One other possibility to consider, if the ABS warning light is either coming on during normal braking, or if you are getting ABS operation even during light braking on dry roads, etc.:

The pick-up of pulses from the ABS sensors on the front wheels can be disrupted or intermittent if the wheel bearings are not adjusted properly (too loose).

I had my car in for front brake pads and new rotors, and when I got the car back the ABS warning light came on at the first application of the brakes. I took the car back, and the mechanic "isolated" the problem to the right side ABS sensor, and replace it. They said that they weren't sure how the ABS sensor got damaged, but they concluded that they must have done it during the rotor replacement.

Only problem was that they problem came back. So, I start poking around myself, and discover that the wheel bearings aren't close to being adjusted properly, following the front rotor replacement. I tightened them up to spec., and the ABS problem went away. The play in the wheel bearing disrupted the spacing between the rotor and the sensor. Replacing the sensor with a new, clean one probably helped the sensitivity of the sensor a little, but the problem was the loose wheel bearing all along.

Bob Schleicher
1991 420SEL, 174,000 miles