Grand Cherokee 2WD V8-4.7L VIN J (2004)
Wheel Speed Sensor: Description and Operation
Wheel Speed Sensors and Tone Wheels
The Mark 20e system uses a new style of wheel speed sensor. The sensors use an electronic principle known as magneto-resistive to help increase
performance, durability and low speed accuracy. The sensors convert wheel speed into a small digital signal. A toothed gear tone wheel serves as the
trigger mechanism for each sensor.
The CAB sends 12 volts to power an Integrated Circuit (IC) in the sensor. The IC supplies a constant 7 mA signal to the CAB. The relationship of the
tooth on the tone wheel to the permanent magnet in the sensor, signals the IC of the sensor to toggle a second 7 mA power supply on or off. The
output of the sensor, sent to the CAB, is a DC voltage signal with changing voltage and current levels. The CAB monitors the changing amperage
(digital signal) from each wheel speed sensor. The resulting signal is interpreted by the CAB as the wheel speed.
The front wheel sensor is attached to a boss in the steering knuckle. The tone wheel is an integral part of the front axle shaft. The rear speed sensor is
mounted in the caliper adapter plate (rear disc only) and the rear tone wheel is an integral part of the rear rotor hub. The wheel speed sensor air gap is
NOT adjustable. Because of internal circuitry, a resistance check of WJ wheel speed sensors will not determine correct or incorrect function.
Correct antilock system operation is dependent on wheel speed signals from the wheel speed sensors. The vehicle's wheels and tires should all be the
same size and type to generate accurate signals. In addition, the tires should be inflated to the recommended pressures for optimum system operation.
Variations in wheel and tire size or significant variations in inflation pressure cam produce inaccurate wheel speed signals; however, the system will
continue to function when using the mini-spare. When driven over rough road surfaces, the rear wheel speed sensor signals may be erratic and cause a
false trouble code (drum brakes only).
