Cause of odd ABS problem tied to cruise control?

[Ron Brooks]

My car is a 88 300SE. About a week ago completely out of the blue I had the following problem: occasionally when I went to stop and got down to 5mph or a little under, the ABS would activate. It occurs approx 1 out of every 50 stops or so. Breaking is completely normal until then. For some odd reason it also seems to occur much more often if the nose of the car is pointed down. I realized this because there is one stop sign I approach at 30 mph that is at the bottom of a hill. The problem does not seem to be affected by the car being cold, warm or hot or whether I am breaking from 70 or 30mph.

I have noticed a wird (to me anyway) correlation. If I start the car, my cruise control works perfectly. But, once driving, if this ABS thing occurs, the cruise system will either not accelerate by using the crsuise stalk or if it does it will never engage in cruise. By this I mean that once you stop accelerating and try to set the speed, the accelerator pedal is released. If you get the speed you want by using the pedal, if you push the stalk to set speed it will also never engage if the ABS thing has happened.

If the ABS thing doesn't happen, the cruise works fine. I can also say that regardless of whether the weirs ABS thing occurs or not, the ABS system otherwise functions fine. We've had some snow and even after the weird thing happens, the ABS system still works fine at all speeds if conditions call for it. The only problem is the 5mph or less activation when not needed.


Any idea's? Could all this be caused by a faulty OVP? The only clue I can give, if it is even related, is that when I check the duty cycle at pin #3 I get a fixed 30. I know this is a fault code for something but I do't know for what.

Thanks for any suggestions.

[Mick J]

When the OVP goes bad it will bring up the ABS warning light and shut the system down.

I would remove the speed sensors and clean them for a start off.

Can't help with that diagnostic code I'm afraid.

[Ron Brooks]

I have to ask...where are the speed sensors located (I assume I might see them if I remove the front tires?) and what is used to clean them? Just spray liberally with brake cleaner and wipe off?

[Mick J]

Yes, you will see them with the wheels off. They are held in place with a single allen bolt. The only time I cleaned any they were covered in metallic swarf (goodness knows where from!) which stuck to the magnet so it was a case of gently wiping them and pulling it off with the fingers.

You may find that on the rear there is one sensor at the front of the diff instead of at the wheels.