Usually there is a big buildup of garbage in the sensor wheels (I think the fancy name is reluctor if you care) that kind of obscures the sensor from reading it. I scrape them out usually with a small drift pin and then blow compressed air at them.

But Chris,they say, you'll DAMAGE the (*ahem*) reluctor wheels!

In theory yes, you would be damaging them by putting insignificant surface scratches in them. In practice, however, the bare (*ahem*) reluctor wheels coated with brake garbage are much LESS readable, as the sensors rely on the impulses from a non-crusty sensor wheel to function right. (Also you could use an appropriately sized dowel or something with an edge that will scrape the debris from the wheels). The more junk is on the wheel, the less defined the impulses will be. If you can't remove the baked on stuff, it still won't function right. ANd some of the junk doesn't blow off of the wheels with compressed air.

But Chris, they say, you should never use compressed air on brake parts because you'll inhale the dust!

The appropriate precautions are assumed.

I've had tremendous success returning the sensor wheels to their original shape, more or less. The typical easy low speed activation of ABS goes away entirely. In fact, on a clean system, you should be able to feel a wheel lose speed before the ABS activates. There is quite a bit of nonconformance built in between each ABS channel. Therefore if you are coming to a stop in snow or ice, you might feel one wheel lose traction, but it may never be significant enough to trip the ABS into activation if the difference is not enough to do so. On a dirty system, it becomes very sensitive and will often activate the ABS at lower speeds, when you barely encounter any slippage whatsoever. That can even happen when turning a corner and braking, or running over a utility hole cover in the road, or some such tiny imperfection.

Now, the fact that the ABS lamp comes on when first started tells me that the problem lies perhaps in another part of the system. The sensors don't read until the car gets moving. That's why you hear a crunch from the ABS when you first start driving (and when you leave a stop on later systems). The system tests integrity of the sensors. That's when you'll see a sensor light very often. If the lamp is on right at the start, you could have a voltage issue rather than a sensor issue. Check the battery cable connections and clean all of them off. And the chassis grounds as well (from the battery). Then see if it still reoccurs.

If a sensor returns no signal or resistance, however, the ABS light might go on right at start. In other words, if a sensor or it's wiring is bad or disconnected. But that is sometihng I'm not totally sure about as it can vary on different (non-Volvo, or other Volvo) cars.
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Chris Herbst, in Wisconsin.