While braking and coming to a stop the front brakes vibrate?

Tiny
OWENKAM01
  • MEMBER
  • 2012 CHEVROLET EXPRESS
  • 4.8L
  • V8
  • 2WD
  • AUTOMATIC
  • 20,000 MILES
Just before I come to a stop the front brakes vibrate.
Monday, December 11th, 2023 AT 1:51 AM

1 Reply

Tiny
CARADIODOC
  • MECHANIC
  • 33,729 POSTS
We call that "false activation". The wheel speed sensors develop rather weak signals to begin with. One of the variables that affects the strength of the signal is speed. Anything else that interferes with those signals makes them even weaker. In this case, the signals are dropping too low for the computer to read them at too high a speed. Most ABS systems stop looking at those signals at either 9 or 15 mph.

A real common problem on GM vehicles has to do with rust build-up under the sensors on the front wheels. That pushes the sensor away from the toothed tone ring causing signals to be weaker than normal. One of them might cut out at 20 or 25 mph, for example. The computer thinks that wheel is locked up and the tire is skidding, so it goes into anti-skid mode. That's the buzzing you hear and feel as brake pressure is being modulated to that wheel.

The solution is to remove the front sensors and clean off any rust that has built up under the mounting flanges. Also look for metal filings or other debris stuck to the tip of the sensor. There's a magnet there, so iron filings will stick to it.

This first photo shows the entire wheel bearing assembly with the sensor bolted on. The red arrow is pointing to the mounting tab where the rust develops underneath. Just the sensor is shown in the second photo.
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Monday, December 11th, 2023 AT 12:47 PM

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