ABS Wheel Speed Sensors
Image: villemk, 2009
Do you ever wonder how your antilock braking system functions during panic stops and low traction conditions? The ABS wheel speed sensor plays a major role, not only in how the ABS works, but also how the traction control and stability control systems behave. Needless to say, the ABS wheel speed sensor is an integral part of the safety systems created to help you maintain control of your vehicle at all times.
How the ABS System Works
Prior to the advent of antilock brakes, drivers had to be careful to moderate the amount of braking force used to slow their vehicles down. Otherwise, the brake pads would grip the brake disc to the point of "locking" the disc and the wheel, causing the vehicle's tires to break traction and the vehicle skid to a halt. This is one reason why drivers were taught to "pump" the brakes in an effort to moderate braking force during a panic stop.
Antilock brakes utilize a variety of automotive sensors and an electronic control unit to help the vehicle's tires maintain traction during a panic stop. The ABS unit actuates the brake's hydraulic valves up to 15 times per second, applying and releasing the brakes continuously to prevent lockup while maintaining braking force. This allows the wheels to continue turning and the tires to maintain traction. The tell-tale pulsation of the brake pedal during the panic stop is a sure sign that the system is at work. Starting in the 1980s, many mainstream vehicles were fitted with antilock braking systems. Today, ABS remains an automotive safety staple.
ABS Wheel Speed Sensor Symptoms
The ABS wheel speed sensor relays information about the vehicle's wheel speed at each wheel to the ABS control module, which looks for rapid deceleration and subsequently modulates brake pressure until the vehicle comes to a stop. The automotive sensor measures wheel speed via a toothed wheel and magnetic sensors. While these particular car parts are designed to last for upwards to 100,000 miles or more, these components can wear out over time.
In the event that the ABS wheel speed sensor fails, the ABS warning light will appear on the vehicle's instrument panel. In most cases, the traction and stability control systems will also be disabled along with the ABS. Drivers may also experience irregular braking at low speeds. Some vehicles may display a particular OBD-II trouble code associated with ABS wheel speed sensor failure.
ABS Wheel Speed Sensor Repair
The first step to replacing the ABS wheel speed sensor is to disconnect the vehicle's battery. Use a lug wrench to loosen the lug nuts on the wheels containing the faulty ABS wheel speed sensor, but do not completely remove them. Place chocks behind the wheels that are to remain on the ground before lifting the end you're working on with the floor jack. Be sure to use jack stands underneath axle or along the car's jack points for added stability.
Next, remove the lug nuts and dismount the wheel. In most cases, the ABS wheel speed sensor will appear as a small black assembly located on the hub. Disconnect the electrical connector by hand and use a socket wrench to remove the sensor's retaining bolt. Remove the old sensor and fit the new sensor in its place, making sure that there is no visible gap between the toothed wheel and the tip of the sensor. Tighten the retaining bolts securely and remount the wheel.
In some cases, the ABS wheel speed sensor can be made to function once more by simply removing oil from the surface of the sensor and its connectors with a soft cloth. If you're not sure about how to access or remove the sensor from your vehicle, consult with your vehicle's factory service manual for more information. If you're in search of car parts for classic or vintage makes, ClassicCar.com is one of the resources to which you can turn.