Wednesday, June 22, 2011

The ABS and Traction Control Lights stays ON dash of car?

Are these lights staying on? If yes, your car has a fault in the system that senses wheel speed and changes the way the car automatically responds to slippery road conditions. Often, successful troubleshooting of these faults will require the use of a special scanner; however, there are a couple of failure modes that a backyard mechanic can discover and correct.

First, let's understand how these systems work. The car/truck must have a sensor at each wheel to know for sure if any wheels are slipping. The usual method is to use a tone ring and sensor as shown below.

The data stream from each of the 4 sensors is evaluated and delivered to the ABS system, which, if the brakes are applied, will essentially pump the brakes for you to prevent skidding. If the brakes are not applied but you have traction control, the traction control system will limit the differental behavoir of the drive wheels to help you get traction. If you don't have traction control, the ABS system may alert you to wheel slippage with a "low trac" light. The logic and electronics associated with the reactive systems is different on different models, and that is why troubleshooting those parts of the system requires special scanners and training.

OK, so what can you do about getting those lights off without paying a specialist big bucks?
You can inspect the wheels for damaged tone rings, broken sensor wires or connectors, and misaligned sensors. The tone ring is usually visible at the end of the CV boot on a drive axle or behind the rotor on a non-drive wheel. For wheels with drum brakes, the tone ring may be inside the drum.
Spin the wheel looking closely at the tone ring for any missing or chipped teeth or a break in the ring. Check the sensor alignment to the tone ring and also the distance. If you can find the specs on the air gap, measure and adjust as necessary. If you can find specs for the sensor, check it with an ohmmeter for shorts and opens. Check the connectors and trace the wires all the way to the ABS module to inspect for damage. Due to the variety of systems and conditions, I'm not going to try to cover repair procedures, but feel free to post questions on these.

If you found anything wrong and corrected it, hopefully your light(s) will go out.

If your ABS AND your brake light (on the instrument panel) are both on, it is likely your basic brake operation has a fault.----------
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