Subaru Legacy ABS Speed Sensor

What is an ABS wheel speed sensor?

There is a crucial part in your vehicle that makes sure your car is working efficiently and safely. It is known as the anti-lock braking system (ABS). When a significant amount of pressure is applied to the brake pedal, the ABS prevents the tires from locking up and losing control. The anti-lock braking system depends on the information from many sensors to detect when the wheels are locked up. The ABS wheel speed sensor (aka ABS sensor or ABS brake sensor) plays an important role here. ABS wheel speed sensors measure the rotation speed of the wheels and pass the data to the ABS control unit. The ABS control unit uses these data to determine when to step in to prevent the wheels from locking.

An ABS wheel speed sensor typically consists of two parts: a reluctor wheel or toothed ring, and either a magnetic or hall effect sensor. The contact between the toothed ring and the magnet produces electric signals for the ABS control unit. The ABS wheel speed sensor is sometimes located on the axle but is mostly installed behind the wheel brake rotor.

In some older cars, The ABS speed sensor is used only to determine if the wheel is locked up. In some newer models with more advanced stability and traction control systems, ABS sensors are also crucial in controlling the movement of the vehicle under all conditions. Under bad driving conditions like rainy or snowy days, your vehicle might lose traction. The ABS wheel speed sensor is able to pass the speed difference of wheels to the traction control system, thereby improving the stability of your vehicle.

Signs of faulty ABS wheel speed sensor

• ABS light: Just like the Check Engine light, the ABS light warns you there may be a problem with the ABS. The ABS warning light is one of the most common symptoms of a bad ABS wheel speed sensor.
• Traction control light: In modern vehicles, the traction control system also heavily relies on the ABS wheel speed sensor. A bad ABS wheel speed sensor can cause the traction control system to malfunction.  
• Loss of anti-lock brakes: If the Powertrain Control Module can’t get the proper data from the ABS wheel speed sensor, it can not tell if the wheels are locked, and the ABS will shut down until the problem is fixed.
• Lack of stopping power under heavy braking: Locked and skidding wheels will make a car decelerate slower. If you find your car takes a long time to stop during heavy braking conditions, there might be something wrong with the ABS wheel speed sensor.
• Less stability under wet or icy conditions: In wet or icy conditions, the traction control system uses the data from ABS wheel sensors to improve stability. Without knowing the wheel speeds, the traction control system can’t work properly.

Possible reasons for ABS wheel speed sensor failure

• Internal short circuits
• Increased wheel bearing clearance
• Breaks in wiring/sensor winding because of vibration
• Damage during replacement of other components