pressure force is greater than the throttle pressure
valve against the valve spring. The valve moves against
force, the shift valve will move, uncovering the clutch
the spring until the vent (return) port is opened enough
port and at the same time closing the servo pressure
to balance the forces acting against the reaction area
port. This causes the clutch to actuate and place the
land and the spring tension acting on the other end of
transmission in an up shift position. Both the servo and
clutch will have some method of venting trapped oil
The regulated oil pressure from the throttle valve is
back to the sump. The method of exhausting the
directed to the spring-loaded end of the shift valve.
trapped fluid varies with the many manufacturers.
Thus, the throttle oil pressure adds to the shift valve
Some use ball-check valves, others use metered
spring pressure to hold the shift valve in the servo
orifices, and still others cut grooves around the valve
position. This action is opposed by the governor oil
lands for oil return. (Notice the servo port land in figure
pressure. Therefore, an increased throttle pressure
2-69 for an illustration of a grooved land for oil return.)
setting requires an increased governor oil pressure
(greater vehicle speed) to cause an up shift.
It is sometimes desirable while operating in direct
The governor valve causes an upshift in the
drive (up shift), and even at full throttle, to downshift
transmission. If there was no opposing or balancing oil
pressure, the transmission would always upshift at the
so on. To accomplish this downshift, the hydraulic
same vehicle speed. However, to give the operator
control system uses a kickdown valve.
more control over the shifting of the transmission and
to lessen overloading of the engine, a throttle valve is
incorporated into the hydraulic control system (fig.
2-70). This valve directs oil pressure to oppose the
The shift valve at this point has two oil pressures
governor pressure, and thus delays upshifting.
(throttle and governor), which act to control the time of
transmission shifting according to vehicle speed and
The throttle valve is operated manually by the
throttle opening. What is needed now is a method of
operator through the accelerator pedal linkage. One
getting the required oil pressure to an area of the shift
end of the valve contains a spring and a moveable plug.
valve that will cause the shift valve to move to a
When the accelerator pedal moves the linkage, the plug
downshift position, even at full throttle. To get the
is moved and thereby varies the tension on the valve
required oil pressure and control its flow to the shift
spring. The force of the spring acting on the spool valve
valve, the hydraulic system uses what is commonly
increases with an increasing carburetor throttle
called a "kickdown valve." The kickdown valve, like
all other valves found in the automatic transmission,
In operation the throttle valve receives oil flow
can vary in design from manufacturer to manufacturer.
from the manual selector valve. Oil enters the area
It may be an integral part of the throttle valve, or a
between the large lands of the throttle valve and also
separate valve arrangement. It may use piston-type
goes into the shift valve oil passage. Oil is also allowed
valves or ball-check valves. For our purpose, a separate
to enter a reaction area at the small end of the valve. Oil
valve arrangement containing a spring and ball is used
pressure builds up in the throttle valve circuits until the
(fig. 2-71). The oil comes from the manual selector
oil pressure in the reaction area is sufficient to force the
va l ve i n t o a c h a m b e r o r h o u s i n g , w h e r e t h e
spring-loaded ball stops oil flow. The ball is connected
to the throttle linkage. The throttle linkage can unseat
the ball and allow oil flow into an oil passage leading to
the shift valve.
TO SHIFT VALVE
The throttle valve, the governor valve, and the
kickdown valve directly control the shift valve. Figure
2-72 shows how these valves function to control the
As shown in figure 2-72, the oil (when the
kickdown valve ball is unseated by the rod) is allowed
to flow from the kickdown valve through a passage to a
Figure 2-70.--Throttle valve.
chamber of the shift valve. At this time the throttle