Fail-Safe Function - Subaru 2002 Impreza Service Manual

FAIL-SAFE FUNCTION
Automatic Transmission
4. 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.
0
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 1st and 3rd speeds.
0
THROTTLE POSITION SENSOR
If the throttle position sensor becomes faulty, the throttle opening is fixed at the predetermined
angle.
0
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 >
0
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.
r .
-
If both the solenoids should fail, the TCM invariably selects and keeps the 3rd gear.
0
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.
In this condition, the usable gears are limited to the 1st and 3rd.
If the lock-up duty solenoid fails, the solenoid is de-energized and the lock-up clutch is disen-
gaged.
0
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
(direct-coupled AWD condition).
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.
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.
LOCK-UP DUTY SOLENOID
2-4 BRAKE DUTY SOLENOID
LOW-CLUTCH TIMING SOLENOID
AT-36