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  #1  
Old 06-28-2002, 06:27 PM
Joseph Bauers
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ABS mystery, or learning what the BS in ABS means!

I have a 1989 300SE. Recently,I had the MB dealer install new ball joints and body strut bushings. After picking up the car,I noticed that the brakes were exhibiting a strange problem. The car would brake normally until it came to a stop, at which point I would feel a pulsing in the pedal briefly, as if the ABS had been activated for a second or so.

My MB tech friend said that this symptom was most likely caused by bad wheel speed sensors. He said he had never changed ABS pumps except in the case of cars that had been in accidents. Other cars showing the same symptoms as mine had been cured with a change of sensors.

I ordered the sensors and installed them myself. When I got the car back on the road, I noticed no improvement. Another tech suggested that perhaps the splines on the hub were dirty--he said that grease or other things getting between splines would cause the sensor to misread the wheel speed.
I removed the front rotors and discovered, indeed, that both hub splines had dirt and what appeared to be grease packed between some of the splines. The techs and I sprayed solvent on them,then dried them with compressed air.

When I reinstalled the rotors and tires,the brakes behaved normally for many applications--perhaps 12 or more. Then, the problem intermittently reappeared, though not as pronounced as it had been initially. Now, I would say that I experience a small pulsing about 40% of the time.
My tech friend suggested disconnecting the battery for a minute,then reconnecting it, hoping that the ABS would reset its codes. This did no good.

He also said that there is a third sensor on the rear differential. He said I might consider pulling it and cleaning it, but he cautioned me that these often were difficult to remove and there was a danger I might break it off in the rear differential. As of yet, I have not looked into the rear sensor.

I am to the point where I am seriously considering unplugging the ABS pump altogether.

Anyone with experience or thoughts on this problem?

Joe Bauers
bauersjo@soltec.net
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  #2  
Old 06-28-2002, 06:47 PM
WRM WRM is offline
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Join Date: Jun 2002
Location: BURBANK CA
Posts: 151
MAKE SURE THAT THE NEW SENSORS ARE REINFORCED WHERE THEY ATTACH TO THE WIRE HOLDING BRACKETS. MERCEDES UPDATED THEM ALONG TIME AGO. THE REAR SENSOR USUALLY DOES NOT GO BAD BUT IT IS POSSIBLE. IT IS VERY EASY TO REPLACE. YOU CAN TEST THEM WITH AN OHM METER. WIGGLE THE WIRES WHILE TESTING TO SEE IF THERE IS AN OPEN.
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  #3  
Old 06-28-2002, 06:59 PM
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Join Date: May 2002
Location: Mass
Posts: 1,127
As you know, there should not be grease on those ABS 'splines'.
It may have come through the hub seal if the seal was damaged, or if the hub was overpacked with grease.
Remove the sensor, shine a light in there, and see if new grease has deposited on the splines.
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  #4  
Old 06-28-2002, 09:43 PM
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Join Date: Jul 2000
Location: British Columbia, Canada
Posts: 827
be aware that by disabling your ABS system, should you get into an accident, and should it be discovered that your ABS is not functioning, you could land yourself in a great deal of trouble.

At the very least, you're going to have to go through a great deal of time and money proving how the non functioning ABS was not the cause.

I'd be a little more of a hard-ass with your dealer and say that the trial and error approach is not acceptable if you have to keep replacing parts that are not the cause.

They should be diagnosing the problem...not guessing and making you buy parts ad nauseum without solving the problem. if they want to go the route otf trial and error, fine, but on THEIR time. THEY try it out and see if the problem is resolved. if it's not, put the original parts back in and proceed to the next possible cause.

Find another car, like yours, and see if you can swap out the ABS computer. See what the result is. if the car behaves normally, it's the computer. if it exhibits the same symptopms, it's something in the braking system. you have new front sensors and cleaned splines. all that's left are 1) rear sensor or 2) wiring.
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