ABS chatter

[Brett Dikeman]

On Jan 16, 2008, at 10:50 AM, gordy.schesel wrote:

> One of my 91 quattros is having ABS chatter when braking to a stop  
> on dry
> pavement.  Odometer has 240k miles.  Is the problem corrosion of one  
> or more
> wheel sensors?

Chatter at low speed is caused by low signal to noise ratio; the ABS  
controller has trouble "seeing" the electrical peak from the sensor  
passing a tooth on the hub wheel, and starts to mis-read where the  
tooth actually is, which causes it to think the wheel has changed  
speed, particularly at low speed.

Causes can be any of the following, I believe:

*Improper spacing (fix: reinstall with new ablative caps)
*rust on the hub teeth (clean, protect with some sort of paint, maybe  
POR15)
*bad ABS sensor
*bad wiring
*bad controller

I may be confusing them with flywheel sensors, but I think filings can  
accumulate on the head as well?

Brett