Brakes have a mind of their own, ABS problem

[FredK]

I have an S50 swapped 318is with an ABS system with a mind of its own. Every 10 stops, it feels solid and exactly as it should. The other 9 stops, the pedal is harder than normal, and the ABS pulses almost immediately and locks up the front wheels unevenly, which unsettles the front end of the car.

The ABS is powered through pin 20 of C101 with a RD/WT power wire.

The system has
1. New reman brake booster
2. Known functioning 23mm MC
3. Brand new soft lines (all six)
4. Rebuilt calipers on all four corners
5. Fresh pressure bleed

Is this a case of a balky ABS solenoid? Or is it a brake proportioning valve problem, where the valve is blocked sometimes, but othertimes not? I think this could be the case because the old MC did have some wear on the rubber grommets, which I assume could have deteriorated, shedding little rubber bits into the brake system?

[UNHCLL]

Have you checked the ABS rings for particulate?

[FredK]

The sensors were cleaned prior to installation, but I'll check and see to make sure!

I guess I'll be smelling a lot of brake fluid this weekend. I have a known good pump and proportioning valve, so I guess I will figure out what the problem is in any event.

[1990m3]

I had a similar problem and it turned out to be a damaged ABS ring on the front of my car. Turns out my dumbass knicked it with the grinder when I was cutting off the factory brake backing plates

[Jand3rson]

Fred, this is the same problem I'm having with my car. I checked the front sensors and cleaned them, but I haven't checked the rears yet, or the rings. I'll have to check those.

[FredK]

Well crap. I spent most of the day watching the Patriots, fabbing an exhaust bracket, and removing a lux carpet from my parts car. Well, there's always next weekend.

[golde30]

on my swap, i didnt power the ABS wire on my harness....however the ABS still works as it should. go firgure that out! HA

[FredK]

I cleaned the contacts on the hydraulic unit and swapped relays with a known-working hydraulic unit. The left front wheel still locks up before the right. Not cool.

EDIT: This did not work.

[FredK]

The problem persists!

Swapped pumps with known good pump. No change.
Swapped ABS computer. No change.
Bled system thoroughly. Pedal has very little compliance now.

I am going to rebuild the brake calipers and replace the proportioning valve and see if that does anything. I have a known-good valve off a parts car, fortunately.

[Knockenwelle]

I'd be interested to hear how this turns out. I have a very similar problem on my '91. It has about 1200 miles on a complete motor/trans/suspension rebuild, so I know all the wiring and sensors are in good shape. Brakes work perfectly sans ABS.

[1990m3]

I had this problem too. FredK, you never posted if you found any damage to the ABS rings. I had the slightest knick, replaced the ring and all is good. May not be your problem but I keep checking this thread to see how your problem was solved and I see nothing.

[Knockenwelle]

You know, I wonder if I have a bad rear ring. I rebuilt the CVs while they were out, and filed a few tiny nicks smooth on one while it was apart. I can't imagine that the sensors are THAT sensitive to anomalies, but that's really the only thing that changed during the build. The ABS worked fine before, and the nicks were present...

[FredK]

I had this problem too. FredK, you never posted if you found any damage to the ABS rings. I had the slightest knick, replaced the ring and all is good. May not be your problem but I keep checking this thread to see how your problem was solved and I see nothing.

Well the rear halfshafts were from a different car, so I am not sure of the condition of the rings as I did not check them prior to installation. Right now, my progress has been slowed because I am dismantling a parts car and it's taking up space in the garage. The parts car has known-good halfshafts so maybe I'll swap those in and see if that changes anything.

The front rings should be fine, because when the strut housings were installed on a different car, the ABS worked fine.

I haven't solved the problem yet, but I'll definitely keep this thread updated.

[1990m3]

You know, I wonder if I have a bad rear ring. I rebuilt the CVs while they were out, and filed a few tiny nicks smooth on one while it was apart. I can't imagine that the sensors are THAT sensitive to anomalies, but that's really the only thing that changed during the build. The ABS worked fine before, and the nicks were present...

they are. I knicked one ever so slightly with my cut off wheel when I was cutting off the front brake backing plates and that was my problem. Took me 6 months to figure it out and I felt like an idiot once I did. You should have seen how small the knick was, it was hard to believe that it affected my ABS system the way it did.

[FredK]

Dang. I think the first thing I'll check is the rear halfshafts, then.