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Home >> Subaru >> 1997 >> Legacy GT Wagon AWD F4-2.5L DOHC >> Repair and Diagnosis >> Brakes and Traction Control >> Antilock Brakes / Traction Control Systems >> Hydraulic Control Assembly - Antilock Brakes >> Description and Operation >> ABS With Traction Control >> For ABS Control

For ABS 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 ABS CONTROL





1) Normal Conditions (non-ABS control operation).
When the Hydraulic control Unit (H/U) solenoid valves and motor are not operating, the section from the Master Cylinder (M/C) to the Wheel Cylinder (W/C) is open (inlet valve open) and the section from the W/C to the M/C reservoir is closed (outlet valve closed). Also, the special valve is open.
When the brake pedal is depressed, the fluid pressure produced by the master cylinder is applied -in the primary system- from TCS1 to FR W/C via FRI and to RL W/C via RLI. At the same time, in the secondary system, the fluid pressure is applied from TCS2 to FL W/C via FLI and to RR W/C via RRI.
When the brake pedal is released, the fluid pressure at each W/C flows back through the same channels and is released. Since the FRI and FIJI channels constrict the oil flow, they are provided with check valves from which the pressure is also released at this time.





2) Pressure-drop conditions.
In order to eliminate the fluid pressure acting on the W/C and release excessive brake force, the inlet valve is ON (closed) and the outlet valve is ON (open), resulting in pressure-drop conditions. Figure shows hydraulic oil channels under these conditions (FRI, FRO--RRI, PRO all under pressure-drop conditions).
Under these conditions the section between the M/C and W/C is closed and the section between the W/C and M/C reservoir is opened by solenoid valve switching. The fluid pressure entering the W/C from the M/C pressure chamber is released by the M/C reservoir, and the W/C pressure drops because of the pressure-drop conditions illustrated below.





3) Pressure-hold conditions.
During ABS control, the fluid pressure is kept constant and pressure-hold conditions are maintained by the valves in front of and behind the pressure rise/drop control, i.e., the inlet valve is ON (closed) and the outlet valve is OFF (closed). The figure shows hydraulic oil channels under these conditions (all channels under pressure-hold conditions). Under these conditions the sections between the M/C and W/C and between the W/C and M/C reservoir are closed by solenoid valves (FRI, FRO--RRI, RRO), and the W/C fluid pressure is kept constant.





4) Pressure-rise conditions.
Once wheel lock can be avoided due to the drop in pressure, the inlet and outlet valves both go OFF in order to increase the W/C fluid pressure again and secure braking force. Hydraulic oil channels under these conditions are shown in Figure. FRI, FRO-RRI, RRO are all shown under pressure-rise conditions. These conditions are the same as normal hydraulic oil channel conditions, i.e., the section between the M/C pressure chamber and W/C is open and the section between the W/C and M/C reservoir is closed. By keeping FRI-RRI open on the M/C pressure chamber side (brake pedal is depressed, so pressure is produced) and on the W/C side, the M/C pressure is conducted to the W/C. The actual control at this time increases the pressure in steps, repeatedly increasing and holding the pressure, until the tendency for the wheels to lock re-occurs. At that time, since the M1C fluid enters the W/C, the brake pedal is stroked in stages. The stroke rate is detected by the stroke sensor, and when it has reached the predetermined value, the motor pump operates and fluid is pressure-fed from the M/C reservoir to the M/C pressure chamber, and the brake pedal returns to its original position. At this time the driver will experience kickback.


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