Hi,

I have weird ABS problem. I bought pre-owned Audi 100 (C4) 2.8 Quattro 1992, with ABS not working.

Situation:

Engine on, ABS light stays on. But after pressing ABS button the light goes off, ABS really doesn't work. After restarting engine ABS light is on again.

After I engage rear differential lock, ABS light goes off (normaly it should go on). When differential lock goes off, ABS light goes on...

It is clear that ABS computer was touched before, either replaced, either repaired. I've tried to replace it to the another unit with the same code. But result was the same like with the old one. Could it be that someone before me replaced computer to the wrong one? Is it possible to check somehow which ABS computer code is the right one for my car?


Another sympthom (might be connected, might be not) After I keep the car with engine off for i.e. one hour, the break pedal becomes hard. It seems to be not normal? There should be leak in vacuum system or is it normal? Could it cause ABS failure?

Thanks,
Algis