Help! Anti-Lock Light is on

[bimmerboy12]

I can almost guaruntee this is a "cookie cutter" problem that has a canned solution.
I start my car, drive, first time I brake, the anti-lock brakes start kicking in like I'm on ice. Then the light pops on and the ABS is disabled. It resets every time I turn the car off and does it over again. I had a Mustang GT and an Audi 90 that did the exact same thing but I never pursued the problem before selling them.
I am guessing wheel speed sensors. How do you test them? They are pricey.
Thanks in advance.
-BH

[bimmerboy12]

Are you kidding me? no one has input on this? I would have thought to be bombarded with possible solutions with how many bimmer gearheads are on here.

[HST]

I know that on the E36's there are two ABS relays that tend to go out, and cause this problem. But the light is always on. It could be the same on the E30. There is no way to test the relay unless you have another car that has working ABS. Also they are not cheep.

[Jscotty]

No canned solution, however my guess is that you have an ABS sensor that is either obstructed by dirt, leaves, or some other debris preventing it from "seeing" the notches. Secondly, one of the sensors could have slipped out of the housing and its looking down at the ground or in some other direction other than towards the notches.

IMO the best way to diagnose this is to just pull every brake rotor to ensure that all 4 sensors are in place. I doubt that its a bad sensor because if this was the case, the ABS light would be on all of the time. But being that the system is activated when you initially brake after starting the car, that is a direct indication that the system is indeed working but it is getting a false reading.

[bimmerboy12]

I will check the sensors and let you know what happens.
Thank you for your help!

[mattdk318i]

http://www.bmwe30.net/cgi-bin/datacgi/database.cgi?file=articles&report=view&ID=00094&Section=06

[bimmerboy12]

Well here's what has happened. I took out all the sensors, which looked good but dirty and cleaned them. The problem cleared up for a little while but could hear the ABS spaz out a little when applying the brakes lightly at low speeds. As of the last couple weeks, it's been doing it more often, to the point where my ABS light will now kick on within 10 seconds of driving. The light will be off with the ABS functioning every time I start the car, so it seems to be resetting every time the car is turned off.
This whole issue seems to be a very gradual degeneration of the system. Any ideas besides throwing parts at it?

[bimmerboy12]

Bump For Help!

[HST]

I'd start with replacing the ABS sensors.

[ddavidv]

Go the the pick-n-pull. I bought a handful of those things for like $10. They are over $100 each new, IIRC.
Also the silver relay on top of the brain box on the driver's side. That's the protection relay for the ABS system and they always pop if the car is jumped or gets any kind of power surge. If you take them apart you can resolder the broken wire on them too. I just keep a handful in stock.

[bimmerboy12]

I checked this ABS relay and it's fine. Another thing I noticed is right after I start the car and start driving, I can hear the ABS pump whir on 3-4 consecutive times, then I get the ABS kicking on when slowly stopping, then the light. Other times, the light will kick on 5 seconds after I start driving with no symptoms. This is killing me. HELP!

[bimmerboy12]

Bump For Help....

[Toms73NovaSS]

I have a similar issue, I think with mine it is the rear reluctor ring on the rear half shafts. They have rust build up and are throwing off the signal. In the next few months I am going to redo my whole rear suspension and when I pull the half shafts I plan on getting rid of the rust and hopefully that will take care of the problem. There is no easy access to them in the car, so for now I just unplugged the abs. I have never had an abs car so I don't feel I am missing much....

And just to inform those who do not have a clear understanding of how the signal generation works.... Leaves and such can not throw off the signal it has to be a ferrite (iron) based material. Every time a tooth passes the sensor it causes a signal spike due to a change in magnetic field. So if the abs computer is not seeing basically on, off, on, off, on, off... it kicks on the abs pump until the signal is restored.

[scobaru]

if you have a multi meter i'd test each sensor and see which one isn't getting the right signal.

abs reads the signals as a sine wave.



so when a wheel locks up abs kicks in because of how it read the sine wave

[bimmerboy12]

if you have a multi meter i'd test each sensor and see which one isn't getting the right signal.

abs reads the signals as a sine wave.

so when a wheel locks up abs kicks in because of how it read the sine wave

Does anyone know of a write-up on testing the wheel sensors? I have a multi-meter.