A basic Antilock Brake System consists of one or more wheel speed sensors, a hydraulic pump/valve body which has the ability to increase or decrease pressure to the brakes, and a control module. When the driver steps on the brakes, the ABS control module monitors wheel speeds for possible lockups. The control module can then operate the pump/valve body to correct this situation, allowing the driver to maintain better control of the vehicle and improving stopping distances.
Wheel Speed Sensors
ABS Wheel Speed Sensors normally consist of a small electrical coil with a permanent magnet, and a toothed ring attached to the brake rotor or hub assembly. On some vehicles, this ring is found inside the differential housing. As the ring spins through the sensor's magnetic field, an AC voltage is produced and transmitted to the control module. The control module interprets this voltage as a wheel speed. On some systems, the Vehicle Speed Sensor in the transmission is used to determine rear wheel speed.
System Operation
On a Rear Antilock Brake System (RABS), only rear wheel speed is monitored. The control module is programmed to respond when the rear wheel speed drops at a rate faster than a preset limit, by reducing the pressure being applied to the rear brakes via a valve body. On a 4 Wheel Anti-Lock (4WAL) system, the front wheel speeds are normally monitored individually, and the rear wheel speeds may be monitored as a single value or separately. When the control module detects that one or more wheel speeds has dropped more quickly than the others, the brake pressure to those wheels is reduced via the pump/valve body, preventing them from locking up.
Summary
ABS systems are an important safety feature on many modern vehicles. Only qualified technicians should diagnose or service these systems, since improper repairs or neglect may cause brake malfunction or failure, leading to crashes and possible death.