False ABS activation at low speeds is a common problem on GM vehicles. On the trucks it's caused by rust buildup between the wheel bearing housing and the wheel speed sensor. The signals generated aren't too substantial to start with, and that rust pushes the sensor away which reduces the signal even more until the computer doesn't see it. To the computer the loss of signal means the wheel has stopped turning. That's not possible while cruising so it turns on the warning light to let you know something's wrong. When the brakes are applied, that loss of signal means the wheel is locking up and the anti-lock function kicks in to try to prevent that.
The sensors can be removed to clean the rust off. You might have to remove the bearing assemblies to get to the sensors. GM likes to sell complete assemblies, as in the wheel bearing, tone ring, and speed sensor all in one package. They aren't concerned with ease of service later.
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Sunday, December 9th, 2012 AT 2:52 AM