Wheel Speed Sensor: Description and Operation

WHEEL SPEED SENSORS AND TONE WHEELS

The Bosch 5.7 system uses one passive WSS on each wheel. The sensor measures the wheel speed by monitoring a rotating tone wheel. As the teeth of the tone wheel move through the magnetic field of the sensor an AC voltage and amperage is generated. This signal frequency increases or decreases proportionally to the speed of the wheel. The CAB monitors this signal to check for a sudden change in single or multiple wheel decelerations. If the deceleration of one or more wheels is not within a predetermined amount, the CAB takes control for antilock action through the HCU. Each WSS has a magnetic inductive pick up coil (WSS) that is mounted to a fixed component. There is an air gap between the tone wheel and the speed sensor assembly. Diagnostically, the coils of the Wheel Speed Sensors have the same amount of resistance. When measured across the CAB harness connector terminals, the resistance should be between 1100 - 1800 ohms.

Correct ABS operation is dependent on Tone Wheel speed signal from the WSS. The vehicle wheels and tires should all be the same size and type to get accurate signals. In addition, all tires should be at recommended tire pressures.