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HCU (HYDRAULIC CONTROL UNIT) DESCRIPTION The HCU consists of a valve body, pump motor, and wire harness. OPERATION Accumulators in the valve body store extra fluid released to the system for ABS mode operation. The pump provides The valves modulate brake pressure during antilock braking and are controlled by the ABM. The HCU provides three channel pressure control to the front and rear brakes. One channel controls the rear wheel During antilock braking, the solenoid valves are opened and closed as needed. The valves are not static. They are During normal braking, the HCU solenoid valves and pump are not activated. The master cylinder and power During antilock braking, solenoid valve pressure modulation occurs in three stages, pressure increase, pressure PRESSURE DECREASE The outlet valve is opened and the inlet valve is closed during the pressure decrease cycle. A pressure decrease cycle is initiated when speed sensor signals indicate high wheel slip at one or more wheels. At Once the period of high wheel slip has ended, the ABM closes the outlet valve and begins a pressure increase or PRESSURE HOLD Both solenoid valves are closed in the pressure hold cycle. Fluid apply pressure in the control channel is maintained PRESSURE INCREASE The inlet valve is open and the outlet valve is closed during the pressure increase cycle. The pressure increase 5 - 256 BRAKES - ABS SERVICE INFORMATION KJ |