Rick's Free Auto Repair Advice

ABS light on

ABS light on –What does it mean?

An ABS light condition means the computer has detected a fault somewhere in the anti-lock braking system (ABS) of your vehicle. The computer conducts a self-test when you start your vehicle. The test checks for proper values in the ABS computer (if equipped) and for proper digital communication between the ABS computer and the main PCM computer. If the system detects a fault, it stores a specific trouble code, turns the ABS light on and disables the ABS braking system. You can still brake normally, but you will not have any anti-lock braking function until you fix the underlying problem.

What types of faults result in an ABS light on condition?

The ABS system monitors the revolutions at each wheel using a wheel speed sensor. Jeep wheel speed sensorIf the ABS system detects that one wheel is rotating at a slower rate than the others, it sees the slowdown as an impending wheel lockup and potential skid condition. The ABS system then commands a brake fluid pressure release to the slowing wheel, which allows it to rotate at the same speed as the others. The ABS system then immediately re-applies brake pressure to the same wheel, causing it to slow the vehicle. It repeats the release/apply cycles multiple times per second to “pump” the brakes to that wheel and prevent it from locking up and skidding.

The ABS system monitors wheel rotation at all times, even when you’re not braking. If the ABS system detects a missing, intermittent, irregular, or out of spec signal from a wheel speed sensor it will set a trouble code and turn the ABS light on. Here are the most common causes of an ABS light.

• Bad wheel speed sensor

• Metallic debris on the face of the wheel speed sensor

• Cracked “tone” ring on axle shaft or hub bearing.

Picture of new ABS tone ring

Check for rust or debris on the tone ring that could interfere with the signal

• Metallic debris on the magnetic ring on hub bearing

• Fault in the wiring harness or connector to a particular wheel speed sensor

• Faulty ABS controller

Other less common problems are:

• Failure in the ABS pump motor

• Failure of one of the ABS apply/release solenoid or valve

• Failure of the ABS controller

How to fix an ABS light on condition?

Many auto forum participants ask how to fix an ABS light on condition without taking the time to find out what codes are stored in the computer. That’s an impossible question to answer. To fix an ABS problem, you first have to know the trouble code. For example, you could simply have a bad wheel speed sensor. But which sensor is bad? The trouble code will test you which sensor set the code. It’s up to you to test that entire circuit to determine whether the root problem is the wiring harness or connector to that sensor, debris on the sensor face, improper spacing between the sensor and the tone ring, a cracked tone ring or a bad sensor. But it all starts with the trouble code. Don’t even think about fixing an ABS light on condition without reading the trouble codes. Otherwise you’re just spitting in the wind.

Can I turn the ABS light off?

You can clear the code with a scan tool. But if you haven’t fixed the underlying problem, it’ll turn on again.

What if the ABS and TRAC lights are on?

That’s a pretty common problem with GM vehicles. The traction control system in GM vehicles gets its data from the ABS braking system. When there’s a failure in the ABS system, the TRAC system automatically shuts down and sets its own trouble code. So a wheel speed sensor failure can turn on the ABS light, TRAC light and Service Engine soon light. Scan the codes and then start your diagnostics. For more information on the GM ABS and TRAC light issue, read this post.

ABS light on, TRAC off, Stabilitrac

©, 2016 Rick Muscoplat


Posted on by Rick Muscoplat


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