Technical information

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Technician Guidelines for Antilock Braking Systems

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Exciter or Tooth Wheel

ABS Sensor Pickup

• keep the supply port closed to maintain existing

pressure, or

• open the supply port to allow brake application

pressure to increase and repeat the cycle.

C. 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.

ABS configuration is defined by the arrangement and

number of sensors and modulator valves used. The most

common configurations for power units are: