I had the same thing happen. Victor rebuilt my unit, by mechanic installed it, light came back on a few weeks later. My mechanic claims faulty wheel sensors are pretty rare and Bay 13 agrees with him (http://www.volvospeed.com/Repair/abstrax.htm).

In my car the fix was simply getting the unit snugged in tight. It's possible yours is a bit loose and when you hit the gas hard, it moves a little, firing off that ABS light. A zip-tie ought to help you hold it in there. Good luck.

--Dan L.
'96 855T, 94k miles
'00 Camry Wagon 115k miles