Rick's Free Auto Repair Advice

False ABS operation

False ABS operation — Causes and fixes

How ABS works and how they can falsely activate

Anti-lock brakes (ABS) are designed to prevent skidding on slick and dry pavement due to a wheel locking up and skidding. A wheel creates the most friction when brakes slow the wheel rotation speed. So ABS brakes monitors the rotation speed of all the wheels and determines if one wheel is about to lock up. To prevent that wheel from locking up, it releases brake pressure to that wheel’s brake to allow it to rotate back up to speed. Then the ABS system immediately applies the braking force to that wheel to slow it again. This cycle repeats rapidly to gain maximum stopping power from the wheel.

How an ABS system senses wheel rotation speed

An ABS system includes wheel speed sensors, notched “tone” rings, a high pressure pump, a valve body, and an electronic ABS module.

ABS brake components

Wheel speed sensors

The wheel speed sensor detects the teeth on the rotating tone ring. The gap between the sensor and the tone ring is critical, as is the cleanliness of the tone ring itself. Rust buildup on the tone ring, a crack in the ring or a chipped tooth can cause false ABS activation; ABS activation when it isn’t actually needed. In addition, since some sensors incorporate a magnet, any ferrous particle buildup on the face of the sensor can interfere with proper sensor operation.

In late model vehicles, car makers have moved away from notched steel tone rings and started using a rubber ring with embedded magnetic material (encoder ring), each section alternating between North and South poles. A Hall effect sensor detects the alternating North/South poles and generates a digital on/off signal to the ABS module.

ABS hall effect sensor with multipole encoder ring
Types of ABS wheel speed sensors and tone rings

Both tone ring and encoder ring styles can create false ABS activation if metallic debris accumulates on either the magnetic encoder ring or the face of the magnetic ABS sensor. Rust buildup on the tone ring or a missing tooth or crack in the tone ring can also create false ABS operation.

ABS activation at slow speeds

Missing tooth on tone ring

Rust buildup on ABS tone ring

Rust accumulation causes false ABS activation

Fix ABS false activation

If the ABS light is on, get the trouble codes read to find out which wheel is having the problem.

If the ABS light isn’t on steady, you might not have stored trouble codes.

In that case, assume you have debris build on either the sensor face or the encoder ring.

Clean an ABS tone ring system

Use a wire brush to remove rust buildup from the tone ring. Blow the rusty debris. Then rotate the wheel and check for missing teeth or a cracked tone ring. If you have missing teeth or a crack, you must replace the part.

Next, wipe the face of the wheel speed sensor to remove any metallic debris.

Clean an encoder ring system

Use a soft rag and gently wipe the rubber ring to remove any metallic debris that may have accumulated on the encoder ring.

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Posted on by Rick Muscoplat

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