Subaru 2002 Impreza Service Manual page 308

ANTI-LOCK BRAKE SYSTEM (ABS)
ABS
1. Anti-lock Brake System (ABS)
A: FEATURE
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The 5.3i type ABS used in the lmpreza has a hydraulic control unit, an ABS control module, a
valve relay and a motor relay integrated into a single unit (called "ABSCM & H/U") for circuit sim-
plicity and reduced weight.
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The ABS electrically controls the brake fluid pressure to each wheel to prevent the wheel from
locking during braking on slippery road surfaces, thereby enabling the driver to maintain the
directional control.
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If the ABS becomes inoperative, a fail-safe system is activated to ensure same level of braking
performance as with a conventional brake system. In that case, the warning light comes on to indi-
cate that the ABS is malfunctioning.
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The ABS is a 4-sensor, 4-channel system; the front wheel system is an independent control de-
sign*', while the rear wheel system is a select-low control design*2.
*I : A system which controls the front wheel brakes individually.
*2: A system which applies the same fluid pressure to both the rear wheels if either wheel starts
to lock. The pressure is determined based on the lower of the frictional coefficients of both wheels.
(1)
ABS control module and hydraulic (5) G sensor
(2) Proportioning valve
(6) ABS warning light
(3) Diagnosis connector (7) Tone wheel
(4) Data link connector ( 8 ) ABS sensor
control unit (ABSCM 8, H/U)
(for SUBARU select monitor)
ABS-2
NF0197
(9) Wheel cylinder
(1 0) Automatic transmission control
module
(1 1 ) Brake switch
(1 2) Master cylinder