Brakes have a mind of their own, ABS problem

[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.

[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.

[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.

[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]

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.

[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.

[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

[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.

[UNHCLL]

Have you checked the ABS rings for particulate?

[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?