Copyright by Goodheart-Willcox Co., Inc.
194 Automatic Transmissions and Transaxles
Pressure regulator valve
Pressure regulator valve spring
Reverse boost valve
Reverse boost valve sleeve
TV boost valve
TV boost bushing
Oil pump reverse boost valve
retaining ring
Cover, pump
Figure 9-10. The main pressure regulator valve is often installed
in the pump housing. Some regulators are installed in the valve
body or a separate auxiliary body. (General Motors)
pressures established by the other transmission valves act on
the ends of each shift valve. In addition, a spring located at
one end of the shift valve puts a preload on the valve.
Hydraulic pressure acting on one end of the valve works
against hydraulic pressure plus spring pressure acting on the
other end. The valve moves back and forth in the bore as
pressures on each end change. When pressures on each end
are equal, the valve is balanced and does not move.
Line pressure is directed to the center of the shift valve. It
is used to apply a planetary holding member. As the passage-
way leading to the holding member is uncovered by movement
of the shift valve, line pressure will pass through the valve’s
annular groove and be routed to the holding member.
Every transmission will have one fewer shift valve than
it has forward gears. For example, a 3-speed transmission
will have two shift valves, a 4-speed transmission will have
three, and so on
The sequence of shift valve operation must be care-
fully controlled so the transmission upshifts through all
available gears. This is done during design by proper sizing
of the springs and valve lands. By designing lands of differ-
ent sizes, or different effective face areas, hydraulic forces
on valves acted on by the same pressure will differ.
Throttle Valve
The throttle valve’s role, in effect, is to sense how hard
the engine is working and to delay upshifting, as necessary.
The throttle valve may be located in the valve body, or it
may be installed in a separate bore in the transmission case.
Movement of the throttle valve modifies line pressure.
The resulting pressure, or throttle pressure, is then trans-
mitted to the shift valve (and a few other valves) at some
pressure less than line pressure. Throttle pressure modu-
lates, or changes, with engine load. As load increases,
throttle pressure in the line to the shift valve increases,
providing greater opposition to the counter (governor) pres-
sure and delaying the upshift. The throttle valve may be
operated by the throttle valve linkage or by the vacuum
modulator.
Throttle Valve Linkage
The throttle valve (TV) linkage is a mechanical con-
nection between the throttle valve in the transmission and
the throttle lever on the vehicle’s throttle body or carburetor,
Figure 9-13. The throttle lever, in turn, is connected to the
throttle plate(s), which are also located in the throttle body or
carburetor. Engine power increases with the opening of the
throttle plate and decreases with its closing.
The TV linkage consists of several levers connected by
a series of rods or a cable. Movement of the vehicle’s accel-
erator causes the throttle lever (and throttle plate) to move
via the throttle linkage, which connects the throttle lever to
the accelerator. The movement is transferred from the
throttle lever to the throttle valve.
The action of the linkage on the throttle valve causes TV
output pressure to change. Throttle pressure, then, varies as
the throttle plate opening is varied, increasing as the throttle
plate is opened and decreasing as the throttle plate is closed.
TV linkage always has some provision for adjustment.
The rod-type linkage is adjusted at either the throttle lever
or where the throttle rod enters the transmission case. The
cable-type linkage is usually adjusted at the throttle lever.
Vacuum Modulator
The vacuum modulator, discussed in Chapter 6,
moves the throttle valve with changes in engine load, caus-
ing the throttle pressure to vary. The basic modulator is a