Impreza L Sport Wagon F4-2.2L SOHC (1999)
Hydraulic Control Assembly - Antilock Brakes: Description and Operation
Description
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The H/U is a fluid pressure controller comprising a motor, solenoid valve, housing, relay, etc. It constitutes two diagonally independent brake fluid
circuits for a cross piping vehicle.
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The pump motor rotates an eccentric cam to let the plunger pump generate a hydraulic pressure.
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The housing accommodates the pump motor, solenoid valve, reservoir, etc., and also constitutes a brake fluid passage.
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The plunger pump is a hydraulic pump which drains off the brake fluid which, when the pressure is reduced, is discharged to the reservoir, and
sends it toward the master cylinder
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The solenoid valve is a 2-position type solenoid valve which switches the brake fluid pas sages between the wheel and master cylinder and
reservoir sides in response to an instruction from the ABSCM.
For each wheel cylinder, a pair of normally-closed and -opened solenoid valves are provided.
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The reservoir is a fluid chamber which temporarily stores the brake fluid to be discharged from the wheel cylinder when the pressure is
reduced.
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The damper chamber suppresses the pulsation of the brake fluid which, when the pressure is reduced, is discharged from the plunger pump,
thereby minimizing the kickbacks to the brake pedal.
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The valve relay controls the solenoid valve and motor relay energizing power supply in response to an instruction from the ABSCM. In normal
(IG ON) condition, the relay is actuated to supply power to the solenoid valve and motor relay. When an error occurs in the system, the valve
relay is forced to OFF to keep the fluid pressure circuit in the normal mode (normal brake mode) and also constitute the ABS warning light
operating circuit.
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The motor relay supplies power to the pump motor to operate the plunger pump in response to an instruction from the ABSCM in the ABS
control mode.
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The H/U has four operating modes; normal mode (control OFF: normal brake mode), "increase", "hold" and "decrease" modes (control ON in all
the three modes).
