Noise and modulation on low speed stops

[KeithV]

Starting today, for some reason, I seem to be having an issue with my ABS.

When slowing to a stop, at around 5mph, I hear a strange "boing"ing sound and the pedal modulates like the ABS is kicking in.

Also, it -seems- like the car pulls a little to one side when I hear the sound, almost like one wheel is receiving far harder braking than the others.

Any ideas?

[PiercedE30]

Make sure all the ABS sensors have the same resistance. Check the brakes (on the side that pulls) and make sure a caliper isn't sticking.

[KeithV]

ABS light came back on and issue stopped. Strange.

[PiercedE30]

ABS light came on because ABS is no longer active, which would also make the issues go away if the ABS system was triggering.

[Jand3rson]

I'm having this exact same issue, just starting a few days ago, after my last auto-x session. Mine is doing it under moderate to heavy braking, and pulling to the left before it engages and actually stops the car. It doesn't seem like David's issue was really resolved, so I want to try to 2 things you suggested, Pierced. How do I check the resistance of the ABS sensors, and how do I check if the caliper is sticking?

[diegom6]

I'm having this exact same issue, just starting a few days ago, after my last auto-x session. Mine is doing it under moderate to heavy braking, and pulling to the left before it engages and actually stops the car. It doesn't seem like David's issue was really resolved, so I want to try to 2 things you suggested, Pierced. How do I check the resistance of the ABS sensors, and how do I check if the caliper is sticking?

I'm in the same boat, altough I have 5 lug swapp but int he past with it, the ABS was working good. All signals are OK from waht we checked. We checked the realy, cleaned it and placed back, no improve....my tech dissambled the ABS module (ECU) , adjusted some weld points , cleaned it and it's ready for test...will report later.

[PiercedE30]

I'm having this exact same issue, just starting a few days ago, after my last auto-x session. Mine is doing it under moderate to heavy braking, and pulling to the left before it engages and actually stops the car. It doesn't seem like David's issue was really resolved, so I want to try to 2 things you suggested, Pierced. How do I check the resistance of the ABS sensors, and how do I check if the caliper is sticking?

The best thing to use is a scope. Put the car in the air and, with the scope connected to the sensor, spin the wheel and check the signal. It will produce a very distinct 5v square wave signal. You can shim the sensors some to make them all have the same amplitude, if need be. Also, check the ABS relay. Those tend to be the weakest point in the system.

[Jand3rson]

Sweet, what about checking the calipers? Should I maybe bleed my brakes, also?

[Jand3rson]

Small update to this, it didn't do it at all today. Possibly because I noticed yesterday that there was dirty fluid residue grime all over the top of my brake fluid reservoir. I cleaned it all off, and noticed that the cap of the reservoir was barely holding on to the first thread, hardly even on. I screwed it back down tight, and nothing today. Could this have been causing it?

[PiercedE30]

I don't see how, but anything is possible.
Hell, if you want your ABS to do the best job it could possible, check out those ABS sensors on a oscilloscope. I think you might find it...interesting...as to how far off some of the signals may be. When one of the mechanics at my work, Winfred over on E30Tech, put his MCoupe suspension on he had to shim most of the sensors. He found that the rear sensors were a good bit off from the fronts. They were also the most sensitive to changes, in relation to how the ABS kicked on, and was able to get them to all have the same amplitude. When I rode in it, I was VERY impressed with how well the ABS system worked. It was almost flawless. It was a whole lot smoother than any of the other E30's I have been in.

[Jand3rson]

It started doing it again tonight. :(

So is it more likely that the sensors are bad/out of whack than the caliper being bad? I know I've never changed them (the sensors).

[PiercedE30]

Maybe try rebuilding the calipers. It may even just be a caliper piston that is gunked up.

[diegom6]

Once my ABS module arrive, I will try it and see what results give...so far it's still not working with the inspection of the module and clean up f the relay.

[Jand3rson]

I need to update this again, after finding some new things out. I went and talked to a couple of guys at German Master Tech, a local shop run by a couple of guys that are BMW Master Techs, and ran this issue by them. They told me that it's very unlikely that it's a stuck caliper, that wouldn't be causing the ABS to freak out.

What they reccomended was pulling the sensors and making sure there wasn't a bunch of crap all over them. When I pulled them, there was tons of stuff on them, the passenger side was so bad, I had to scrape it all off with a razor blade. I got everything back together, and it did it right when I got to the end of my street, and then several more times while I was out driving.

Has anyone else had this issue, and how did you fix it? I've checked most of the things suggested here, and I'm still not any closer to getting this fixed. I don't know much about brakes, please help!

[PiercedE30]

Pull the ABS relay?