Anti-lock brake chatter
 

Hello all,
My sons 1986 300D brakes started acting up about a month ago. As the brake is depressed, and just before the car comes to a full stop, the front brakes "chatter" and brake ability is reduced, you can feel the "pulsating" through the pedal. Pretty scarry ...
I've removed the two magnetic sensors ( which were totaly full of brake debri) and cleaned them and reinstalled them.
The problem went away, and the drake system acted normal - for about a month...
The problem is back, the sensors are clean, and I've sprayed inside the hole with brake cleaner, but it didn't fix anything, it's still doing it.
Any ideas ???

Thanks: George

Sounds like the sensors have gone bad!! Follow the wire from the sensor up to the engine compartment. Unplug both senors and perform a ohms test on both. They should be the same. Next check for AC voltage output when the wheel is spinning( I used to do this while driving along at a steady speed so that both wheels where turning at the same velocity) to see if AC voltage is generated by both wheels. If one is not showing voltage then you will know which one is bad!!

Signal drop out . You will need to scan the sensors to see if they are dropping out at low speed...a common problem.

If cleaning them made it work again, then it is Tone Ring bridging or sensor sensitivity going away.

Tone Ring bridging is where dirt bridges across the teeth of the tone ring and the sensor sees a rapid slow down of that wheel because two teeth appear like one...the abs then tries to dump brake pressure to that brake.

A loose wheel bearing can make this happen, but usually you can feel the wheel has a problem.


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how to clean the tone ring? do i need to take off the brake rotor first? thanks

http://www.picotech.com/auto/articles/diagnosing-abs.html
This may be of interest, if you happened to miss it last week. I think Arthur Dalton posted it here, originally.