Jim Grant's Tech Tips
‘98 Pontiac Grand Am GT, ABS Light Even After Brake Work
Q: What would cause my ABS light to come on my ‘98 Pontiac Grand Am GT. I have replaced the brake pads on and it still comes on.
- Carmilita C.
A: It is a common misunderstanding that the Anti-Lock Brake System (ABS) some how knows about the condition of the brake pads or shoes. The reality? The ABS knows nothing about it! The ABS system, or should we say computer, makes the assumption that the brake pads and/or shoes and the braking surface (rotors and/or drums) are in good condition. Assume is a dangerous word. A few manufacturers do have wear indicators built into the brake pads that will turn on a brake warning light. However, your vehicle like the majority, does not have a wear sensor. The ABS computer uses in-puts and out-puts to operate. For in-puts there will be a wheel sensor at each wheel to report wheel speed (not true to all vehicles); a stop light switch signal; and there may or may not be a level sensor in the brake reservoir. As for out-puts the ABS computer controls a group of solenoids/valves that are in the brake lines, and in some cases a pump and the ABS warning light. This is a generalization of the ABS system. When the key is turned on the ABS computer wakes up and looks around at all the things that the computer’s wires run to, (note there are no wires to the brake pads or shoes). It wants to know if most of what it controls or receives information from is connected. If the computer is happy, the ABS warning light will turn off in 3 to 5 seconds. Once the vehicle starts moving and your foot is off the brake the computer runs another check for responses from the solenoids that it controls. If there is a problem with this test, the ABS warning light will be turned on. Lastly, while the vehicle is being driven the computer monitors for the quality of the signal it receives including the wheel speed sensors. If there is a signal problem, once again the warning light will be turned on. So as you see, the ABS system is really all about electricity. Brake pads and shoes are a mechanical thing. If your ABS light is on you need to have the computer checked for codes to truly know what is wrong with the system to perform the correct repairs.