What you described does sound like the anti-lock system is activating. GM has tried a number of different sensor designs and all of them commonly cause weak signals and false activation. On one design it's caused by the normal play that develops in wheel bearings in as little as 15,000 miles. On trucks rust builds up under the sensor and pushes it away from the tone ring.
When the yellow warning light turns on, that means a problem has been detected, and usually that's an electrical problem like a broken wire to a sensor. When the system is activating falsely, no problem has been detected, and the activation is in response to a loss of signal making the computer think that wheel has stopped rotating. The first thing your mechanic is going to do is connect a scanner that can access the ABS Computer to view the wheel speeds. On most GM systems they saved money by controlling the two rear wheels together. The scanner will show the two front wheel speeds and a single rear wheel speed. You have to watch those speeds on a test drive and see which one drops to 0 mph. The sensor usually is not the cause, but replacing it often takes care of the cause, like that rust buildup. Your mechanic will determine the best repair once the cause is identified.
Friday, January 24th, 2014 AT 7:01 AM