>>>ABS - Antilock Braking System<<<

Four-wheel ABS
The four-wheel ABS or Anti-lock Braking System is designed to help the driver maintain steering control during hard braking, especially in slippery conditions. How it works:
Imagine, you are driving a car without ABS on a slippery road (e.g. after a rain or snow). Suddenly, you notice something on the road right in front of you. You hit the brakes, and try to turn aside, but the steering doesn't work; the car just skids out of control (see the picture). Why, because all the wheels are locked up while you are holding down the brake pedal. As a result, you lose the ability to steer the vehicle.
The four-wheel ABS can help in situations like this. It prevents the wheels from locking up, helping you maintain steering control during braking. In a similar situation, driving a car equipped with four-wheel ABS, it would be easier for you to steer your vehicle while braking.

Braking on the snow without ABS

ABS hydraulic control unit
Essential to know:
- The four-wheel ABS system can help to slightly reduce the braking distance in some situations. However, under certain conditions (e.g. on loose snow or gravel), the braking distance may be longer. The main purpose of ABS is to help the driver maintain steering control during hard braking.
- Some trucks may have the two-wheel ABS system that controls only rear wheels. The rear wheel ABS system does not help to maintain steering control during braking. To gain full advantage of the ABS system you need to know what type of ABS you have in your vehicle and operate it correctly. Check your owner's manual for details.
- Good tires play an important role; for example, if the car has bald or wide sport tires the ABS can hardly prevent it from skidding on the snow.

How the actual ABS system works
Major components of the typical ABS system include four speed sensors (one at each wheel), an electronic control unit (ABS computer) and a hydraulic control unit (see the picture). The ABS computer constantly monitors the signal from each wheel speed sensor. When it senses that any of the wheels are approaching lock up during braking, the ABS computer sends the signal to the hydraulic control unit, which modulates the braking pressure for a corresponding wheel(s) preventing it from locking up.
When the ABS is activated, it feels like rapid brake pedal pulsation with a buzzing noise, which is normal.
ABS system problems
If the ABS light comes on while driving, there is a problem with your ABS system and you need to visit your dealer as soon as possible.

ABS at Work
There are many different variations and control algorithms for ABS systems. 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 would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 kph) 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 an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the tire can actually significantly change speed. The result is that the tire slows down at the same rate as the car, with the brakes keeping the tires 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 and closing of the valves. Some ABS systems can cycle up to 15 times per second.