Applying brakes too hard, or on a slippery surface, can cause the wheels to lock. When wheels lock, steering control is lost and, in most cases, it produces longer stopping distances. The antilock braking system prevents wheels locking or skidding, no matter how hard brakes are applied, or how slippery the road surface. Steering stays under control and stopping distances are generally reduced.
It consists of a brake pedal, a master cylinder, wheel speed sensors, the electronic control unit or ECU, and the hydraulic control unit, also called a hydraulic modulator.
The wheel speed sensor consists of a notched or toothed rotor that rotates with each wheel, and a pickup. As the wheel turns, a small voltage pulse is induced into the pickup and sent to the electronic control unit. When the brakes are applied, the wheel's speed of rotation changes. This sends a new signal to the ECU.
If the control unit detects that a wheel might lock, it sends a signal to the hydraulic control unit. In a three-channel system, the hydraulic control unit uses three solenoid valves to control brake pressure and prevent them locking.
The valves are in series with the brake master cylinder and the brake circuits. One operates for each of the front wheels and one controls both rear wheels. At the start of a journey, the ABS automatically checks itself. Any failure in the system lights up a warning light in the dash-panel.
