Subaru 2002 Impreza Service Manual page 313

ANTI-LOCK BRAKE SYSTEM (ABS)
ABS
E: ABS CONTROL MODULE AND HYDRAULIC CONTROL UNIT (ABSCM & H/U)
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ABS CONTROL MODULE SECTION (ABSCM)
The ABSCM contains two microcontrol modules (MCMs): master and slave. Both the MCMs pro-
cess the same program and each MCM monitors the other's outputs. If a mismatch occurs be-
tween their outputs, the ABSCM cuts off the system and activate the fail-safe function.
The ABSCM can store a maximum of 3 trouble codes in an EEP ROM. If more than 3 faults have
occurred, only the 3 most recent failures are stored and others are erased. Trouble codes remain
stored until they are internally or externally erased.
The ABSCM has a test routine (sequence control pattern) which facilitates checking of the hydrau-
lic control unit.
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ABS control
Using primarily the wheel speed data from each ABS sensor and secondarily the vehicle decelera-
tion rate data from the G sensor as parameters, the ABSCM generates a simulated vehicle speed
when there is a risk of wheel lock-up. Using the simulated vehicle speed (called "dummy" vehicle
speed) as a reference, the ABSCM determines the state of the wheel in terms of the tendency to-
ward lock-up. If the result shows that the wheels are about to lock, the ABSCM issues commands
to energize or de-energize the solenoid valves and activate the motor pump of the H/U to modulate
the brake fluid pressures that act on the wheel cylinders, thereby preventing the wheels from lock-
ing.
The ABSCM controls the right and left front wheel fluid pressures independently and the rear wheel
fluid pressures based on the wheel which is the most likely to lock (select-low control).
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Functions available using SUBARU select monitor
When the SUBARU select monitor is connected, the ABSCM allows it
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To read out analog data
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To read out ON/OFF data
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To read out or erase trouble code
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To read out data showing conditions under which a trouble code has been stored (Freeze
frame data)
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To initiate ABS sequence control pattern
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Indication functions
Under the control of the ABSCM, the ABS warning light provides the following three indication func-
tion:
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ABS fault alerting
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Trouble code indication (by flashing in the diagnosis mode)
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Valve ON/OFF indication (when sequence control pattern is initiated)
ABS-7