Rear
REAR WHEEL SPEED SENSORDESCRIPTION
The antilock brake system uses two-wire wheel speed sensors, known as active wheel speed sensors. The sensors use an electronic principle known as magnetoresistive to help increase performance and durability. The sensors convert wheel speed into a small digital signal. A wheel speed sensor (WSS) is used at each wheel. The gear (tooth) type tone wheel serves as the trigger mechanism for each sensor. At each wheel of the vehicle there is one wheel speed sensor and one tone wheel.
The rear wheel speed sensors are mounted through the disc brake adapter (Fig. 9) (Fig. 10) or drum brake support plate. The rear tone wheels are mounted to and rotate with the hub and bearing assemblies.
The WSS air gaps are not adjustable. Each WSS is serviced individually. The tone wheels are serviced as an assembly with the hub and bearing assemblies.
OPERATION
The CAB (MK20e) or ABM (MK25e) sends 12 volts to power an Integrated Circuit 01C) 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.