Hydraulic Control Unit

HYDRAULIC CONTROL UNIT
The hydraulic control unit (H/U) is a brake fluid control assembly composed of a DC motor, plunger pump, pump housing, solenoid valve, valve housing, relay, etc.

In vehicles with cross brake piping, the fluid pressure channel corresponding to the two independent brake fluid channels that are installed diagonally to each other is constructed to control the front wheels respectively and to control the rear wheels together with in the ABC control mode, while it controls fluid pressure of the driving wheels (front wheels) individually in the TCS control mode. The H/U is mounted in the engine compartment.

Operation condition of the H/U is roughly classified into three modes, that are the normal mode in which the ABS/TCS is not functioning, the ABS control mode, and the TCS control mode.

The normal mode in which the solenoid valve and the motor pump are stopped corresponds to the vehicle conditions of normal running and parking. In this mode the H/U is used as a fluid channel between the master cylinder (M/C) and the wheel cylinder (W/C).

In the ABS control mode, if a wheel is about to be locked owing to the fluid pressure applied to the W/C by stamping the brake pedal, the solenoid valve of the H/U switches the operation mode of the fluid pressure channel to decreasing, retaining, or pressurizing according to a command of the C/M in order to control fluid pressure of the W/C suitably to the wheel condition. Therefore, braking without wheel lock is realized.

Switchover of the control modes is made by varying fluid pressure performed by combined operation of the two solenoid valves of the inlet valve that is regular open and the outlet valve that is regular close. The motor is actuated intermittently according to output of the stroke sensor.

In the TCS control mode, the TCS valve (TCS) is actuated with start of the motor pump by a command of the C/M, and brake fluid pressure controlled by the W/C control solenoid valve is applied to the driving wheels (front wheels) from the reservoir of the M/C through the suction pipe. Thus, wheel slip is prevented.