Lord_Galva

So I've been struggling with this issue for a while and feel like I have not made any progress. The ABS is not working properly and its causing me to have a longer brake distance. So first I want to say the light is not on and the ABS relay was replaced with a new unit. When I brake at speeds under say 40mph, the brake pedal pulses violently and stopping takes longer than with the relay removed. Any thoughts?

chudson

LG - you are describing an ABS unit that is being convinced to engage because it thinks stopping conditions support such. When the ABS activates as designed this does not cause illumination of any warning lamps. Best GUESS is an issue with a wheel speed sensor. The signal may be weak and at low speeds the ABS computer interprets this as a wheel that has stopped rotating. When you activate the brake switch by depressing the brake pedal ABS engagement can occur.

This is a common issue on the VW and Audi where the depth of the wheel speed sensor is adjustable. During a repair the sensor will not get positioned correctly and the resulting symptom is exactly as you are experiencing. The sensor depth in 968s is not adjustable. Someone with the early style Bosch tester needs to check your vehicle. The KTS 300 (aka Bosch Hammer) does not perform this function on our vehicles. The required tool is the Bosch ABS 2 tester # KDAS 0003.

Jfrahm

You can also probe the inputs to the ABS brain to read the voltage from the wheel sensors while you are driving. You should see a small AC voltage from each wheel and if one is significantly different than the others you can clean or replace that wheel sensor. One wheel might appear to 'stop' when it is still rolling and that is causing the ABS unit to try to prevent lockup.

A good digital multimeter can do this. You can find the wire colors and pin numbers in the wiring diagram. This thread has a schematic but you might have the 968 electrical manual also, which would be better and easier to read:

https://rennlist.com/forums/924-931-...out-there.html

-Joel.

Paul Waterloo

As chudson suggests, sounds like a similar issue that I had seen many times on Audi's. First step I would do if you are handy is to pull the sensors at the wheels, clean them, and then re-seat them. See if that fixes it. That almost always worked on Audi's.

Is this recent? Has the car been worked on prior to it starting?

Lord_Galva

It has been so long that I cannot remember when it started. Pulling every sensor and cleaning sounds like something I can try that I have not yet done.

thomasmryan

+1 on cleaning the sensors and tone rings.

Paul Waterloo

Forgot I wrote procedure.

http://www.audipages.com/Tech_Articl...eshooting.html

968gene

Something simple like...do the tire diameters match?

chudson

Gene makes a good point and indeed something to confirm, but the diameter difference would have to be noticeable (i.e., an incorrect size installed) for this to be the cause and this would fool the ABS at any speed and should not cause the symptom to appear only under 40 MPH.

RajDatta

+2
Check the sensors. They get pretty cruddy.

MB968

Had a speed sensor issue on my Mazda years ago. I used an old style volt meter, one with an actual dial and needle. I disconnected the sensor, connected the leads to the meter, and spun the wheel by hand. The bad one had a voltage way below the others, so it was easy to identify.

Lord_Galva

Update:
So I cleaned all the sensors and I was getting the right front wheel to lock during hard braking causing a pull in that direction.
The ABS light came on and back home I went.
After restarting the car the ABS light was off and it was business as usual with the knocking sound at about half strength.
I then swapped left and right sensors and the knock followed so looks like I need to pony up the dough this time.