Anti-Lock Warning Comes and Goes

Here is an interesting one. From time to time the Anti-Lock brake warning light will come on (along w/center console master warning). I can turn off the master warning, but not the Anit-Lock warning.

If I put on the emergency brake and then release it (while vehicle is stopped) the Anti-Lock warning goes away.

Sometimes it will not appear for weeks, then start showing up regularly.

I don't see a Relay (unless I missed it) for this function. Anyone have a thought on this one?

Thanks in advance.

Ed Boudreau
Dallas
'87 928S4

Hope Sean and Roger are reading this and can chime in... I have never had that problem *knocking on wood* with my S4....

I presume you know the parking brake is a separate system from the disc brakes.

I suppose it could be an electrical connection that they have in common, so perhaps tracing the wires for the sensors to some common connector.

I would check the rear abs sensor, perhaps you have a poor connection that the parking brake helps clear temporarily.

Stan.Shaw@Excell.Net;2375 wrote: I presume you know the parking brake is a separate system from the disc brakes.

I suppose it could be an electrical connection that they have in common, so perhaps tracing the wires for the sensors to some common connector.

I would check the rear abs sensor, perhaps you have a poor connection that the parking brake helps clear temporarily.

Right I knew about the sep. systems. Here is what I found on an earlier thread....
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porsche928forums.com/viewtopic.php?f=5&t=630

sharkythesharkdogg wrote:
All right. After some brainstorming and talking with other people I came up with a method of testing the ABS speed sensors that doesn't involve the ABS tester. (No local dealerships have it, no other Porsche specialists have it. Even if the dealerships had one it would cost $120 bucks to get it diagnosed. Lame.)

I figured since spinning the brake rotor through the magnetic field of the ABS speed sensors produces a small amount of power (like a D/C generator) that I could read it with a volt meter. So I unplugged the sensors, spun the wheels, and read the minuscule amount of voltage produced. 3 of the 4 produced voltage. The 4th produced no readable amount. Not a bit. To me that's a good indication there's a break in continuity and that the sensor is bad.

The sensors are over 200 a piece, though. The price we pay for superior German engineering

Good job on the diagnostic Ed.
You can check those or any (hall sensors) by checking the resistance but its nice to have the know correct Ohms/resistance to compare it to.
And yes, spinning a reluctor wheel or in this case the rotor will induce voltage into the sensor that as you have found out is a very quick way to check just by checking voltage as you spin the wheel.

I would then check the sensor for the correct clearance (compared to the others) For play in that bearing, and debree or metal fillings stuck to the magnet

Good luck, nice job and hope its an easy repair.

Brad

Just me, but I would start by pulling all of the ABS sensors and cleaning them. They are semi-magnetic and will get gummed up after time. All it takes is pulling the wheels are an allen key, then some cleaner.

If that that does not clear it up, then you are looking at an electrical diagnosis for the issue.