In the end, this turned out to be a very different problem than I expected. There was nothing at all wrong with the ABS itself. Instead, the positive terminal on the battery was so badly corroded that one of the wires had come disconnected. Turns out it was the wire that supplies battery + to the ABS solenoid valves and return pump motor via the respective relays. So, when the ECU self-test closed the relays and saw no voltage, it concluded that the relays were defective!

I agree that it can be quite a bag of worms to fix an ABS problem on an older car.

A tech at my dealer worked on my '90 744 TI in his spare time - he even put his ABS box in my car to see if it could be the problem (above and beyond the call of duty IMHO).

I had to put the trunk and dash panels back to pitch in on the work.

He checked the wheels, under the hood, under the dash and in the trunk - no luck - the trouble did improve as only about 1 out of 4 trips resulted in the light coming on some time during the trip and it always went out for the start of the next trip.

ONE THING I DID FIND the right front tire (Turbo 16" wheels with wide tires) rubbed on the wire to the ABS until it hit the wire in the center of the rather fat insulation, I was able to reposition it in its holder to avoid further damage.

I repaired it with a nice patch of silicon caulk and a wrap with electrician's tape
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 485,000 miles put on 9 bricks

After fiddling with it a bit, the symptoms have changed slightly. Now the light is off for a few moments after start and then comes on. The OBD reports error codes 4-1-1, 4-1-3, and 2-1-5, which the FAQ says mean open or short in the front wheel inlet valves (4-1-1 = left, 4-1-3 = right) and open or short in the valve relay (2-1-5). I removed and tested both relays under the cover in the ABS unit and they both seem fine. The FAQ does not mention the inlet valves. Where are they, and how can they be tested? And, if the relay tests out ok, where else should I look for a short/open? Everything looks clean and in good condition - no visible corrosion or damage, no recent repairs or work that would have pinched a wire...

Could be a bad solder joint or two on the instrument cluster flexible board.
Not to uncommon and pretty easily fixed.
Need to have a soldering iron though.

HTH
mila

Have you looked at the ABS Brakes section in the FAQ? If you have not cleaned the wheel sensors, start there.

The ABS system is quite robust, there is no reason to suspect some sort of hideously expensive parts-replacement is called for.

john
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1989 245 236K / 1993 945 127K

The 1993 940 my wife drives will often have the ABS light on when she first starts driving the car for the day. The light always goes off in less than a mile. There are no stored codes. I should try cleaning the speed sensors at the front wheels. Then again, she needs to give me a little time to do that.
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john