Anti-Lock Brake System: Description

The Electro-Hydraulic Control Unit (EHCU) consists of the Electronic Brake Control Module (EBCM) and the Brake Pressure Modulator Valve (BPMV). The EHCU regulates hydraulic pressure in the brake system during an ABS event. The EBCM is the electronic portion of the EHCU assembly. EBCM receives and processes numerous sensor and switch inputs and then controls BPMV function if it is necessary to respond to these inputs. Inputs to EBCM include, wheel speeds, brakelight switch, brake fluid level switch, transfer case switch (4WD), differential pressure switch, traction assist switch (if equipped with traction control) and delivered torque signal (if equipped with traction control).

Outputs of EBCM include, system relay, isolation solenoids, dump solenoids, ABS indicator, BRAKE indicator, LOW TRACTION indicator (if equipped with traction control), TRACTION OFF indicator (if equipped with traction control), requested torque signal (if equipped with traction control) and pump motor, BPMV is the hydraulic/mechanical portion of the EHCU, consisting of a valve body and pump motor. The BPMV is actuated by EBCM in order to regulate hydraulic pressure in the following brake circuits; left front, right front and rear circuit. Each circuit within the BPMV has an isolation valve and a dump valve. These valves are used to hold, decrease or increase hydraulic pressure within the brake circuits. There are 2 low pressure accumulators located within the BPMV, which are used to store brake fluid in the unit during ABS operation. The pump, also located within the BPMV, is used to return fluid from the accumulators to the master cylinder or fluid reservoir.

The front Wheel Speed Sensors (WSS) are magnetic coil/pickup type. Each WSS produces an AC voltage signal and is used by EBCM to calculate wheel speed. Any imperfections in the toothed ring, such as a missing or otherwise damaged tooth, can cause an inaccurate wheel speed sensor signal. The rear wheel speed input signal originates at the Vehicle Speed Sensor (VSS) and is supplied to EBCM by the Powertrain Control Module (PCM) or Vehicle Control Module (VCM) . PCM is used on diesel models and VCM is used on gasoline models.