Diagnosing ABS

[BS87]

My ABS is starting to go crazy. The light will come on sometimes, but stay off others, it seems a 50/50 occurance. The wheel sensors all look good, etc, so what could be making it come on half the time?

[AdamF 88iS]

Loose or dirty connections and grounds, maybe even broken wires.

[robbieosterman]

My ABS computer light will turn on when there is low voltage on the battery.

[BS87]

Just checked today, and could not find a bad ground. This is pissing me off.

[AndrewBird]

There was an E30 tech site that had step by step procedeures for finding what was wrong, but I don't know what it was. Anyone know what I am talking about?

[JHZR2]

http://www.ibmwr.org/ktech/abs-diagnostics.shtml

[The Dough]

^ That is for BMW bikes.

[jlevie]

Troubleshooting the ABS system isn't very difficult. It is a stand alone
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 wheel 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.

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 wriing fault in that circuit or a
bad input channel in the ABS unit.