The Car Care Guide

Wheel Speed Sensors

The wheel speed sensor has two main components: the exciter and the pickup. Other components include associated wiring and mounting equipment.

• Exciter - The exciter is a ring with notched teeth. The most commonly used exciter has 100 evenly spaced teeth, but the number of teeth can vary depending on the system design. The component is known by several names: sensor ring, tooth wheel, tone ring, and exciter.
• Pickup - The pickup is commonly called “the sensor.” It contains a wire coil/magnet assembly, which generates pulses of electricity as the teeth of the exciter pass in front of it. The ECU uses the pulses to determine wheel speeds and rates of acceleration/deceleration. The strength of these electrical pulses decreases rapidly with slight increases in the gap between the pickup and the exciter.

Wheel speed sensor location varies. It can be located anywhere on the axle to sense wheel speed. The sensor can be an assembly containing both the exciter and the pickup with a fixed gap. Or, the pickup and the exciter can be mounted separately on different parts of the axle assembly. The sensor pickup is a sealed unit and typically of elbow or straight design.

On most ABS air-braked vehicles, the pickup is located in the mounting flange on the wheel end. The exciter usually is either mounted on - or integrated with - the wheel hub. Since the output of the pickup decreases so rapidly with slight increases in exciter-pickup gap, it is imperative that the wheel end and sensor gap be maintained within the manufacturer’s specification.

When the wheels of only one tandem axle have wheel speed sensors, they are usually placed on the axle whose wheels are most likely to lock-up first during braking. On a tandem with a four-spring suspension, the sensors are generally on the lead axle. On a tandem with air suspension, the sensors are generally located on the trailing axle.