Wheel Speed Sensor: Description and Operation

SWITCHES / SENSORS

Wheel Speed Sensors and Tone Wheels: One active Wheel Speed Sensor (WSS) is located at each wheel. The sensors use an electronic principle known as magneto-resistive to help increase performance, durability and low speed accuracy. The sensors convert wheel speed into a small digital sound. A toothed gear tone wheel serves as the trigger mechanism for each sensor.

The CAB sends 12 volts to power an Integrated Circuit (ICC) in the sensor. The IC supplies a constant 7 mA signal to the CAB. The relationship of the tooth on the tone wheel to the permanent magnet in the sensor, signals the IC of the sensor to toggle a second 7 mA power supply on or off. The output of the sensor, sent to the CAB, is a DC voltage signal with changing voltage and current levels. The CAB monitors the changing amperage (digital signal) from each wheel speed sensor. The resulting signal is interpreted by the CAB as the wheel speed.

Because of internal circuitry, correct wheel speed sensor function cannot be determined by a continuity or resistance check through the sensor.

Correct antilock system operation is dependent on accurate wheel speed signals from the wheel speed sensors. The vehicle's wheels and tires should all be the same size and type to generate accurate signals. In addition, the tires should be inflated to the recommended pressures for optimum system operation. Variation in wheel and tires size or significant variations in inflation pressure can produce inaccurate wheel speed signals; however, the system will continue to function when using the correct factory mini-spare.