Those warning lights are telling you the anti-lock brake function has a problem and the computer turned the system off. That leaves you with the base brake system that every car has had for decades. The scraping you're hearing is the system engaging when it shouldn't be, and that is typically due to a wheel speed sensor problem. Once the computer detects the problem and the warning light turns on, you won't hear the system engage any more until you turn the ignition switch off and restart the engine.
There's two different styles of front wheel speed sensor depending on whether you have standard or heavy duty brakes. The speed sensors used with standard brakes have been causing a real lot of trouble on GM front-wheel-drive cars. They develop rather wimpy signals to start with, then as a little normal play develops in the wheel bearing, it allows the tone wheel to move away from the sensor, and that results in a still weaker signal. It gets too weak for the computer to read, so it sets a diagnostic fault code and turns on the yellow "ABS" light. With this system, the entire wheel bearing assembly has to be replaced to solve the problem. I suspect this is not what is on your car because the new wheel bearings commonly cause a repeat failure in as little as 15,000 miles.
The heavy duty brake system uses a different style of front wheel speed sensor that causes very little trouble. The most likely thing to go wrong is rust builds up under the sensor and pushes it away from the tone wheel. The fix is to remove them, clean off the rust, then reinstall them.
There's two things to be aware of with this system. The first is there were some scanners that have a software glitch that causes them to read the wrong fault codes. When there's a diagnostic fault code for "left front wheel speed sensor", the culprit could be the right front sensor. I can't remember the years and models this pertained to, but I think it was solved by '99.
The second thing applies to your car since you've been driving it so long like this. The ABS Computer is constantly performing numerous tests on the system, both before you start moving and while you're driving. One wheel can have a fault code set for an electrical problem with its sensor, as in a broken wire. That will be detected right away and turn on the warning light before you ever start moving. It can set a different fault code for loss of signal from that sensor due to the two things I mentioned, rust under the sensor or play in the wheel bearing. This is the condition you have, and this is where the problem can arise. The computer constantly watches all four wheel speeds and compares them to each other. When one sets a fault code and turns on the warning light, the computer knows it can't rely on that sensor's signal to compare to the other wheels, so it suspends those tests. When rust buildup affects one sensor, you know it is likely to have affected the other one the same way, but since the tests have been suspended, that second wheel's problem will not set a fault code.
All the mechanic has to go on when writing up a repair estimate is the one diagnostic fault code in the computer. He will replace that wheel bearing, clean the rust, or do whatever repair is needed to address that fault code. THAT'S when he goes out on a test drive and the rest of the tests resume, and THAT is when the problem with the second sensor is detected for the first time. The warning light turns right back on again, but this time the fault code indicates the other wheel has the problem. The mechanic has to tell you more diagnosis, more parts, and more money are needed. We really hate having to do that. It's frustrating for mechanics and car owners, and is a common reason people lose faith in the mechanic. The real fault lies with the owner who waits many months to get the problem repaired. Another way to look at it is if you get a diagnosis, then wait a long time to get the problem fixed, be prepared for the mechanic to find more wrong than you expected. In the case of the wheel speed sensors that have the rust buildup issue, even though only one has set a fault code, ask to have both front sensors repaired for this right away. Even if the second one hasn't developed this problem yet, it has gone through just as much rain, snow, and salt. It's just as old, so you know it's going to have just as much rust as the first one. If the mechanic just repairs one sensor, then the second one develops the same problem a week or two later, the typical car owner is going to be angry and incorrectly assume the car wasn't repaired properly.
The place to start is by having the diagnostic fault codes read and recorded. They will indicate the circuit that needs further diagnosis.
Monday, February 16th, 2015 AT 3:14 PM