If you have anti-lock brakes, you're probably hearing the system activate falsely when it's not needed. GM has a lot of trouble with this on their front-wheel-drive cars and on their trucks, but for a different reason. Rust builds up under the front wheel speed sensors and pushes them away from the tone rings. Their sensors develop pretty wimpy signals to start with, and when that rust causes an increase in the distance between the sensor and the toothed wheel, the signal gets even weaker. Lower speeds decreases the signal voltage even more to the point the computer can't read it, so it assumes that wheel is locked up.
The sensors need to be removed to clean off that rust. It's important to understand first that electrically everything is okay, as in no broken wires. That means no diagnostic fault code will be set. Once the signal dropout gets bad enough, there won't be any signal at higher speeds. THAT'S when the problem will be detected, a code will be set, the computer will turn the system off, and it will turn on the yellow ABS warning light to tell you.
The other important note is this is happening on both front wheels. The computer constantly watches all the wheel speeds and compares them to each other. When a diagnostic fault code finally sets for one of them, one of the signals used for comparison is missing or can't be trusted, so some of the self-tests are aborted. That means if you wait a while to have the first problem fixed, a second one can develop that doesn't get detected. The mechanic has to base his estimate for repair on what the computer tells him, which is a problem with one front wheel. It isn't until that repair is done that the self-tests resume, and THAT is when the second problem is detected. The warning light comes right back on during the test drive, and the mechanic has to tell you more diagnosis and repairs are needed. We hate having to do that, and it's frustrating for car owners. They assume incorrectly that the mechanic didn't do a proper job of diagnosis, they didn't diagnose it correctly, or they're trying to sell unneeded parts. This can all be avoided by having ABS problems repaired right away, or in this case, servicing the sensors on both front wheels, not just the one that is developing a weak signal.
Monday, March 2nd, 2015 AT 12:28 PM