Audi A6 False Anti-lock brake activation

I had a problem with my 2000 Audi A6 2.7T Quattro. When I came to a slow stop, like to a stoplight or stop sign, as the car's speed dropped below 5 miles per hour, the anti-lock brakes came on at the front wheels. The car would lurch slightly from the front end as the brakes were modulated at the wheel. There was also the typical ABS grinding noise as the pump motor worked. This was a frustrating problem.

My online research revealed a condition called "False ABS activation at low speeds." Though this article pertains to Chevy, Ford, and GM 4x4's, this sounded like my problem.

http://www.mightyautoparts.com/pdf/articles/gb0804.pdf

The gist of the article is that corrosion around the wheel speed sensors can interfere with an accurate reading of the wheel's speed. This, is turn, can trick the ABS computer into thinking one of the wheels is slipping; it thinks the front wheels are turning at different rates. The ABS module is probably not faulty; its doing its job properly.

The wheel speed sensors are basically electro-magnets which are mounted close to a notched metal ring near the wheel bearings. The notches in the ring cause the electrical current passing through the sensor to vary proportional to the speed of the wheel (in accordance with the Hall effect.)

Lacking an oscilloscope, I took the experimental approach. I pulled off a front wheel to see how hard the sensor is to reach. Luckily, the wheel sensor is located on the forward side of the rear of the wheel spindle. It has a 90 degree elbow electrical connector attached to it. I looked all around for a nut or something holding it in but found none. Trying to wiggle it, I discovered the sensor is "pressed" in or friction-fit. Wiggling it some more, I found out I could remove it by pulling and wiggling gently. It popped out and I couldn't wait to visually inspect it for corrosion.

Sure enough, the sides of the sensor body were coated with a light layer of orange. The tip of the sensor was hairy with what appeared to be corrosion as well as some metal flakes. I'm not sure what the flakes were from but I've read that residue from a brake job (turning and grinding the rotors specifically) can sometimes fall into the bearing area. I'm not sure about that but there were a few flakes on the sensor tip.

I cleaned the sensor barrel and copper sleeve with some brasso, blew out the hole with some compressed air, and re-inserted the sensor. Much to my delight, a quick test drive showed that the condition was gone. Now I can take passengers around without scaring them that my brakes are on the fritz.

Now that I know what I'm doing, I could do that repair without pulling a wheel in a matter of minutes.