For TCS Control

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 Anti-lock Brake system (ABS) control mode, while it controls fluid pressure of the driving wheels (front wheels) individually in the Traction Control System (TCS) control mode. The H/U is mounted in the engine compartment.

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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 closed. 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.

FLUID PRESSURE CONTROL OF HYDRAULIC CONTROL UNIT FOR TCS CONTROL





1) Pressure-rise conditions.
When the driving wheels slip, TCS1 and TCS2 are operated and the hydraulic oil channel is switched over to TCS mode. At the same time the motor pump operates and sucks in brake fluid from the Master Cylinder (M/C) reservoir, and hydraulic pressure is imparted to the Wheel Cylinder (W/C) via FRI and ELI. This generates braking force and suppresses the wheel slip.





2) Pressure-hold conditions.
Pressure-hold mode is entered in order to focus temporarily on the behavior of the wheels following pressure rise/drop control and to maintain a proper degree of slip conditions due to engine output drop control and pressure rise/drop control of brake pressure. Under these conditions FRI, FRO--FLI, FLO are in hold conditions, and the motor pump pressure-generating side is cut off from the W/C side. The motor continues to rotate at this time and the pressure inside the H/U rises. Once it exceeds the stipulated value, however, PLV is released and the pressure is relieved to the suction circuit.





3) Pressure-drop conditions.
When the driving wheel slip is reduced, FRI and FRO are turned ON and ELI and FLO are also ON, and the fluid pressure channels enter pressure-drop conditions. The W/C pressure is released to the M/C reservoir and the braking force is alleviated and vehicle acceleration conditions are maintained.