ABS not working
 

The ABS on my 1991 965 is not working. I heard that it might be one of the 4 sensors. Is there a way to identify which sensor is bad or if it is anything else other that the sensors?:confused:

I dont have specific experience with them but you could probably find which one was bad by disconnecting one at a time and see which one makes no difference when disconnected.

How do you know your ABS isnt working? Which wheel locks?

Take a digital multi meter + turn it to AC voltage readout, get the car on a lift, un hook all of the sensors, hook the digital multi meter to the two prong plug for the abs sensor and spin the wheel that it reads. If you get a signal, chances are the sensor is good, if not- try a new sensor.
It could also be that you got some dirt in there or also a fuse may have popped.

John,
As far as I have notice both fronts are locking, probably all 4. The ABS light is on so I assumed the system will not work if one sensor is bad, not sure.

Kris,
Thanks for the advise, I will check the sensors and run the test as soon as I have time.
Thanks both for your help.......R

Have the brakes been bled recently or brake fluid level dropped to a very low level at any time?

Dave,
The answer is yes to both but the ABS light was on before that (??).
I did run the test Kris recommended. After testing all 4 sensors they all produce AC voltage when the wheels are turning. Something that might be an indication of a problem is a result after the way I tested the rear sensors. With the engine running and the rear wheels turning there is a voltage difference between right and left sides, in 1st gear 0.4 vs 0.6, in 5th gear 0.8 vs 1.1 V AC. I wonder if that can be an indication of a bad sensor (system can be confused by the voltage difference) or if it can be considered normal. If it is I will try to find out if the problem is related to the ABS relay.
Thanks to all ......R

.4-.8VAC? Should you be seeing voltage that low? I would have guessed 4.0-5.0VAC or 10-12VAC. I don't have a shop manual with me at the office, maybe others will chime in?

The low voltage is from disconnected sensors while the wheels were manually rotated, measured connecting the voltmeter to both sensor terminals. I guess it is the AC voltage (someone more electrically savvy can correct me) produced when the ABS dented wheel is rotated on the sensors magnetic field. If that is the case and the wheel speed was similar on both rear wheels I would have expect the voltage to be about the same but it was different so I am not sure if that is an indication of a faulty sensor. I guess that’s the test Kris recommended.
I will like to know others experience while testing the sensors that way.
Any other recommendation (or existing thread) on how to identify the faulty parts (sensors, control unit, relay etc.) on a 964 ABS before expending money replacing working parts is welcome.

Pardon my ignorance, I don't have ABS on my Porsche (too early). Would onboard diagnostics (OBD) help in giving you a clue what ABS device is giving the fault?