|Brake Job Tips: Hose Clamp; ABS Sensors||ABS Pulses at Low Speeds|
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
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|
(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.
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 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.
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.
[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