Anti-lock braking systems (ABS) are designed to help bring a car to a stop on slippery surfaces. When a driver slams on the brakes, all four wheels will lock up, which will cause the car to skid. This can cause the driver to lose control of their vehicle, and it can actually increase the stopping time.
The ABS is essentially a computer monitoring system that senses when you panic and works the brakes to give you more control. What it does is pumps the brake so the wheels continues to spin. This allows it to have more traction, which means you will still be able to steer the car and you will stop faster.
The system consists of four main components - speed sensors, pumps, valves and a controller. The speed sensors, which are either located at each individual wheel or at the differential that powers them, keep track of the wheel speed. When you slam on the brakes, the sensors will notice a sudden reduction of speed and kick the rest of the system into gear.
The ABS has its own special valve in each wheel's brake line. The valves often have three positions. The first is open so the regular brakes can function normally. In the second position, the valve blocks the line so that it is no longer accessible by the master cylinder, which provides pressure to the brakes. This way drivers cannot exert any further stopping force, which allows the ABS to take over.
The third position allows the valve to relieve some of the built-up pressure on the brakes. Yet, more pressure is needed to pump the breaks and keep the wheels spinning, which is where the pump comes in.
Controller is essentially just another name for the computer that monitors the speed sensors and controls the valves. It is responsible for signalling the valves when the speed sensors indicate that the wheels are accelerating and decelerating.
You can tell when the ABS system is engaged because the brake pedal will give off a rapid pulsing sensation. Even though the ABS is braking the car for you, it is important to remember to never pump the brakes or release them when the ABS is activated - at least not until the car has come to a safe and complete stop. Pumping the brakes is a technique reserved for cars without anti-lock brakes to prevent the wheels from locking. ABS is designed to eliminate wheel-locking all together, and it pumps the brakes for you.