Subaru 2002 Impreza Service Manual page 314

ANTI-LOCK BRAKE SYSTEM (ABS)
ABS
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HYDRAULIC CONTROL UNIT SECTION (H/U)
The H/U is a fluid pressure controller consisting of, among others, a motor, solenoid valves, a hous-
ing and relays. It also constitutes passage of the two diagonally split brake circuits.
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The pump motor drives an eccentric cam which in turn moves the plunger pump to generate
hyd rau I ic pressure.
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The housing accommodates the pump motor, solenoid valve and reservoir. It also constitutes
a brake fluid passage.
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The plunger pump, when operated, draws the brake fluid from the reservoir, lets the fluid in a
wheel cylinder drain into the reservoir, and/or forces the fluid into the master cylinder.
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The outlet solenoid valve is a 2-position type. It opens or closes the brake fluid passage between
a wheel cylinder and the reservoir according to commands from the ABSCM.
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The inlet solenoid valve is duty-controlled to reduce brake fluid pulsation for minimum ABS op-
eration noise.
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The reservoir temporarily stores the brake fluid drained from a wheel cylinder when pressure
"decrease" control is performed.
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The damper chamber suppresses brake fluid pulsation which would occur during pressure "de-
crease" control in the fluid discharged from the plunger pump to minimize kickbacks of the brake
pedal.
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The valve relay controls power supply to the solenoid valves and motor relay in response to a
command from the ABSCM. In normal (IG ON) condition, the relay is closed to supply power to
the solenoid valves and motor relay. When an error occurs in the system, the valve relay is turned
OFF to keep the fluid pressure circuit in the normal mode (non-ABS mode).
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The motor relay closes and supplies power to the pump motor in response to a command from
the ABSCM during the ABS drive mode operations.
The H/U has four operating modes; normal mode (non-ABS mode), and three ABS active modes,
i.e., "increase", "hold" and "decrease" modes.
ABS-8