I realize that there is a sensor at each wheel that operates the ABS (87 S4) and I suspect one (or more) of them ia/are bad.

Can anybody give a procedure for replacement of same. I've been thinking that the way to find the bad unit is to use a rubber mallet to pound on a wheel, one at a time, while the car is running hoping that this will cause the faulty unit to turn on the yellow light. Does any of this make sense?

 

The car is an 87 S4

 

One way to check the ABS sensors is to check the resistance on the four sensors.
There should be resistance between the two sensor wires (about 1000). And infinate from each wires to ground. If you get resistance between the sensor and ground, the sensor is bad.

 

Replacement of the sensor is easy. The sensor is only held in place by one allen head screw. It's connected to the ABS wiring harness by a plug that disconnects by pulling out.

I had one sensor that went bad (wire on the sensor broke in a spot that couldn't be fixed) and replaced it. Another time my ABS light was on due to a ground wire that broke. After fixing the wire, the system worked correctly.

Fix it soon if you can, the ABS just saved my car in a panic stop situation recently.

 

I thought "continuity" was done with a test light and if it lights that only tells you the wiring is intact and grounded properly. I'm guessing a reading of "1000" must be ohms you are measuring and that would require a multi meter.

I thought "infinite" meant a broken wire or short circuit.

This I don't understand at all. Are you saying if one lead of the test lamp is connected to one of the sensor wires and one lead to ground and the lamp lights, that means the sensor is bad?

 

Steve,
Yes you are right. I use a multi-meter to mesure the resistance in ohms. My mistake.
... In this case infinite means very high resistance. About 20 something thousand ohms if I remember correctly.
... If you get resistance between the sensor wires and ground, then your sensor is bad.

"These sensors normally fail by shorting internally to the case of the sensor while still showing an acceptable resistance between the sensor wires. This will show up as a resistance between one sensor wire and ground = to X where X is between 0 and roughly 1000 ohms. The other wire on the sensor will measure roughly 1000-X ohms - all depnds upon where the short to ground is. "

Thanks to Jon for this good explanation. Way better than mine.
Post #6 here:
https://rennlist.com/forums/928-foru...it-is-bad.html

Hope it is a bit clearer now. Sorry for using wrong terms.
Changed "continuity" for "resistance" in my post.

 

Thanks for finding that Bertrand - I failed this morning.

BJ

The first thing to check is if you get an ABS warning before the car has moved - If you do the most likely culprit is the ABS relay on the CE panel. Fix this first before you bother doing any other checking. You can bridge the relay temporarily for testing purposes but don't leave it bridged as the ABS will be on all the time (keys in ignition or in your pocket) and wil flatten the battery.

Once that test is out of the way jack all 4 wheels off the ground and do the spin the wheels test - ign on (and relay bridged if you have to) spin one wheel at 4 mph or more - if it produces an error the sensor for that wheel is good. Then ignition off and relay unbridged then back on again to reset the ABS brain and repeat on another wheel etc. This is the quickest way to find which wheel has and ABS sensor faults.

OPnce you've narrowed it down to one wheel then start testing as described in Bertrands link to my earlier post.

HTH

 

If the car still fails the "ignition on" test that Jon mentions above after changing the ABS relay in the CE panel, then there are two more relays on the ABS hydraulic unit. Access is by removing the LH inner wheelarch cover. These two relays are heavy duty, but they have Bosch P/N on them which means you can buy them much cheaper than Porsche will try to charge.