The most common cause is a cracked or rusted reluctor ring.
This is a toothed wheel mounted on the drive shafts of the front wheels, on the hubs of the rear wheels. Sensors count the teeth as they pass, the ABS computer uses these signals to calculate the speed of each wheel.
The front ones seem to fail most often.
Cleaning them with a small wire brush and checking the ends of the sensors are clean can, sometimes, be enough to fix it.
There are, however, other potential problems.
If it was mine I'd start with the free checks, eg-
give the rings and sensors a clean.
check the wiring for any obvious damage.
disconnect the multiplug at the ABS pump, clean the pins with electrical contact cleaner spray (or WD40 if that's all you have).
If there's no improvement you might have to get it on a diagnostics machine to find exactly where the fault lies. Most independent garages have these, and will usually be cheaper than Vauxhall dealers.
sammy, January 2011
