Subaru XT 1988 Service Manual page 520

3-2
AUTOMATIC TRANSMISSION AND DIFFERENTIAL
condition in which the impeller shaft (engine side) alone
rotates and the turbine shaft does not rotate, is called the
"stall point" (the vehicle is at a standstill). At this stall point,
4) LOCK-UP CONTROL - MECHANISM
when in full throttle, the torque ratio of the turbine shaft and
impeller shaft is at a maximum. The torque ratio at this point
is called the "stall torque ratio", and the engine speed is called
the stall speed.
~L---1L-::::::::::r~~~_---_---lmpeller
cover
Lock-up clutch
Torsional damper
Friction washer
&
wave spring
Drive plate
crankShaft~
Fig.
8
The control unit controls the 50 Hz duty control (pulse width
modulation) solenoid by changing its on/off time ratio. The
lock-up control valve actuated by this solenoid switches the
oil passages, and controls the hydraulic pressure applied to
the lock-up clutch.
Pulse Width Modulation
Fig. 9
VO,,,,,ONTIll
OFF EJA
.1- -
• Hydraulic preuure is controlled by changing the ratio of B/A .
• Period A IS constant. (Frequency IS 50 Hz)
ON
Duty ratio 25 %
~
ON
Dutyratio50%~
ON OFF
o u t y r a t i o 7 5 % _
I
AOFt
~
---+-
Time (tl
L3-587
6
Impeller
_~l._I--ilL--ff---Turbine
LJ.\-_-ff----Stator
One-way clutch
L3·586
The duty control solenoid is operated by applying on-off
voltage pulses; and the percentage modulation is used to
indicate its performance. This percentage modulation is also
called the duty ratio, which is represented by the following
expression.
Duty ratio = ON . ON
t~m:F'
x 100 (%) = AB x 100
(%)
time
+
time
The repetition period A is defined as follows:
Period A
=
ON time
+
OFF time
Pulse width modulation meims variation of duty ratio while
keeping period A constant .