Wheel Speed Sensor
The ABS controller uses wheel speed sensors to detect the rotational speed of each wheel. The sensors used by most of today’s vehicles are the permanent magnet type. A permanent magnet sensor is a coil of wire wound around a magnet core whose magnetic lines of flux change as a toothed ring rotates past its tip.
Every wheel controlled by the ABS unit will have a speed sensor to inform the control unit of the wheels speed. These sensors will be mounted on the steering knuckle of front wheel drive vehicles and the axle housing of the rears. The toothed ring is usually mounted on the CV side of the axle and sometimes (trucks) on the differential ring gear.The ABS control unit receives the low AC voltage from these sensors which increases with wheel speed. It then compares these values to the values it has stored in its memory.
Example: If the wheel were to lock up inducing a skid the ABS unit would be able to sense this by the decrease in frequency and activate the appropriate solenoid removing hydraulic pressure from the brake unit.
