.  [Query: 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.  [Query 1:] The Anti-lock light illuminates on my 1990 760 a while after I start driving, before that ABS seems to work OK. The light seems to become lit when I exceed 25 mph, but lately it seems like the speed has been higher, yesterday I 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 insulation. 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.

[Query 2:] The ABS light on my '90 740T Wagon came on as soon as I started the car after lunch today (very hot, very humid). And it stayed on after I 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 fact 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

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.  [Query:] I 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 I need to remove the wheel bearing. Is that correct? Are the ABS sensors difficult to remove?. The manual mentions an internal toothed ring which has to be removed from the old rotors 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.  I 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.

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