What is an ABS sensor and where is it located?

In today's automotive world, anti-lock brakes have become standard components in vehicles and are an extremely important safety feature.  When driving on a slippery road and firmly engaging your brakes, your vehicle's wheels could potentially lock up, causing the vehicle to skid or spin uncontrollably from the loss of traction.  This is obviously not ideal as far as safety goes.

The main purpose of the anti-lock braking system (ABS) is to prevent a vehicle's wheels from locking up at a time when momentary wheel rotation is needed.  By allowing the wheels to maintain their traction with the road surface, the anti-lock braking system is able to stop the vehicle faster, and allow the driver to steer away from potential hazards.  The anti-lock braking system consists of several components, one of which is the ABS sensor.

The ABS sensor, also commonly referred to as a wheel speed sensor, ABS speed sensor, ABS brake sensor, or ABS wheel speed sensor, is part of the anti-lock brake system and is used to measure the amount of times a wheel rotates (i.e. its speed).  This information is used by the system to determine if a wheel is slipping more than the others and if the brakes need to be engaged in an emergency.  In newer models, ABS speed sensors can be part of electronic traction control systems and in the stability control systems of some trucks and SUVs.  Older designs use an external brake sensor near the wheel in conjunction with a ring gear located either on the axle or the back of the brake rotor, while newer versions incorporate these pieces into the wheel hub and bearing assembly.

A vehicle can have up to four wheel speed sensors.  Some vehicles have 2 sensors: one on the left and right wheel in the front or rear of the vehicle.  Others will have 3 wheel speed sensors, with a shared one located in the rear axle, in addition to one on the left and right wheel in the front of the vehicle.  Lastly, some vehicles will have 4 ABS sensors, one on the left and right wheel in both the front and rear of the vehicle. The number of ABS sensors that a car or truck has depends on the type of brakes that the vehicle has.

How do I know if my wheel speed sensor needs to be replaced?

The most common reasons for replacing a wheel speed sensor are damage or contamination.  Since the sensor is close to the ground, road debris can cut or fray the wiring.  If not held securely in place, the harness may chafe on nearby chassis parts.  Since the brake sensor is electrical, exposure to the elements (such as ice and moisture) can cause connectors to corrode or internals parts to short out.  A defective or malfunctioning ABS speed sensor will typically set an ABS light on your instrument panel, and trigger an ABS diagnostic code in your vehicles computer.  It also may throw a traction control light on as well.

Can I replace an ABS speed sensor myself?

Replacing the ABS speed sensor varies depending on the vehicle.  For vehicles that have the external style sensor, replacement is as simple as unplugging the connector from the vehicle harness and unbolting the sensor from its location.  On vehicles that have the ABS brake sensor integrated into the wheel hub and bearing assembly, the job is more involved since the entire assembly will have to be removed, requiring at least the brakes to be taken out of the way.