Subaru 2002 Impreza Service Manual page 241

TRANSMISSION CONTROL MODULE (TCM)
Automatic Transmission
D: LOCK-UP CONTROL
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The TCM has pre-programmed lock-up clutch engagement and disengagement conditions for
each gear and shift pattern. In addition, it specifies engagement of the clutch whenever the 4th
gear is selected in the D range. The engagement and disengagement conditions are defined in
terms of the throttle valve position and vehicle speed.
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The TCM controls the operation of the lock-up clutch by means of the duty solenoid which in turn
controls the lock-up control valve as described below:
1. NON-LOCK-UP OPERATION
The duty solenoid allows the pilot pressure (supplied from the pilot valve) to be applied to the "dis-
engaging" end of the lock-up control valve spool. The lock-up control valve then opens the clutch
disengaging circuit port to allow the lock-up operating pressure (torque converter clutch regular
pressure) to build up in the circuit. On the other hand, the valve opens the clutch engaging circuit's
port and allows the fluid in the circuit to flow to the ATF cooler, thus lowering the pressure in the
circuit. As a result, the lock-up clutch is disengaged due to difference in pressure between both
circuits.
This operation is performed for all the speed gears except the 4th gear of the D range.
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2. LOCK-UP OPERATION
The duty solenoid allows the pilot pressure to be applied to the "engaging" end of the lock-up con-
trol valve spool. The lock-up control valve then opens the clutch engaging circuit's port that com-
municates to the torque converter's impeller chamber, allowing high pressure fluid to flow to the
lock-up clutch. The clutch then engages.
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The TCM controls the current to the duty solenoid by gradually changing the duty ratio. As a
result, the lock-up control valve also moves gradually, so the clutch engagement pressure in-
creases smoothly. This causes the lock-up clutch to become initially in a half-engaged state and
then in a fully engaged state, thus preventing shock during engagement.
This operation is performed for all the speed gears and always when the 4th gear is selected in
the D range.
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