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7-56
GROUP 7-AUTOMA TIC TRANSMISSION
pressure to the
2-3 shift
valve and
1-2 shift valve. In addition, the port
which delivered line pressure to the
qifferential
in
area of the shift valve
lands, is
closed.
The shift valve will be
reopened
(moved up) causing a
3-2
downshift
under one or more of the following
conditions:
GOVERNOR PRESSURE
REDUCED
If governor pressure
is
reduced to
the point where
it
can no
longer
hold the shift
valve
down against
spring force and
T
.V. pressure force,
the valve wiU move up causing a
downshift. Under closed throttle
conditions, the 2-3 shift valve will
close at approximately 10 mph
(speed at which governor pressure
is cut off). Since governor pressure
is cut off at this speed the 1-2 shift
valve train also
closes
at the same
time. This will provide a 3-1 down-
shift when coasting in D range.
THROTTLE PRESSURE
INCREASED
If throttle pressure is increased
sufficiently, it will move the throttle
modulator valve and consequently
the 2-3 shift valve up, causing
a
3-2
torque demand downshift.
THROTTLE PRESSURE
INTRODUCED BELOW
2-3 SHIFT VALVE
If the downshift valve is moved
through detent, boosted throttle
pressure
(T.V.
pressure on
250 2V
&
302) is directed
to
the underside
of the 2-3 shift valve, forcing the
valve up and causing a forced 3-2
downshift. Maximum
3-2 forced
downshift
speed is controlled
by
governor pressure.
(D to 2 Range)
If the manual vafve is moved to
2 range, the force created by 2 oil
pressure acting on the area differen-
tial of the 2-3 shift valve and on
the spring end of the valve plus the
force of the throttle modulator valve
spring will exceed the force created
by governor pressure. The 2-3 shift
valve will then close (move up)
allowing the direct clutch apply-
intermediate servo release oil to
exhaust, permitting the
intermediate
band to apply causing a 3-2 down-
shift. In addition, D2 oil pressure is
introduced between the 1-2 shift
valve and D2 valve, and regulated
downshift valve oil pressure is intro-
duced between
the
lands of the 1-2
shift valve. This opens (moves
down) the D-2 valve and holds it
open and closes (moves up) the 1-2
shift valve and holds it closed; thus
preventing a 2-1 downshift. The
transmission will remain in inter-
mediate or second gear until an-
other mode of operation is selected.
(D to 1 Range)
If the manual valve is moved to I
range
at
vehicle speeds greater than
approximately 39 mph, a 3-2 down-
shift will be accomplished in the
same manner as in 2 range, except
that 1-R oil pressure instead of 2
oil pressure is directed to the 2-3
shift valve.
At vehicle speeds of approxi-
mately 39 mph or lower, the force
created
by
regulated downshift
valve pressure acting on the area
differential of the 1-2 shift valve
plus the force of the 1-2 shift valve
spring will exceed governor pres-
sure force holding the 1-2 shift
va~ve
train open (down). The 1-2
shift valve train will then close
(move up), exhausting intermediate
servo apply and allowing 1-R oil
pressure to pass through the D2
valve to the spring end of the D2
valve,
and to the reverse and low
band via the low
servo
modulator
valve. The transmission will remain
in low or first gear until another
mode of operation is selected.
(2 Range)
In 2 range, 2 oil pressure is intro-
duced between the 1-2 shift valve
and D-2 valve, and regulated down-
shift valve oil pressure is introduced
between the lands of the 1-2 shift
valve. This action opens the D2
valve and holds it open and closes
the 1-2 shift valve and holds it
closed providing a second gear start
and preventing a 2-1 downshift.
The transmission will remain in
intermediate or second gear until
another mode of operation is
selected.
(1 Range)
In 1 range, regulated downshift
valve pressures enter the modulated
throttle pressure passage to provide
a manual low downshift to first
gear. Once the transmission is in
low gear 1-R oil pressure, which is
directed to the D2 valve, passes
through the D2 valve and is
delivered to the spring end of the
D2 valve, preventing an upshift.
1-R oil pressure which passes
through the D2 valve also applies
the reverse and low servo via the
low servo modulator valve.
(R Range)
In reverse 1-R oil pressure
is
directed to and through the D2
valve to the spring end of the D2
valve and to the reverse and low
servo applying the reverse and low
band. The force created by 1-R oil
pressure on the spring end of the
D2 valve and the force created by
regulated downshift valve oil pres-
sure on the area differential of the
1-2 shift valve is added to the force
of the 1-2 shift valve spring,
preventing any movement of the 1-2
shift valve train
regardless
of
governor pressure.
R oil pressure is directed to and
through the 2-3 shift valve to the
reverse-direct clutch applying the
clutch and to the release side of the
intermediate servo. The force created
by regulated downshift valve oil
pressure on the spring end of the 2-3
shift valve, and 1-R oil pressure on
the area differential of the lands
adjacent to the top valley of the 2-3
shift valve is added to the force of
the throttle modulator valve
spring,
preventing any movement of the 2-3
shift valve regardless of governor
pressure.
2-3
.
BACK-OUT VALVE
The.
purpose of the 2-3 back-out
valve is to provide smooth upshifts,
when the throttle is suddenly closed
while accelerating in second gear.
Operation is as follows
:
Normal Throttle-On 2-3 Upshifts
When the 2-3 shift valve moves to
cause a 2-3 upshift, D oil pressure
passes through the valve to apply
the direct clutch and release the
intermediate servo. This same pres-
sure is also directed to the end of
the 2-3 back-out valve. However,
with throttle open, T.V. boost
pressure (T.V. pressure on
250 2V
&
302) on the opposite end of the
2-3 back-out valve, assists spring
force in holding the valve up, so
that there will be no valve move-
ment until after the 2-3 shift has
been completed.
Back-Out 2-3 Upshifts
When the throttle is closed during
a 2-3 upshift, and before the shift is
completed, there may be enough
pressure in the direct clutch cylinder
to apply the clutch at the reduced
engine torque input, but not enough
pressure to release the intermediate
servo. This condition could cause a
harsh 2-3 shift. However, if the
throttle is closed during a 2-3 shift,
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