1992 300D False ABS reaction at near 0 mph
 

Has anyone ever experienced such a reaction from the ABS brakes?

Whenever I drive the car and hit the brakes for a stop sign, just before stopping (I'd say at 2-3mph), the ABS kicks in, even if I'm on dry tarmac.

I would imagine this is due to a defective ABS sensor, but which one then?

Since I do not notice any pull towards left or right, I would think the rear sensor on the differential is the source of the problem.

Any advice welcome, I'll keep searching around for infos.

Bye,

Two thoughts....

Are you certain its not the trans clunking down a gear?

Does diameter of front vs rear rubber match exactly?

Try This
 

I had the same problem on my 90 300sel. Take each sensor out at the wheel and clean them they will have brake dust and little bits of metal on them from the brake pads and rotors. What happens is the sensor misses a count from the rotor and thinks it is locking up so it triggers the ABS unit

No, it is not tranny related.


Yes, they are identical and almost brand new. I've put on 4 Goodyear Assurance TripleThread. Threadwear index of 740, I'll see how long they last.

Thanks,

Ok. I'll report my findings.

Bye,

I have the same problem on my 124. That was a great question and great advice. I'd much rather clean something than replace it:D But one question, why would it only do it when you are about to stop?

Well, I did remove both front sensors today. I could indeed notice quite a bit of iron "dust" gathered around the tip of the sensor.

I cleared that out on both sides.

Then I looked down the hole with a flahlight. I could see the toothed wheel. It is rusted quite a bit, but I can clearly see that all teeth are still there.

So I used a steel pick (kind of screw driver but with a pointed end), and scraped every single "valley" between the teeth.

And, unfortunately, it didn't do a thing. Same symptoms.

I need to test everything before I start throwing expensive parts in there.

To be continued!

Bye,

My 1986 300sdl is doing the exact same thing. I took off both front sensors and they were very dirty with lots of metal filings on them. I cleaned them and sprayed brake cleaner into the holes where the sensors live. It looks pretty clean to me. This did not help. It still does this on occasion.
I have not pulled the rear sensor and cleaned it yet.
On an interesting side note, my alternator is not putting out the correct voltage. At ilde with a?c on and l;ights on and rear defogger on it barely puts out 11.5 volts. At 2000 rpm it barely gets to 12 volts. I have ordered a new alternator. I wonder if the voltage to the ABS computer is too low at idle speeds and throwing the system off. In hot sunny Louisiana, my a/c is always on and I find this happens mostly after I first drive away in the morning and I come to my first stop. Occasionally my ABS light will flash on at idle. My Overload protection relay is brand new.

I am going to change my alternator this weekend and see if this changes anything. If it still does it I will pull the rear sensor and clean it.

John

Rear sensor on the differential is also on my list. But I will replace the oil at the same time, while in there. I bet it's been in there since 1992!

Bye

when was the last time the brake fluid was flushed?

I flushed my brake fluid about one month ago. I flush fluid every spring.

FWIW, I am involved in railcar maintenance and this is a pretty common problem. One or more of the sensor output voltages is dropping too low to detect at low speed and the system sees that as a zero speed wheel. Most likely it is caused by the magnetic dust on the sensor reducing its output. The system works by detecting frequency of course, but if the voltage is too low, it won't read it. At higher speeds the voltage goes above the threshhold and the system works fine.

Thanks for that diagnosis JohnS, I'll remember that :) Makes perfect sense for stray magnetic fields to reduce the output, after all its no different to a speed sensor on a bicycle computer :)

In my case, the brake fluid age is unknown.

I have just bought new bleed screws for the front calipers. They are quite rusted, but the calipers otherwise works fine.

I will change this brake fluid soon as well.

But could it affect the ABS at all?

Thanks

Not sure I understand. The ABS sensor works by counting pulses, right?

So it reads an induced voltage from magnetic sensor (like an Hall Effect), something like +5VDC, 0VDC,+5VDC, 0VDC, +5VDC, etc. etc.... When the wheel stops, it suddenly sends only 0VDC, thus triggering the pressure release from the ABS unit.

OR, do you mean that the voltage supplied to the sensor gets to low at low engine RPM (like brandlj mentionned here earlier on)?

How does one test these ABS sensors by the way? I'll work on this a little later.

Bye,