Anti-Lock Warning Comes and Goes

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11 Dec 2010 15:52 #2318 by timberhollow9310
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

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11 Dec 2010 16:24 #2319 by pcar928fan
Hope Sean and Roger are reading this and can chime in... I have never had that problem *knocking on wood* with my S4....

James
78 Silver / Black-white #295
84 Ruby Red / Black AO84
88 Dark Blue / Linen-Black
92 Polar Silver / Dark Blue 92EURO
93 Arrow Blue / Black

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11 Dec 2010 16:45 #2320 by srshaw3
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.

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11 Dec 2010 16:51 #2321 by timberhollow9310

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....
___________________________________________________________
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

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12 Dec 2010 14:19 #2328 by 928mac
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

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16 Dec 2010 22:07 #2355 by SeanR
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.

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17 Dec 2010 00:22 #2358 by 928mac
Hey Ed did you made any head way?

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25 Dec 2010 08:39 #2451 by the flying scotsman
The ABS sensors are on each wheel. As Sean describes pull the wheels, locate the sensors and clean with brake cleaner.......they get coated in brake dust.

If that doesn't fix the issue then there's 3 relays to check (there's a fuse also but in this case the issue is intermittant, worth a quick check but unlikely). The easy relay is the one on the main fuse/relay panel in the passenger side footwell.

The next 2 relays are on the ABS system itself visible under the hood drivers side near the power steering reservoir. The easiest way to get at he se relays is to remove the drivers side front wheel and inner fender cover. Then you'll see a cover over the ABS system that when removed you can view the complete control system.

All 3 relays are expensive so use one of the few diag. tools to check the system...........I use Theo's PDT999.

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