Subaru XT 1988 Service Manual page 522

3-2
AUTOMATIC TRANSMISSION AND DIFFERENTIAL
5) LOCK-UP CONTROL - OPERATION
Lock-up control valve
Lock-up apply circu it
Lock-up release circ
I
Vehicle speed
Throttle opening
Engine speed
!I'I'"--::::=o Lock-up duty pressure
or pilot pressure
D
. (8) (A)
rain
Pi lot pressure
ATF temperature
sensor signal
Range signal
AfT control unit
Fig. 11
The lock-up control valve is pushed downward by torque
converter REG pressure and pilot pressure. It is pushed up-
ward by lock-up duty pressure and spring force.
(1) During Lock-up Operation
Oil pressure at the lock-up control duty solenoid valve is
drained (duty ratio 95%) by a signal from the automatic trans-
mission control unit so that no lock-up duty pressure is
developed and the lock-up control valve remains in condition
(A). As a result, hydraulic oil flows into the lock-up apply
circu it. On the other hand, the lock-up release circuit drains.
This causes a pressure differential across the lock-up piston.
The piston is then forced against the impeller cover and turned
as an integral unit with the cover. Thus, power from the engine
is directly transmitted to the transmission input shaft. That
is, the transmission is directly coupled to the engine.
(2) During Non-Lack-up Operation
In this mode, the lock-up control duty solenoid is driven at
a 5% duty ratio. This causes the lock-up duty pressure (pilot
pressure) to be generated. With this pressure, the lock-up
control valve is set to condition (8), and hydraulic oil flows
8
Filter
Plunger
Duty solenoid 8 (lock-up)
L3-589
into the lock-up release circuit. On the other hand, the lock-up
apply circuit is connected to the oil cooler in the radiator.
Accordingly, the relationship between "lock-up release pres-
sure lock-up apply pressure" is established. As a result, the
lock-up piston is forced to separate from the impeller cover,
and power is transmitted from impeller to turbine to input
shaft, as with an ordinary torque converter coupling.
(3) During Smooth Control
When the lock-up clutch activates, the clutch partially engages.
Lock-up apply pressure increases smoothly to engage the lock-
up clutch.
(4) Non-Lack-up Operation During "1st Speed",
"N", "R"
and
"P"
In this mode of operation, pilot pressure is generated, and the
lock-up control valve is set to condition (8) where lock-up
is inoperative.