There's two common problems on GM vehicles. On yours, the wheel speed sensor is built into the wheel bearing assembly. That makes the part much more expensive but it goes together quickly on the assembly line which is all they care about. This type of sensor is only used by GM and it produces a really wimpy signal to start with. Next, a little play develops in the bearing assembly which is normal, but that play allows the sensor to produce an even smaller signal until it reaches the point it drops out and can't be read by the computer. If that happens while you're braking, that is interpreted as that wheel has locked up and the brake pressure to it needs to be reduced, or "modulated". That's the pulsing you're feeling in the brake pedal. The pulsing is a normal feeling when the system is reducing brake pressure. In this case the condition causing it to activate is not normal.
Next, while you're driving and not applying the brakes, the computer is watching the wheels speeds. If that signal drops out from one sensor, the computer knows there's a mechanical problem, sets a diagnostic fault code related to it, turns the system off, and turns the yellow warning light on to tell you. It will not activate when a tire is skidding so you won't feel the pulsing in the brake pedal. Typically you have to turn the ignition switch off and restart the engine to get the anti-lock system to reset and turn on again.
The fix for this is to replace the wheel bearing assembly. I don't think you can buy just that sensor for the front wheels. Even if you could, the sensor isn't the cause of the problem. It's the slop in the bearing that is allowing the sensor to move away from what triggers it. To add to the insult, it is real common for new wheel bearings to develop the same normal play in as little as 15,000 miles and you'll have the same problem again. The dealer's scrap metal bins are full of these bearings.
Another source of frustration for mechanics is to start the diagnosis, all they have to go on is that fault code. Once a code is set for one wheel, the computer knows it can't rely on that sensor's readings to compare to the other wheels so it suspends some of the other self-tests it runs. If the same thing happens to a second wheel, no fault code will be set. You can easily have TWO wheel bearings that are causing a problem, but there will only be a fault code for one of them. Experienced mechanics will take a test drive with a scanner connected that lets them view live data. They'll watch the wheel speed readings to see if more than one doesn't look right, then they can provide a more accurate estimate for repair, which includes two wheel bearings.
If they just go by the one fault code, that is all they'll tell you is needed. It's not until it is replaced and they go on a test-drive that the computer can resume its self-tests. THAT'S when the second fault code shows up and the mechanic has to start all over with the diagnosis, then tell you more parts are needed. You incorrectly assume the mechanic made a mistake with the first bearing, and the mechanic is frustrated because he didn't diagnose the complete causes of the problem.
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Thursday, October 10th, 2013 AT 12:38 PM
