Weird ABS light behavior after replacing brake MC

[ghrays]

I had a leak in my brake MC as well as at the connection of the driver's side brake hose and the brake caliper. Since these all had over 200K miles on them, and I hate working with brake fluid, I replaced everything i could think of at once: both front brake hoses and calipers, the brake master cylinder and booster (both ATE as Girling is NLA) as well as the clutch master and slave cylinders. I did bench bleed the brake MC before installing it to the booster.

After bleeding the brake pedal and brakes seem fine with one exception: The ABS light is on when turning the ignition on, but does not go off right away after starting the motor. When test driving, the ABS light goes off intermittently when hard braking and then comes back on. I don't feel the ABS system engaging when hard braking.

I tested both front ABS brake sensors, they are normal (1100 Ohms) resistance and 0.1 volt AC when rotating the wheel.

So, my question is, could there be air in the ABS pump even though I don't see any when bleeding at the caliper bleed screws, and could this cause the erratic ABS light illumination? Also, I am pretty sure the ABS pump is not engaging when hard braking, whereas before it would always turn on appropriately.

[ghrays]

I tried again today to purge the presumed air from the ABS pump by driving and repeatedly hard braking from 30 MPH or so. First, the ABS light came off, and only came on again when hard braking. Second, the ABS system seemed to be trying to engage, albeit very slowly with a distinct grabbing/clanging sensation at 1-2 cycles per second. I tried to bleed the brakes after this and found no more bubbles to bleed from any of the four caliper bleed screws.
So I guess there is a little improvement although the ABS is clearly not functioning normally. I will continue to try this hard braking over the next few days and see if the problem resolves itself. I am reluctant to attempt any kind of electrical stimulation of the ABS pump itself and would see that as a very last resort.

[moatilliatta]

What is your charging voltage?

Check the ABS Relay?

I'm pretty sure ABS light fault logic doesn’t care about if there is air in the system.

[ghrays]

What is your charging voltage?

Check the ABS Relay?

I'm pretty sure ABS light fault logic doesn’t care about if there is air in the system.

I checked the two ABS relays in the ABS pump unit (1990 model does not have a relay above the ABS control module) with a 12V power source and multimeter and they both work as they should. Driving again today, the ABS light went out after starting the car but came back on intermittently. When braking hard enough that the ABS system should kick in, it does but very anemically with only one or two slow pulses. I wonder if the ABS unit could have gotten stuck in some way with the feeding brake lines open to the air from the time it took from disconnecting the old master cylinder to installing the new one (a couple of weeks).

[bmwman91]

All of the original ABS units are usually all gunked up by now, due to the brake fluid being very hydrophilic. It may very well be that opening it all up and pushing a bunch of new fluid through dislodged a big piece of crud which has jammed up the pump's shuttle valves or one/multiple of the solenoid diverter valves. The ABS control unit does run a test on the motor and valves, although I am not sure how much it is able to determine faults beyond simple open-circuits. Based on what I have observed of Bosch Motronic units, the power drive ICs do have current sensing ability in many cases, and they can determine if either something is not drawing the proper load, or if a flyback spike is missing. Again, not sure about the ABS computer, but if the fault light is on now and the actual functionality is not as good as before, I'd bet that crap has lodged itself in there somewhere.

If that is the case, I'd say get a Jordan-rebuilt ABS unit. It's definitely not a simple DIY thing, and there are dozens of little seals, filters and other stuff in there that probably do not have a PN. It is one of the best "upgrades" I have ever made to my car...I had no idea how poorly the ABS unit was working until I put in a good-as-new one. It works fast, modulates wheel slip very nicely, and the pedal does not drop to the floor like almost everyone says it is supposed to (because every E30 ABS unit out there is gunked by now). Check this thread for info...not cheap, I know.

https://www.r3vlimited.com/board/forum/e30-classified-forums/for-sale-wanted/parts-for-sale/9933031-refurbishment-service-for-e30-abs-pumps

[moatilliatta]

I’d also consider changing all the wheel speed sensors and making sure the reluctor wheels are ok.

Wheel bearings tight?

[ghrays]

Each time I drive the car, the light is on less and less. Today with hard braking, the light did not come on and I could feel a few pulses of the ABS kicking in. In 30 mins of driving today, the light only came on twice, both times for a fraction of a second.

bmwman91, I read your thread of installing a rebuilt ABS pump. Did your ABS light come on intermittently like this before you went through your ABS bleeding procedure you described?

I may end up getting one the rebuilt pumps from Jordan. I just wanted to see if I could get the system working before tearing it apart again and then having the same issues.