Rick's Free Auto Repair Advice

Most common ABS problems — ABS light on

ABS problems — ABS light on

How to diagnose and fix ABS problems

Here are the most common ABS problems:

• Metallic debris on the face of the wheel speed sensor
• Cracked tone ring or chipped teeth on the tone ring
• Excessive air gap between the tone ring and wheel speed sensor, usually caused by rust buildup under the sensor
• Metallic debris buildup on the multi-pole magnetic ring
• Broken or failed wheel speed sensor
• Broken or intermittent wiring harness from the wheel speed sensor to the ABS module

Step 1 to determine what’s going on with your ABS system

Get the trouble codes read

Symptoms of ABS problems

ABS light on  and/or ABS and Trac OFF lights on

Any time the ABS module can’t read a wheel speed sensor, it sets a trouble code for that sensor. The ABS module then turns on the ABS and traction control warning lights and disables ABS and traction control.

False ABS Activation without trouble code

The most common ABS problem is false activation where the ABS kicks in at low speed on almost every turn and during almost every stop, even though the pavement is dry.

False ABS activation is caused by a faulty image of corrosion on the ABS tone ringABS sensor, debris on the face of the ABS sensor, an air gap that’s too large, rust on the tone wheel, or debris on the magnetic multi-segment magnetic ring.

Chipped tooth on ABS tone ring

Broken tone ring tooth

First generation ABS systems used a passive wheel speed sensor that contained a magnet and a toothed “tone” ring on the axle. As the tone ring rotates past the wheel speed sensor, it creates a fluctuating AC voltage that’s read by the ABS module.

Since both the sensor and the tone ring were exposed to the elements, the sensor face would collect metallic debris and the tone ring would rust. The metallic debris collection changes the air gap between the sensor and the tone ring, resulting in a weaker signal to the ABS module which it intercepted as a wheel that’s about to lock up, so it activated ABS to prevent wheel lock up.

If there’s no trouble code, here’s how to proceed

• Clean the face of each wheel speed sensor using a clean dry rag. If you find metallic debris on the rag, you’re on the right track.
• Clean rust off the tone ring using a wire brush. Check for cracks in the tone wheel or chips on the teeth. Replace the tone wheel if you find any.
• Refer to a shop manual to find the recommended air gap between the sensor and the tone ring. If the gap is too large, chances are there’s rust buildup under the wheel speed sensor mounting. Remove the sensor, sand off the rust, apply a light film of dielectric grease and remount the sensor. Then test. If you still get false activation, test the sensor.

Test a passive wheel speed sensor

Using a digital multimeter, disconnect the electrical connector to the wheel speed sensor. Then connect the meter leads to the sensor connector. Set your meter to the lowest AC volts setting and spin the wheel by hand. The AC voltage must match the spec published in the shop manual. If it doesn’t, replace the sensor.

If the sensor tests good. Disconnect the electrical connector at the ABS module and locate the terminals for the problem ABS sensor. Repeat the test at that main connector to ensure that the signal is getting through. If the signal at the main connector is weaker or missing, replace the wiring harness from the wheel speed sensor to the main connector.

Diagnostic technique and fix for active wheel speed sensors

Active wheel speed sensor require power and generate a digital signal instead of a fluctuating AC signal. You’ll need a scope to test an active wheel speed sensor signal. The only thing you can test with a multimeter is the presence of power and ground and an average of the digital signal.

• Active wheel speed sensors don’t accumulate metallic debris like a passive wheel speed sensor, but it’s still a good idea to check for debris, air gap spacing, and the condition of the tone ring or multi-pole magnetic ring. If the vehicle uses a multi-pole magnetic ring, wipe the face of the magnetic ring with a clean rag or nylon brush. Do NOT use a wire brush on the magnetic ring.

cut away driagrman showing active wheel speed sensor and multipole magnetic ring, along with a wheel hub with magnetic ring

Active wheel speed sensor with multipole magnetic ring

Less common ABS problems

In many vehicles, the vehicle speedometer works in conjunction with the ABS module. So a failed ABS module can cause a dead speedometer and multiple warning lights to come on. In that case, the most common cause is a shorted ABS valve solenoid or a circuit board solenoid driver issue. The best way to correct this is to send your ABS module to a rebuilder for a rebuild and updated program. Just do a search for ABS module rebuilders. Modulemasters.com is a reputable supplier.

©, 2021 Rick Muscoplat


Posted on by Rick Muscoplat

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