Service Manuals, User Guides, Schematic Diagrams or docs for : . Car Manuals Subaru Impreza 2001-2002.2004-2007 Approved Subaru Impreza 2004 WRX STi 08. MECHANISM AND FUNCTION 33. AUTOMATIC TRANSMISSION 15. Fail-safe Function

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W1860BE.book Page 110 Tuesday, January 28, 2003 11:01 PM




                                                           FAIL-SAFE FUNCTION
 AUTOMATIC TRANSMISSION


 15.Fail-safe Function
 The fail-safe control function ensures minimum level of driveability even if a fault should occur in the
 vehicle speed sensors, throttle position sensor, inhibitor switch, or any of the solenoids.
     Front and rear vehicle speed sensors
 A dual speed-sensing system is used. The speed signal is taken from the transmission (by the out-
 put shaft speed sensor). Even if one sensor system fails, the vehicle can be controlled normally with
 the other normally operating sensor system.
 If both the front and rear vehicle speed sensors become faulty, the vehicle is made to operate only
 in the 3rd speed.
     Throttle position sensor
 If the throttle position sensor becomes faulty, control will be maintained by assuming that the throttle
 opening is at a certain angle.
     Inhibitor switch
 If the TCM receives different signals simultaneously from a faulty inhibitor switch, it selects a range
 in the following priority:
 D > N (P) > R > 3 > 2 > 1 >
     Shift solenoid 1 and 2
 If a fault occurs in either of solenoids 1 and 2, both the solenoids are de-energized, and the gear is
 held in the 3rd. If both the solenoids should fail, the TCM invariably selects and keeps the 3rd gear.
     Line pressure duty solenoid
 If the line pressure duty solenoid fails, the solenoid is de-energized and the line pressure is raised
 to the maximum to enable the vehicle to operate.
     Lockup duty solenoid
 If the lockup duty solenoid fails, the solenoid is de-energized and the lockup clutch is disengaged.
     Transfer duty solenoid
 When the transfer duty solenoid becomes faulty, it is de-energized. This causes maximum oil pres-
 sure to be applied to the transfer clutch so that the power is always transmitted to the rear axle (di-
 rect-coupled AWD condition).
     2-4 brake duty solenoid
 If a fault occurs in the 2-4 brake duty solenoid, the solenoid is de-energized and the usable gears
 are limited to the 1st and 3rd.
    Low-clutch timing solenoid
 If a fault occurs in the low clutch timing solenoid, the solenoid is de-energized and the usable gears
 are limited to the 1st and 3rd.
    2-4 brake timing solenoid
 If a fault occurs in the 2-4 brake timing solenoid, the solenoid is de-energized and the usable gears
 are limited to the 1st and 3rd.
    Torque converter turbine speed sensor
 If a fault occurs in the torque converter turbine speed sensor, the usable gears are limited to the 3rd.



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