Troubleshoot the ABS system/ Sensors?

[downforce22]

My ABS has not been working on my project car since I got it running and I am trying to figure out how to determine what the problem is.

Symptoms include the ABS light turning off when the car is started. DUring this time if you hit the brakes they barely work and provide a lot of resistance without much slowing. After the car has been moving for about a minute the ABS light will illuminate and the pedal will be spongy but will work (ie lock up) if you press hard enough.

I have switched the abs control unit and that did not fix it. I believe the problem is one of the ABS sensors but do not want to replace all four as they cost $100 each.

If I disconnect all sensors but one, would that illuminate the light? Can I connect only one at a time to determine whether each sensor individually is working properly?

Thanks for the help guys!

[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.

[downforce22]

Thanks jim. To follow up on this, both my rear sensors were rusted in the housing. I pulled them out and the first was all jacked up. The second I had to drill out as it snapped in half.

I bought two replacement rear sensors and replaced them. The light still came on.

I disconnected all four sensors and did the individual test on them. Each sensor set off the light after the car was moving.

I am going to verify the front sensors are not backwards as apparently they are different from left/right on the front. I'll report back with results if the weather cooperates.

http://spece30.com/forum/39-brakes-and-abs/53217-a-different-kind-of-abs-problem?limit=10&start=10

For reference

[philakamrbean]

If you've got the ETM, look for the ABS ECU pinout. (I had it somewhere). It will tell you which sensors are which pins, and then just measure resistance. Had to do this to my car last year when a front sensor died.

[barry]

It's the number 20 pin in the 101 Look for brown earth wire or red yellow white make sure the battery is disconnected. If you have the brown wire it is live.

Go to www.E30zone.net engine swap m20 Scroll down the article to ABS