Wheel Speed Sensor

1. Front Left Wheel Speed Sensor 
2. Front Right Wheel Speed Sensor 
3. Rear Left Wheel Speed Sensor 
4. Rear Right Wheel Speed Sensor 

A Wheel Speed Sensor is an essential component of the ABS ECU (Integrated Brake Actuation Unit/IBAU) because the IBAU calculates vehicle speed and determines whether wheel lock occurs based on data from the sensor.

For example, front-wheel vehicles use speed of a faster rear wheel as a reference when determining vehicle speed, and, if the vehicle detects excess difference in speed between front and rear wheels, the ABS control sets in.