Wheel Speed Sensor
Wheel speed sensors are located individually at each wheel or axle. These sensors generate a signal that changes with wheel speed. Imagine one wheel on dry pavement and one wheel on ice. A speed sensor signal that suddenly drops off indicates that one or more wheels are approaching lock up. This information is being monitored by the computer, which in turn commands the hydraulic modulator to regulate line pressure at the affected wheels. When accessible during normal brake service, wheel speed sensors should be cleaned and air gap checked.