300E Antilock Light
 

I just replaced the four brake hoses and flushed the brake fluid in my 1986 300E. It was quite a job to get all of the air out, but the Motive preesure bleeder made it quite a bit easier.

Prior to this, the "Antilock" light would occasionally come on after driving a bit, primarily in the morning. While I had the wheels off, I removed both front ABS wheel sensors, cleaned both and cleaned the internal gear teeth with brake cleaner. Both had a fair amount of metal filings attached. Both had about 250 ac mv output. I did not remove the sensor on the differential. Later, while beginning the test drive, the light came on again. I shut the car down, restarted and drove for quite awhile and the light did not reappear.

Any thoughts on what may cause this intermittent problem?

Brent Smith
1986 300E 273,000 miles

The OVP relay can play a part in this behavior. If it hasn't been changed out in a long while it might be time for a new one.

Also, one of the relays in the ABS control box can cause problems as well if it's old. From memory I think it's the black one - at least it was in my old 300E I think.

ABS Problem Solving
 

The overvoltage relay has been replaced in the past year. If this was a problem, I would think the light (and others) would be on from starting the car. In this case, the light sometimes comes on within a minute or two of driving. IT also usually happens only when the car is first running in the morning. Later, it is usually fine.

It appears that there are two black relays: return pump motor relay and the solenoid valve relay.

I have a complete stack of manuals for my 300E. None of the factory manuals say much about ABS. The electrical troubleshooting manual has circuit diagrams. But the Chassis manual has nothing about ABS. I do notice that the little Technical Data book has a section on testing the ABS with a Bosch Tester--which of course I don't have. Does anyone know of any tech support writeup on troubleshooting the ABS?

Thanks.

Brent Smith
Atlanta, GA
1986 300E 273k miles - bought in 1990

I would check the ABS sensor in the rear, that is more likely the culprit. My ABS light occasionally comes on in the winter if I've been spinning the rear tires in snow (to get unstuck, etc).

Wheel Sensors seem okay
 

I just pulled the rear bench seat out and checked the rear sensor at the connector. The resistance looks okay at about 750 ohms. I then connected the digital ac voltmeter and went for a short drive. The ac volts went up smoothly to about 1.5 volts at 25 mph. I reconnected the sensor and put everything back together.

After driving the car daily for a while, the pattern remains the same. When the car is first started in the morning, the light comes on (and latches on) within a minute or so of driving. If I stop and restart within five minutes, it repeats the same problem. However, once the car is warmed up, the light stays out when restarted--even if the car sits for a few hours and is driven later in the day. The problem is driving me batty.

I have done a few things over the past year in the neighborhood of the sensors, like replace the struts and drain and refill the differential with synthetic. But the ABS worked fine for several months before this intermittent light started.

Any more thoughts out there?

Thanks.
Brent Smith
1986 300E 273,000 miles

Maybe it's the ABS ECU, near the engine ECU? It's a big black box. I have a spare...

Could be.......
 

It could be a bad ECU (cold solder joint or break in the circuit board) or maybe a bad connection between the ECU and the multi-pin connector. I guess the underhood heat could affect the problem. Before I replace the ECU, I'll try pulling it, inspecting and cleaning the connecting points with contact cleaner.

Thanks, JohnM.

Brent

Try checking the earths as well. The FSM gives all the locations. Anywhere an earth is subject to heat and vibration such as in the engine bay could be a culprit.

Ground Check
 

Good thought. I'll go over the ground connections when I check the ABS ECU this weekend.

Thanks Benzme.

Brent

You really need the diode to draw the error codes. Buried somewhere DEEP in the bowels of this forum is a schematic for making this with simple items from Radio Shack. You use this to count the flashes on the correct terminal of the diagnostic connector near the battery.

The ABS is not diagnostic capable.

This thread where I worked on the same system on my Audi might be helpful: http://www.peachparts.com/shopforum/tech-help/288450-trying-troubleshooot-bosch-abs2.html

As others noted the ABS computer lives in the engine bay by the battery, but you could put the car up on jackstands and spin the wheels. I think you could use a simple multimeter and look at the AC voltage coming off of the sensors to get an idea which one might be giving trouble.

-J

Checked Wheel Sensors
 

I've already pulled the two front wheel sensors, cleaned and inspected and replaced them. The resistances were within tolerance and they generated a nice ac mv when the wheels were spun. The rear sensor was tested as I mentioned above. In my case, the problem is likely underhood heat related. It could still be a sensor, but they look okay so far. I don't have a scope (and haven't used one in forty years), but if I did have one, I would certainly like to see the waveforms.

If I knew someone in the Atlanta area that had an older Bosch ABS tester, it would be worth a try. My 1986 M-B tech data book lists the outcomes using the Bosch tester. It apparently plugs in series with the ABS ECU and the connector.

Thanks all.

Brent

Fixed the problem!
 

I remembered when I had removed and inspected the two front wheel sensors that the left sensor had a dent in it. Even though it seemed to generate an okay millivolt signal, I decided to replace it if I could find a used one. I bought a used sensor on eBay for about $40 and installed it today. It now works fine and the "Antilock" light stays out! :)