Anti-lock braking systems (ABS) became common in the 1990s. The technology is now found on every modern vehicle.
If you are old enough, you may remember having to "pump the brakes" in an emergency to keep the wheels from locking. Well, guess what? Thanks to ABS, the car does all the pumping for you. All you have to do is step on the brake pedal.
At the heart of an ABS system you will find a computer called the control module. The module uses ABS speed sensors to measure the rotation rate of your car's wheels. If the sensors indicate that wheel lockup is about to happen, the control module pushes brake fluid to the sliding wheel to prevent the lockup.
An ABS speed sensor — also known as a wheel speed sensor — measures rotation by reading a toothed ring. Typically the ring is fixed to the axle shaft or to the hub.
Most late-model cars have four ABS speed sensors — one at each wheel. Older applications, however, may have only one or two such sensors. An example is the rear-wheel anti-lock system (RVAL) often found on pickups, which uses only one speed sensor on the differential.
