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PART 7-2- C4 AUTOMATIC TRANSMISSION
7-55
Modulated throttle pressure in these
areas provides shift delay in relation
to throttle opening.
CUT BACK VALVE
Increased line pressure is required
to prevent clutch and band slippage
under stall conditions. As vehicle
speed increases, the requirements
for increased line pressure are con-
siderably reduced. The cut back
valve provides for the reduction in
line pressure. When governor pres-
sure acting on the end of the cut
back valve exceeds the force of T.V.
pressure and throttle boost pressure
opposing
governor
pressure
the
cut back valve will move, cutting
off primary T.V. pressure being fed
to the pressure booster valve and
intermediate band accumulator
valve. The cut back valve movement
will therefore vary with engine
throttle opening and vehicle speed.
See Figure 22 for line pressure
variation with output shaft rpm
(vehicle speed) at constant vacuum
values.
WIDE OPEN THROTTLE-
I" VACWM
NOM.
160l-----.
120
CLOSED THROTTLE-ABOVE
17''
VACUUM
o~-----~-~~-~--
o
5
10
15
20
25
30
OUTPUT SHAFT RPM
x
10
D1569·A
FIG. 22 - Line Pressure Versus
Output Shaft RPM
LINE COASTING
BOOST VALVE
The line coasting boost valve is
provided to boost line pressure un-
der light throttle or closed throttle
driving conditions in 2, 1 or R
ranges. The boosted line pressure
controls the 2-1 downshift point in
1 range.
Primary T.V. pressure is delivered
to the end of the line coasting boost
valve and to the upper valley of the
pressure booster valve. 1-R or 2 oil
pressure is directed from the manual
valve through a ball shuttle valve to
the line coasting boost valve. Under
throttle off conditions, the force
created by 1-R or 2 oil acting on
the area differential will be directed
through a ball shuttle valve to the
end of the pressure booster valve.
The output of the coasting boost
valve decreases with an increase in
primary T.V. pressure. Primary T.V.
pressure acting on the pressure
booster valve tends to increase line
pressure, however coasting boost
pressure is decreasing, resulting in a
slight reduction in regulated line
pressure from 20" to
s·
manifold
vacuum at Zero or low vehicle
speeds. Below 8" manifold vacuum,
the output pressure of the coasting
boost valve will connect to primary
T.V. pressure from the cutback
valve, which will increase regulated
line pressure. At high road speeds
(after cutback) the output of the
coasting boost valve will decrease
from 20" through
o·
manifold
vacuum which slightly decreases
regulated line pressure.
DOWNSHIFT VALVE
(fn
1, R and 2 Ranges)
1-R or 2 oil pressure is delivered
to the downshift valve. The dif-
ference in diameter between the
lands provides an area differential
for regulation. This pressure will
be referred to later as regulated
downshift valve oil pressure and is
used primarily to control the 2-1
downshift point in 1 range. When
the throttle is opened through de-
tent the downshift valve is forced to
the right against spring pressure.
Boosted throttle pressure (T.V. pres·
sure on 250 2V and 302) now re·
places regulated downshift pressure
and is directed to the 2-3 and 1-2
shift valves.
(In D Range)
Because there is no control (line)
pressure feed to the downshift valve
in this range it is ineffective until the
throttle is opened through detent.
At this point boosted throttle pres-
sure (T.V. pressure on 250 2V and
302) enters the valve and is directed
to the 2-3 and 1-2 shift valves to
give a 3-2 or 2-1 kickdown shift
depending on road speed.
1-2 SHIFT VALVE TRAIN
AND
2-3
SHIFT VALVE
The 1-2 shift valve train is com-
posed of the 1-2 shift valve, 02
valve, and the 1-2 shift valve spring.
Operation of the 1-2 shift valve
train, 2-3 shift valve, and the down-
shift valve in the various modes is
as follows
:
(D Range: 1-2 Upshifts and
2-1
Downshifts)
In 0 range the 1-2 shift valve is
held closed (up)
by
modulated
throttle pressure acting on the dif-
ferential area between the two lands
of the 1-2 shift valve, by 0 oil pres-
sure acting on the differential in
area between the two lands at the
spring end of the 02 valve, and by
the 1-2 shift valve spring. Governor
pressure tends to move the 1-2 shift
valve train against these forces.
When force created by governor
pressure exceeds the forces holding
the 1-2 shift valve train closed, the
1-2 shift valve and 02 valve will be
opened (moved downward), closing
the exhaust port and allowing 0 oil
to pass through the 02 valve to
accomplish the 1-2 shift. When the
02 valve is moved downward 0
oil is exhausted from the differential
in areas provided by the lower two
lands of the 02 valve. This action
eliminates the force created by 0
oil which tends to hold the 1-2 shift
valve train closed.
If governor pressure is reduced to
the point where spring force and
modulated throttle pressure force
exceeds governor pressure force, the
1-2 shift valve train will move up
(close) cutting off the flow of 0 oil
through the valve and opening the
exhaust port allowing a downshift
to low gear.
If the throttle is open to the point
where modulated throttle pressure
acting on the 1-2 shift valve plus the
1-2 shift valve spring force creates a
force greater than that provided by
governor pressure, the 1-2 shift
valve train will be closed, providing
a torque demand downshift to low.
If the throttle is open through
detent, the downshift valve moves
to allow boosted throttle pressure
(T.V. pressure on 250 2V & 302)
to enter the modulated throttle
pressure passage at the 1-2 shift
valve to provide a forced 2-1
downshift.
(D Range:
2-3
Upshifts and
3-2 Downshifts)
The 2-3 shift valve is held closed
(up) by throttle modulator valve
spring force, modulated throttle
pressure force, and by line pressure
force acting on the differential in
area of the lands of the valve to
which is it is delivered.
Governor pressure tends to open
the 2-3 shift valve. When force
created by governor pressure ex-
ceeds the forces holding the valve
closed (up), the valve will move
downward allowing 0 oil .Pressure
to apply the direct clutch and
release the intermediate servo to
accomplish the 2-3 shift.
With the shift valve open (down)
the throttle modulator valve is held
down cutting off modulated throttle
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