Signs of a Bad ABS Wheel Speed Sensor

Signs of a failing ABS speed sensor

These are the most common problems associated with a faulty ABS speed sensor:

• Warning lights appear on the dashboard: If the ABS computer finds a problem with a wheel speed sensor or its circuit, it will trigger the ABS warning light on the dash. It may also turn on the traction and stability control warning lights, if the car has those systems. Occasionally, the speedometer may stop working, as well.
• Loss of anti-lock brakes: Usually, the ABS computer will disable the ABS system if it detects a problem with a wheel speed sensor or its circuit.
• Loss of stability and traction control: If it detects a bad wheel speed sensor, the ABS computer will usually disable the stability and traction control systems, as well. On some vehicles, a bad wheel speed sensor may affect other functions as well, such as hill-start assist and roll stability.

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Wheel speed sensor design

Most wheel speed sensors have a coil of wire wrapped around a magnetic core. The sensor is paired with an ABS ring — a ring that has teeth — attached to the axle shaft or hub assembly. As the wheel spins, the toothed pattern of the ring creates an electric signal in the sensor. This signal increases in frequency and amplitude with wheel speed.

There is another type: the Hall-effect sensor. With this design, the ABS computer sends a reference voltage to the sensor. As the wheel spins, the sensor produces a voltage signal that increases in frequency with wheel speed. Hall-effect sensors are often referred to as “active wheel speed sensors” and are more accurate than the magnetic type.

Wheel speed sensor use in ABS

On most vehicles made in the last couple decades, there are four wheel speed sensors — one at each wheel. Signals from these sensors are used by the ABS computer to determine how quickly the car is slowing. If the computer senses that wheel lockup is about to occur, it commands the hydraulic modulator to pump the brakes. This helps prevent skidding and decreases the chance of an accident.

Wheel speed sensor use in traction control

On vehicles equipped with traction control, wheel speed sensors are used by the ABS computer to sense wheel slippage. If a slip is detected, the computer will make sure brake pressure is applied to the affected wheel. This helps the vehicle maintain traction, especially during heavy acceleration.

Wheel speed sensor use in stability control

Electronic stability control improves vehicle handling and steering. To accomplish this, the ABS computer monitors changes in vehicle speed, momentum, steering and acceleration. This information is provided by a number of sensors, including the wheel speed sensors. Using this data, the computer applies the brakes at certain wheels to optimize vehicle stability.

» LEARN MORE: Get an estimate for your wheel speed sensor replacement

How to fix the problem

Before spending time and money to replace a wheel speed sensor, a thorough diagnosis should be performed. Circuit or ABS ring problems can lead to unnecessary wheel speed sensor replacement. A professional mechanic can properly diagnose the issue using tools such as a scan tool, oscilloscope and digital multimeter.

A faulty wheel speed sensor should be replaced as soon as possible. On some cars, this is a relatively simple procedure a that most people can handle — as long as they have a safe way to get under the vehicle. Typically, replacing a wheel speed sensor involves removing its retaining fastener and unlatching its electrical connector. Once the sensor has been replaced, any diagnostic trouble codes should be cleared with a code reader or scan tool.