ABS Troubleshooting (activates w/ brake activation), Soft Brake Pedal - Need Help!

[Cloudbase]

'89 325i. 199,000 Miles. Here's what I need help with - ABS Troubleshooting, Sensor and Ring Cleaning, Soft Pedal Diagnosis. I've searched, but could would still appreciate your help.

ABS Activates and Light

Ever since I got the car I would get occasional ABS activation just after starting while reversing out of my driveway.

Recently my Antilock light will come on after a descent bump disabling the system. It has slowly been happening more consistently.

Today I put the car on jacks to see if it was a loose wheel speed sensor or bad wire. I giggled and pulled gently on the wires to try to get the light to activate. It didn't and when I drove it afterwards my ABS activated with every brake application.

My left front wire has a nasty kink in it where the rubber has cracked. I'm thinking this is probably the culprit as I've read very small changes in electrical current will affect the control module. I've got a friend with an '87 parts car that I can pick up a sensor from.

I'd also like to clean my rings on all four wheels while I'm in there. I can't find any pics or a techinique for this. I was thinking Brake Cleaner or Mass Air Flow Cleaner, but I don't want to assume and mess it up. The only DIY in the DIY section involves the ABS relay. The Bentley Manual shows a diagram of what the ring looks like, but it isn't very helpful. Any tips on this would be much appreciated.

If that doesn't fix it I was thinking replacing the remaining sensors. Then the Relay, then the control module, and lastly the Hydralic Unit.

If you've gone through this, please point me in the right direction.

Slightly Soft Pedal

Ever since I got the car my brake pedal has been slightly soft. If you hold the pedal down with engine running it will sink slightly towards the floor. It seemed like a classic bad Master Cylinder. I've replaced the MC twice and pressure and manually (following the Bentley 12 pump ABS procedure) bled the system at least 5 times. I've also gone out on a gravel road and gone deep into the ABS to get the pump to cycle.I've replaced all the rubber lines with new SS ones. I've never noticed any fluid loss in the reservoir or at any of the components. Any suggestions? Next I'm going to start rebuilding calipers.

[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.

If the brake pedal sinks to the floor while you are holding constant pressure on the pedal with the engine running, either there is a leak at a line or caliper or the master cylinder is bad. When you replaced the master cylinder was it with a new (not rebuilt or used) part?

[Sagaris]

Not to threadjack but where are the connectors for the rear ABS wheel speed sensors? I wasn't able to find them last time I was under the back of the car.

[Cloudbase]

Thanks for the reply jlevie. Tonight I unplugged the left front sensor, which I suspect is bad, and got an anti lock light. When I plugged it back in it the ABS activated continuously again. I realize this isn't removing the other imputs to the ABS unit. I will do the proper sensor test tomorrow.

I also couldn't find the rear wheel sensor connectors when I had the car on jacks, but I didn't take the rear wheel off. Do you know where they are off hand.

The way the pedal sinks to the floor is very subtle. The few E30 guys who I've had drive the car barely noticed it. It sinks ever so slightly about 1/3 of the full pedal travel from where it should normally stop towards the floor. I can never get it to go all the way. The Master Cylinder was the second replacement. It was used, but known to be working. It was my buddies from his M52 swap so I know that braking system worked before he pulled it. I bench bled it thoroughly. It could be bad, but I suspect it is somewhere else. I'm going to bleed it one more time tomorrow as my friend will be over to help. I've heard that manually bleeding with the pressure bleeder hooked up will sometimes dislodge a stubborn air bubble. Any thoughts on that.

If that doesn't work I was going to start rebuilding calipers.

[jlevie]

The connectors for the rear sensors are under the rear seat.

[Cloudbase]

Okay jlevie, I did the individual wheel speed sensor test above. I got an ABS Light with both front sensors individually and no light from either rear sensor individually. Following the logic of the test that would point either bad:

- left and right rear wheel sensors (I wonder if it's slush and crud in my rear wheel rings).
- bad circuitry in the rear wiring harness
- or bad channels in the Module.

What are your thoughts? I'm thinking of pulling the rear wheels and trying cleaning the rings. Then swapping out my Module for a working one just to isolate that. I've got a buddy with a parts car, I bet he'd let me swap the modules.

[Cloudbase]

Update: So I cleaned the left and rear wheel sensors and rings with brake cleaners. They were both filthy with road grime. I had to start the car, put it in 1st and let the wheels spin to rotate the rings. I used a soft dremel brush tool mounted in the dremel (not running of course) to gently brush the rings with brake cleaner. Then I blew it out with air and made sure it was dry.

On the first drive the left rear tested good, but the right rear did not. I decided to test them both individually again and that time the right rear tested good and the left rear did not? WTF?? I repeated the tests with both individually and neither tested bad again. I'm inclined to think that cleaning them didn't matter and the good tests were erroneous. Thinking it's a bad Module channel or rear circuitry wiring. Perhaps I'll try replacing both rear sensors just to rule that out.

[Cloudbase]

Update: I like to close out my problem threads if I find a solution. It turned out to be a left front ABS sensor. It was odd because using the individual test procedure it tested great and then I couldn't get a conclusive test on the rear sensors. Sometimes with just one rear sensor plugged in I wouldn't get an ABS light until after 30 minutes of driving. Swapping my control module didn't change anything. The left front wire was had several kinks with close to 90 degree bends. When I hit a bump it would cause it flex and fail the channel. As for the soft brake pedal I'm going to start rebuilding calipers, but that might take a while.