Re: brake trouble lights on E39

[Elcrinv@xxxxxxx]

Last week I solicited suggestions on what to do to my 99 528iT when the ABS 
and AST trouble lights turn on. I also discovered the speedometer, odometer 
and MPG gauge were inoperative. Most thought the sensors were dirty as this 
could happen on older models.   
Obviously, I was not getting a speed signal from one or more sensors. Pulling 
a wheel and removing a rotor I found that the sensors on an E39 are located 
in the hub and are essentially sealed from dirt and brake dust. Dealer did a 
diagnostic readout which indicated the left rear pulse generator was faulty. 
Technician verified this by switching the left and right rear sensors (or 
pulse generators, if you will). Removal is done by removing the wheel, 
unscrewing an Allen bolt which holds in the sensor located on top of the hub. 
Easily accessible. To unplug it the plastic wheel well housing shield has to 
be removed. Got the part this weekend ($103 from dealer, $88.95 after club 
discount) and switched it with the old one. The old one had broken at the ear 
through which the Allen bolt goes. The hole where the sensor goes had some 
dirt in it. I suspect that when it was originally installed the worker just 
stuck the sensor in halfway and tightened it with an air wrench. The sensor 
hung up and the plastic ear broke. Failure occurred at 57,000 miles.
I'm told the sensor uses a Hall effect or field. It is different from the 
electromagnetic sensors which measured teeth in the brake rotor. All four 
wheel sensors are needed to give the OBC a proper signal for operation of the 
ABS, AST, etc.
Fred Callender RMC

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