Hi budman39. Welcome to the forum. The ABS light can turn on by itself but usually it will also turn on when the red light turns on. The red light can be due to loss of pressure in one of the two hydraulic circuits, the parking brake not fully releasing, or simply low fluid level in the reservoir. Any of those things can adversely affect the operation of the anti-lock system. That's why the yellow light turns on too.
There should be diagnostic fault codes stored in the ABS computer once the light has turned on. They will lead you to the circuit or system with the problem, not necessarily the defective part. GM has been having a huge problem with ABS sensors that are built into the wheel bearing assemblies, but that affects front-wheel-drive cars. I haven't heard of that problem with the trucks. If you have separate bolt-on sensors, examine the toothed tone rings on the cv joints, or rotor and drum hubs for cracks between two of the teeth.
There are a few things to consider. The computer performs some system checks right after turning on the ignition switch and other problems are detected while driving. Electrical tests are performed on the wheel speed sensors when the ignition switch is turned on. Related problems will cause the yellow warning light to turn on right away before you even start moving. It can take a mile or more to detect a cracked tone ring. The extra pulse per tire revolution makes it look like that wheel is turning faster than the other three. The computer knows that all four wheels will be turning at different speeds when you cruise around a curve, but it isn't happy when it sees three wheels going the same speed and one is different. That can also be caused by having one tire that is a different circumference than the other three. Those types of problem take a while to be detected.
A signal dropping out from one wheel speed sensor can occur anytime. The most common cause is a broken flexible wire to one of the front sensors. Other causes include metal filings or rust buildup on the end of the sensor, and corrosion on the pins in the electrical connectors.
Wednesday, September 8th, 2010 AT 12:50 AM