Rick's Free Auto Repair Advice

ABS and TRAC OFF lights on GM vehicles

Why are my ABS and TRAC OFF lights on GM vehicles

Shops are reporting huge failure rates on ABS sensors in some late model GM vehicles where the ABS sensor is integrated with the wheel bearing/hub. These sensors are in two styles. One style bolts the sensor directly on the wheel bearing. The sensor counts the notches on the ABS tone ring inside the wheel bearing. The clearance between the sensor and the tone ring is critical. The problem occurs when rust builds up between the sensor and the wheel bearing. This phenomenon is called “rust jacking,” where the rust literally pushes the sensor upwards, increasing the gap between the sensor and the tone ring to the point where it reads intermittently. If you have this style sensor, remove it and lightly sand the rust (lightly being the key word). Then apply a light coat of marine grease on the bare metal and re-assemble. You may want to try applying a thin bead of RTV silicone around the outer edge to prevent water from getting under the sensor.

The second and more common style has the ABS sensor built into the bearing/hub. This unit CANNOT be serviced. You must replace the entire bearing/hub.


ABS sensor, wheel bearing. hubABS sensor, wheel bearing

When either type of sensor goes on the fritz, it lights up the ABS light AND turns off the Traction Control System. That’s because the traction control system gets all its information from the ABS computer. No input means a shutdown of the entire system. Plus, this will set a code in the PCM.

Get a new Timken wheel bearing with hub and ABS sensor for these vehicles

CHEVROLET MALIBU (1997 – 2003)
OLDSMOBILE ALERO (1999 – 2004)
PONTIAC GRAND AM (1999 – 2004)

GM is also having a huge failure rate on the wiring harness that goes from the ABS computer to the sensors. The harness can short to ground. Because the harness must flex with each movement of the lower control arm, the harness can also develop an open condition where the ABS signals never reach the ABS computer. This is such a large problem that aftermarket supplier Dorman now makes replacement wiring harnesses for about half the price of the GM version.

Get an aftermarket ABS Wiring Harness for the front left sensor on these vehicles.

2005-97 Chevrolet Malibu;
1998-97 Oldsmobile Achieva;
2003-99 Oldsmobile Alero;
2005-99 Pontiac Grand Am
Dorman Part #970-008

Get an aftermarket ABS Wiring Harness for the front right sensor on these vehicles.

2005-97 Chevrolet Malibu;
1998-97 Oldsmobile Achieva;
2003-99 Oldsmobile Alero;
2005-99 Pontiac Grand Am
Dorman Part #970-009

If you have an ABS and TRAC OFF light and want to troubleshoot the system, start by examining the connectors at each front wheel bearing. You may find that the entire connector has broken off the bearing is just hanging in mid-air. If the wiring at the sensor appears good, disconnect it. Then disconnect the main connector at the ABS computer (usually located on the driver’s side—just follow the harness back to the computer). With the wiring harness disconnected, use your meter to test each of the wires that go to the ABS sensor. Check for continuity of each wire and then check for a short between the wires. Also check for a short to ground. If you find any of those conditions, replace the harness. Do NOT attempt to fix the short/open. The system operates on such low voltage that the increased resistance of a splice may prevent communication. Also, a splice make actually break again due to the flexing required.

970_008, wiring harness, dormanproducts.com, dorman wiring harnessIf the harness checks out, your problem is most likely in the ABS sensor wheel bearing/hub assembly. You can test the ABS sensor by connecting a digital multimeter to the ABS sensor connector and setting your meter to AC voltage. Then spin the wheel. You should see AC fluctuations. If you don’t then it’s safe to assume the wiring is broken. If you see fluctuation, you cannot assume that the sensor is good. The multimeter test is not considered the most reliable test. A more accurate test requires a scope.

To replace the wheel bearing/hub, you will need to remove the axle nut. That will require a large socket and a long ½” breaker bar. You can use an impact wrench to remove the axle nut, but you CANNOT use the impact wrench to install the new one. The impact can destroy the new bearing. Also, make sure you install a NEW axle nut. The old one CANNOT be reused. Find the torque value for the new nut and use a torque wrench to set it properly.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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