causes for the ABS light to stay on

[DrMookinstein]

Newly purchased an '86 325e. [Anti-Lock] light is illuminated at all times. I replaced the ABS relay and the light is still on. I pulled the cover off the ABS module under the hood and couldn't find anything wrong. The Bentley Manual is surprisingly unhelpful when working on this car. What are the causes for the ABS light to come on? Thanks.

~Mike

[jlevie]

Troubleshooting the ABS system isn't very difficult. It is a standalone system
that only gets input data from the brake light switch, wheel sensors, and
system power. The parts of the system are the ABS module located next to the
steering column, the ABS relay mounted above the ABS module, the ABS unit in
the left front of the engine bay, and a speed sensor at each wheel.

If the ABS light comes on with ignition, but doesn't go out when the engine
starts, a wiring problem, bad module, or bad ABS relay are the likely
candidates. A failed or weak alternator will also cause this because the
system voltage won't rise enough. There is a fusible link inside the ABS relay
that can be blown. The relay can be repaired, or better yet, replaced. The
relays in the ABS unit can be replaced.

If the ABS light comes on when you turn on the ignition, goes out when the
engine starts, and then comes back on before the car is moving the cause could
be a bad module, a bad ABS unit, or a wiring fault.

If the light goes out after engine start and only comes on when the car is
moving, there's a problem with one (or more) of the sensors. The simple test
to find out which sensor(s) is the cause is to disconnect all but one of the
sensors and drive the car. If the light comes on, that sensor is sending a
speed signal to the ABS and is good. Repeat with each other sensor to find the
one(s) that don't cause the light to come on. Since the ABS module has no
speed input besides the wheel sensors, if only a bad sensor is connected the
module can't tell the car is moving and thus doesn't fault on missing speed
signals from the other wheels. Once a bad speed sensor is found, if a new
sensor still doesn't work, there could be a wiring fault in that circuit or a
bad input channel in the ABS unit.

[drulle]

^^^ thank you for such great and detailed explanation. I have same issue as OP. Will look into it following your advice

[Roysneon]

Troubleshooting the ABS system isn't very difficult. It is a standalone system
that only gets input data from the brake light switch, wheel sensors, and
system power. The parts of the system are the ABS module located next to the
steering column, the ABS relay mounted above the ABS module, the ABS unit in
the left front of the engine bay, and a speed sensor at each wheel.

If the ABS light comes on with ignition, but doesn't go out when the engine
starts, a wiring problem, bad module, or bad ABS relay are the likely
candidates. A failed or weak alternator will also cause this because the
system voltage won't rise enough. There is a fusible link inside the ABS relay
that can be blown. The relay can be repaired, or better yet, replaced. The
relays in the ABS unit can be replaced.

If the ABS light comes on when you turn on the ignition, goes out when the
engine starts, and then comes back on before the car is moving the cause could
be a bad module, a bad ABS unit, or a wiring fault.

If the light goes out after engine start and only comes on when the car is
moving, there's a problem with one (or more) of the sensors. The simple test
to find out which sensor(s) is the cause is to disconnect all but one of the
sensors and drive the car. If the light comes on, that sensor is sending a
speed signal to the ABS and is good. Repeat with each other sensor to find the
one(s) that don't cause the light to come on. Since the ABS module has no
speed input besides the wheel sensors, if only a bad sensor is connected the
module can't tell the car is moving and thus doesn't fault on missing speed
signals from the other wheels. Once a bad speed sensor is found, if a new
sensor still doesn't work, there could be a wiring fault in that circuit or a
bad input channel in the ABS unit.

That takes a lot of the mystery out of diagnosising my ABS system. I had no idea how it really worked but that really clears things up.

[mcskibadee1]

What could it be if it comes on every time after driving for about 4 minutes and stays on. Car sits over night starts up and again, after 4 minutes the light comes on again.

I recently replaced the grommets that hold the brake fluid reservoir so could air in the system cause this?

[jlevie]

What could it be if it comes on every time after driving for about 4 minutes and stays on. Car sits over night starts up and again, after 4 minutes the light comes on again.

I recently replaced the grommets that hold the brake fluid reservoir so could air in the system cause this?

See the diagnostic above... The master cylinder & reservoir has nothing to do with the ABS light being on.