I've got a 92 745 NA with unknown mileage (I think around 150,000) and ABS brakes. Sometimes when backing down the driveway with my foot applying medium pressure to the brakes, I'll get a single muffled "clunk" sound that I can feel both in the pedal and the seat of my pants, followed by what sounds like a VERY FAINT buzzing for a split second. I can sort of imagine it coming from the right front area of the car. At first I imagined a loose caliper or the like, but everything seems snug there. Also that doesn't explain the faint buzzing. Everything acts OK when going forward.

I've cleaned both front ABS sensors - lots of fuzzy iron particles built up on the magnets - no fix.

Is there some sort of ABS reset or calibration or ?? that is supposed to occur when backing up?

Anyone have more ABS experience that can offer an opinion?

Thanks once again.

Seem to remember reading somewhere that the system performs a brief test on occasion. Could it be that a wheel is skidding while you are backing and of course the ABS is doing just what it should.
--
David Hunter

Dry concrete - all wheels turning the same speed.

It didn't do it this morning. I'm going to pay attention to how often it happens since I cleaned the sensors.

Thanks.

It's your ABS system performing a self test. Every time the car is started, upon reaching aprox 7 mph, the abs system runs the pump for a second. If there is a problem, it turns on the warning lamp. This is a normal function. In later years the test was changed to occur at 12 mph to make it less noticeable.

So if I were to start up the car and drive FORWARD vs backward, I would experience the same thing - one time?

Given this info, I'm going to pay more attention to the conditions under which I experience it (once per engine start, how fast am I rolling, etc.).

Thanks for the insight.

I am not sure that you would feel and hear it so dramatically going forward without your foot on the brake.
--
David Hunter