Stopping a car in a hurry on a slippery road can be very challenging. Anti-lock Braking System (ABS) increases your steering ability and can prevent you from skiddding whislt braking . In fact, on slippery surfaces, even professional drivers can't stop as quickly without ABS, as an average driver can with ABS.
The theory behind anti-lock brakes is simple. A skidding wheel (where the tyre contact patch is sliding relative to the road) has less traction than a non-skidding wheel. If you have been stuck on ice, you know that if your wheels are spinning you have no traction. This is because the contact patch is sliding relative to the ice. By keeping the wheels from skidding while you slow down, anti-lock brakes benefit you in two ways: you'll stop faster, and you'll be able to steer while you stop.
ABS needs some way of knowing when a wheel is about to lock. The speed sensors, which are located at each wheel provide this information.
Since the valve is able to release pressure from the brakes, there has to be some way to put that pressure back. That is what the pump does. When a valve reduces the pressure in a line, the pump is there to get the pressure back up.
There is a valve in the brake line of each brake controlled by the ABS. On some systems, the valve has three positions:
The controller is called an ECU (computer) in the car. It watches the speed sensors & controls the valves.
There are many different variations & control algorithms for the ABS. We will discuss how one of the simpler systems works.
The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel will stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph under ideal conditions, but a wheel that locks up could stop spinning in less than a second.
The ABS controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees acceleration, then it increases the pressure until it sees deceleration again. It can do this very quickly, before the tyre can actually significantly change speed. The result is that the tyre slows down at the same rate as the car, with the brakes keeping the tyres very near the point at which they will start to lock up. This gives the system maximum braking power.
When the ABS system is in operation you will feel a pulsing in the brake pedal; this comes from the rapid opening & closing of the valves. Some ABS systems can cycle up to 15 times per second.
You absolutely should not pump the brake pedal in a car with ABS. Pumping the brakes is a technique that is sometimes used in slippery conditions to allow the wheels to unlock so that the vehicle stays somewhat straight during a stop. In a car with ABS, the wheels should never lock in the first place, so pumping the brakes will just make you take longer to stop.
In an emergency stop in a car with ABS, you should apply the brake pedal firmly & hold it while the ABS does all the work. You will feel a pulsing in the pedal that may be quite violent, but this is normal so don't let off the brake.