Rick's Free Auto Repair Advice

Trac off light on

Trac off light on and ABS light on GM vehicles

Shops are reporting huge failure rates on ABS sensors in some late model GM vehicles. When the ABS system detects a fault the computer lights the ABS light and turns off the traction control notifying you with a trac off light on. The problem with the ABS light and Trac off light on is most often a bad ABS sensor inside with the wheel bearing/hub. These sensors are made in two styles:

Exterior ABS wheel speed sensor

This style ABS wheel speed sensor bolts the sensor directly wheel bearing/hub assembly. The wheel speed 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 speed sensor and the wheel bearing. This phenomenon is called “rust jacking,” where the rust literally pushes the sensor upwards which increases 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 off 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.

Integral ABS wheel speed 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, the intermittent or missing ABS wheel speed sensor signal causes the ABS brake system to shut down. This turns ABS light on and the Trac off light illuminates. 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.

In most cases replacing the wheel bearing/hub assembly fixes the problem. However, we’re seeing some cases where ABS light on and trac off light on can be caused by a faulty ABS wheel speed sensor wiring harness.

To find out which wheel is causing the problem, use a code reader or scan tool that’s capable of reading B, C, and U trouble codes. If you don’t have a code reader or scan tool, find one here.

ABS sensor wiring harness

GM is also having a huge failure rate on the wiring harness that goes from the ABS computer to the wheel speed 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 light on and TRAC OFF light on 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

Save

Save

Save

Save

Posted on by Rick Muscoplat


Custom Wordpress Website created by Wizzy Wig Web Design, Minneapolis MN