(4) When the gearshift lever is placed in
forced downshift by pushing the accelerator to the floor
drive (D) (fig. 23-21), the manual valve delivers line
(fig. 23-23). This action causes the throttle plug to
pressure directly to the low band and to the governor.
contact the shift valve spool, forcing it into the low-gear
Because the vehicle is not moving, the governor does
position. This action cuts off line pressure to the direct
not supply pressure to the shift valve and the compound
clutch and the release side of the low-band servo,
planetary gear train (para 23-9) remains in low gear.
causing the direct clutch to release as the low band
applies. This places the compound planetary gear train
(5) As the vehicle begins to move with the
(para 23-9) in low gear.
gearshift lever in drive, the transmission is in low gear,
f. Auxiliary Devices. The hypothetical hydraulic
providing the engine with the necessary mechanical
advantage to accelerate the vehicle to speed. As the
system illustrated above is a good learning tool;
vehicle speed increases, the governor pressure rises
however, to actually function, many more devices are
proportionately. When vehicle speed reaches a point
necessary. The following devices are necessary in
where the governor pressure on one side of the shift
addition to the basic operation of the hydraulic system:
valve spool can overcome the throttle spring pressure on
the other side of the shift valve spool, the spool moves to
(1) Devices called accumulators (fig. 23-
the direct or high-gear position (fig. 23-22). As the valve
24) are connected into the pressure supply lines of
spool moves, the shift valve begins to deliver line
selected planetary gearset holding elements. An
pressure to apply the direct clutch and release the low
accumulator is a spring-loaded piston that causes line
band. The low band, so that it can be released while it is
pressure to build gradually when the element is applied.
still under apply pressure, uses a double-acting servo.
This gives a cushioning effect to its application, resulting
The double-acting servo can be released by applying
in smoother shifting of the transmission.
pressure under the piston. The diameters of the piston
and the bore of the double-acting servo are stepped so
(2) Valves are installed to prevent the
that the pressure under the piston will act on a greater
transmission from being shifted into reverse during
surface area and therefore overcome apply pressure.
forward movement and to prevent a forced downshift
The application of the direct clutch and the release of the
above a predetermined speed.
low band place the compound planetary gear train (para
23-9) in high gear or direct drive. The pressure exerted
(3) A modulator (fig. 23-25) is used in
by throttle spring against the shift valve spool increases
some automatic transmissions in place of throttle
proportionately as the accelerator is depressed. This in
linkage. The modulator is a diaphragm device that uses
turn will proportionately raise the vehicle speed at which
engine manifold vacuum to indicate engine load to the
automatic upshifts occur.
(6) If a situation arises when quick
To get an idea of how complicated the hydraulic system
acceleration is needed, the operator can initiate a
really is, a schematic view of an actual hydraulic system
is shown in figure 23-26.