I'm not that experienced in fault finding the C4 ABS system, maybe someone here have seen this before.

While driving, the ABS control unit will often fail, resulting in Diff and ANTI LOCK warning lights as well as the audible warning signal.
In this state there is no response using a PST-2 diagnostic system. If the unit is reset by briefly turning off the ignition, the warning lights go out and unit works until this problem occurs again. OBD connection is OK, but never with the engine running. There's no stored fault codes, but we once saw a code 15. (Control unit defective) This has not occured again, despite a lot of fiddling. Unit looks good with no corrosion issues in the main connector.
I suspect a problem in the ABS control unit power supply, or an internal control unit fault. Obviously, I would like to rule out all possible external causes before we proceed with any repair of the control unit. Both GND and +12V connections looks OK, but we will double check this.
Has anyone seen this behaviour before and maybe have a tip or two?
Cheers,
Tore

 

First thing you should do is check your brake fluid level, make sure you are checking it while the car is running and its ALL THE WAY FULL TO THE TOP MARK- if it's a little low you will get these warnings. Good luck, and hope this is all it is.

 

Hi Tore
I have a 964 that had similar errors, I finally found out that it was one of the acelroments that was faulty (I tried to clean them repeatedly) after switching to new acelrometer the failure was gone.

 

Thanks guys.
The fluid level is slightly above the high level mark, but I would expect a low fluid level to not inhibit the ABS control unit to respond to OBD diagnostic requests.
I would also expect a failing accelerometer to produce a OBD fault code in the ABS system. Saab96, did you read out any codes using a Porsche compatible diag tool?
Cheers,
Tore

 

Hi Tore
Unfortunately I do not have any diagnostic tools (wish I had) I used the workshop manual and google. I systematically worked, tested, and eliminated all equipment associated with the ABS controller. Finally I switched the two accelerometers and then the error changed ... I thought, one or both of the accesometers had to be faulty. My first fault occurred after driving 2-400 meters (ABS switched off, no ABS breaks and no 4w drive, both warning lights and alarm), after I switched the accelerometers, the error happend if there was a bump, offen combined with a turn (4w drive light and alarm). After changing the accelerometer to a new one, this last error is also gone :-) No alarms and red lights.

 

Tore,

While I can't talk about the ABS unit and it's functioning, the symptoms look very similar to when my airbag unit was defective.

Intermittent issues, testing power, no connection to diagnostic tool, sometimes it did with no codes. I mean, it sounds identical.

After a year of trying to fix it I got a used hood unit, plugged it in and everything works fine.

By all means try out the other recommendations and I hope that's it, rather than the control unit.