Hi ibericaman. Welcome to the forum. This sounds typical of the signal being lost from one of the wheel speed sensors. GM has had a huge problem with wheel bearings with the ABS sensors built in. I don't know if that extends back to '99 models, but on the newer cars, a little play develops in the bearing that would not cause any other problem or even be noticeable on any other car. But GM is famous for packing a lot of stuff into what becomes expensive assemblies. In this case, to get the new sensor, you have to buy the entire wheel bearing assembly.
Before you do that though, you'll need to verify that's the problem and which wheel is causing it. Your mechanic will connect a scanner that can access the ABS computer and he will watch the wheel speed sensors in real time during a test drive. The goal is to find one that's losing its signal. There might also be a diagnostic fault code stored in the computer's memory but one thing to be aware of; there is a glitch in the software of some aftermarket scanners that causes them to report the wrong wheel. You might get a fault code for the right rear wheel speed sensor when in fact it's the right front one. Sorry I can't remember the scanners, car model, or wheels affected but I know it's a GM car.
Most scanners also have a record / playback feature that allows the mechanic to make a recording of the sensor readings by pressing the "record" button when the problem occurs. Because the data is traveling through the scanner's memory, the recording actually starts a few seconds before the button is pressed. Later, those recorded signals can be viewed in an attempt to determine the cause of the problem.
Older cars used separate, inexpensive, bolt-on wheel speed sensors. If you have that style, check for frayed or broken wires and for metal chips building up on the end of the magnet that interfere with the generation of the signal. Your mechanic may also verify a fault code for a defective sensor by measuring its resistance. The most accurate measurements are taken right at the connector at the computer, because that also includes the car's wiring harness and any related electrical connectors in the measurement. With this type of sensor, also check the toothed tone rings around the outer cv joints to see if they are cracked. Some systems are somewhat forgiving of the extra pulse per wheel revolution caused by the crack, but some cars do strange things in response to them.
Wednesday, July 21st, 2010 AT 2:14 PM