transmission to start in low range and automatically up
prevent one pump from trying to drive the other in case
of failure. (If the check valves were not there and a
shift to a higher range at the proper time. In selecting
pump failed, the other pump would pump its oil
the drive position, the manual selector valve is moved
through the failed pump and into the sump.)
to cover the oil passage that feeds line oil pressure into
the kickdown valve chamber of the shift valve, as
Oil from the front pump is directed to the torque
shown in figure 2-75.
converter (TC) control valve. The TC valve regulates
the oil flow and pressure for the torque converter and
The shift valve will remain in the low range
the transmission lubrication circuits. Oil is also
position (held by spring tension and throttle valve oil
directed from the pump to the regulator valve and the
pressure) until the governor oil pressure is sufficient to
manual selector valve. Oil is directed from the manual
move the shift valve against the throttle valve oil
selector valve back to the primary reaction area of the
pressure. Governor oil pressure increases proportional
regulator valve. When the oil pressure is high enough
to the vehicle speed.
in the reaction area, it will move the regulator valve
When governor oil pressure is high enough, the
against the valve spring and, at the same time, vent
shift valve will move. Movement of the shift valve
some of the oil flow back to the sump. When the valve
closes the servo oil passage and uncovers the clutch oil
has positioned itself to balance the spring tension and
passage. Notice in figure 2-75 that when the clutch is
the oil pressure in the reaction area, the line oil pressure
engaged there is also a release of oil pressure applied to
is regulated.
the servo. This prevents the clutch from engaging
Figures 2-73, 2-74, 2-75, 2-76, and 2-77 show a
before the servo is completely released. With the clutch
drawing of the clutch, kickdown (low) servo band,
engaged, the transmission is operating in a 1 to 1 gear
reverse servo band, and planetary gear sets (including
ratio, or direct drive.
the reverse planetary unit). Notice in the drawings how
If, while operating in direct drive, it is desirable to
the planetary units are held or driven by use of hubs,
m a ke a f o r c e d d ow n s h i f t -- t h a t i s , g o i n t o
housings, and hollow shafts. As each operating range is
kickdown--the operator simply pushes the accelerator
discussed, take note of the power flow through the
pedal through full throttle (detent). When the throttle is
planetary units.
forced through detent, mechanical linkage moves the
kickdown valve rod, forcing the kickdown valve ball
LOW RANGE.--Low range (L) is the gear ratio
off its seat, as shown in figure 2-76.
selected by the operator when it is advisable to keep the
transmission in low gear for an extended period of
When the kickdown ball is unseated, main-line oil
time. Low range is selected manually, by placing the
pressure is directed to the kickdown chamber of the
selector lever or push button in the L position. This
shift valve. At the same time, maximum throttle oil
operating range is shown in figure 2-74.
pressure is acting against the up shift end of the shift
valve. The combined force of line pressure from the
Through linkage from the selector lever, the
kickdown valve, assisted by throttle pressure and
manual selector valve has been moved to a position that
spring force at the up shift end of the shift valve, is now
opens oil passages going to the shift valve and then to
sufficient to overcome the high governor oil pressure.
the servo. One oil passage goes to a check valve and
Therefore, the shift valve is forced into the downshift
then to the kickdown chamber of the shift valve, which
position, the clutch is released, and the low-speed
causes the shift valve to remain in a downshift position.
servo band applied. The transmission is then in gear
The other oil passage passes oil through the shift valve
reduction, or low range. When the accelerator is moved
servo passage to the low/kickdown servo unit. Thus,
to a position less than full throttle, the kickdown ball is
the transmission is in low gear and will remain in this
again seated by spring force. Oil pressure is no longer
condition as long as the selector lever is in the L range.
directed to the kickdown chamber, and once again the
Governor oil pressure is developed during this
shift valve is moved by governor pressure to an up shift
operation, but it is not able to move the shift valve
position.
against the main-line oil pressure in the kickdown
chamber. Throttle valve oil pressure is also applied to
REVERSE RANGE.--Most transmissions
the shift valve, which aids the kickdown chamber oil
operate efficiently at a regulated line pressure of 80 to
pressure in opposing the governor oil pressure.
90 psi. This is true for forward drive conditions only.
For reverse operation the working line pressure must
DRIVE RANGE.--The operator, by moving the
selector lever to the drive (D) range, allows the
be increased to handle (prevent slipping) the high
2-57
