Introduction:

An ABS (Anti-lock Braking System) is an enhanced braking system commonly found in newer automobiles. An anti-lock brake sensor or ABS sensor is a type of tachometer that measures the rotational speed of a wheel and passes it to the car's Engine Control Module (ECM). The ABS sensor is also called the wheel speed sensor or ABS brake sensor. Since all the wheels do not turn at the same speed, the ABS sensors report the speeds of all four wheels to the ECM. Based on this data, the ECM determines if the wheels are locking up. The application of the ABS brake is quicker than manual brakes. Due to this, when the ABS is engaged, it may emit a grinding noise in some automobiles.

Location
Older configurations of ABS had the sensors located outside the wheel hub in parts such as the steering knuckle and differential housing. The ABS sensor is coupled with a ring gear (or tone ring). This ring is mounted on components such as the brake rotor or brake drum. The ABS sensors in newer systems are often integrated into the wheel hub assembly itself. Depending on the type of braking system, vehicles may have as few as one or up to four ABS sensors.

Mechanism
A typical ABS sensor usually consists of a toothed ring (tone ring) and a magnet enclosed within a coil. As the wheel rotates, the teeth of the ring pass by the magnet, which induces a changing magnetic field and generates an alternating current (AC) signal in the coil. This signal's frequency corresponds directly to the wheel's speed. The signal is then sent to the ABS control module, which interprets it to determine the rotational speed of each wheel.

The sensor described is a common design, but the exact look and size can vary from car to car.

Most later-model vehicles are equipped with some type of traction control and stability control, where the ABS sensor is a key component. During a loss of traction, the speeds of the wheels on one side of the car will differ from those on the other. In these situations, the vehicle's computer uses data from the ABS sensors to regulate braking power and engine torque to individual wheels, helping to achieve the desired traction and maintain vehicle stability.

The ABS sensor is thus the primary sensor that monitors wheel lock-up. If the sensor transmits information indicating a wheel is locked, the computer will momentarily reduce braking force to that wheel. As the braking force decreases, the wheel can turn again and regain grip. This process happens rapidly and repeatedly during hard braking.

ABS Sensor Failure Symptoms
Over time, exposure to road debris, dirt, and corrosion can cause an ABS sensor to fail. Cleaning and inspecting the sensors at recommended intervals is vital for optimal performance.

Key symptoms of a faulty ABS sensor include:

1. The ABS warning light or traction/stability control light illuminating on the vehicle's dashboard.

2. A loss of anti-lock braking functionality during hard stops, potentially causing wheels to lock up.

3. Erratic or incorrect operation of traction or stability control systems.

4. In some cases, a considerable loss of traction when driving on slippery roads due to disabled driver aids.

It is important to note that the ABS sensor may not always be the culprit. The associated wiring or connectors can also degrade, failing to transmit the correct signals to the ECM. Therefore, before replacing an ABS sensor, it is good practice to inspect its wiring harness and connections to accurately diagnose the fault.