Rear Wheel Speed Sensor: Operation

The CAB (MK20e) or ABM (MK25e) sends 12 volts to power an Integrated Circuit (IC) in the sensor. The IC supplies a constant 7 mA power supply to the CAB/ABM. The relationship of the tooth on the tone wheel to the permanent magnet in the sensor, signals the IC to enable a second 7 mA power supply. The output of the sensor, sent to the CAB/ABM, is a DC voltage signal with changing voltage and current levels. The ground for the IC and the current sense circuit is provided by the CAB/ABM.

When a valley of the tone wheel is aligned with the sensor, the voltage signal is approximately 0.8 volts and a constant 7 mA current is sent to the CAB/ABM. As the tone wheel rotates, the tooth shifts the magnetic field and the IC enables a second 7 mA current source. The CAB/ABM senses a voltage signal of approximately 1.6 volts and 14 mA. The CAB/ABM measures the amperage of the digital signal for each wheel. The resulting signal is interpreted by the CAB/ABM as the wheel speed.