Copyright by Goodheart-Willcox Co., Inc.
196 Automatic Transmissions and Transaxles
Throttle
lever
Valve body
TV link
TV lever
Throttle
cable
Throttle body
Figure 9-13. The throttle valve on many automatic trans-
missions is operated by a cable from the engine throttle plate.
Changes in throttle plate position change the position of the
throttle valve and, therefore, the throttle pressure. Throttle
pressure is sent to the shift valves, pressure regulator valve,
and sometimes, other valves. (General Motors)
Clip-on modulator
Vacuum
chamber
Diaphragm
Vacuum
connection
Bellows
Plunger
Sleeve
Screw-in modulator
Plunger
Transmission
case
Modular valve
sleeve
Modulator
valve
Gasket
Vacuum
modulator
Figure 9-14. Two different kinds of vacuum modulators are
shown here. Both use a vacuum-operated diaphragm and spring
to affect valve position. Note that the internal bellows, which is
evacuated, and the smaller spring in the one design are used to
compensate for power loss that occurs at reduced atmospheric
pressures, such as at higher elevations. (General Motors, Fiat)
sealed container with an internal, vacuum-operated dia-
phragm. The modulator is threaded into the transmis-
sion case or is held to the case with a bolt or clip,
Figure 9-14.
The diaphragm divides the container into two regions,
sealing one off from the other. One side of the modulator is
connected to engine vacuum by tubing connected to the
intake manifold; the other side is at atmospheric pressure.
In operation, the vacuum side of the modulator contains
a spring that pushes on the diaphragm, opposing manifold
vacuum. Changes in manifold vacuum cause the diaphragm,
working against spring pressure, to move back and forth. The
other side of the diaphragm is connected to the throttle valve,
either directly or through a plunger. The throttle valve is
moved by the diaphragm as engine vacuum changes.
Engine vacuum will vary with changes in throttle
opening and engine load. If the load is high, the manifold
vacuum will be low and throttle pressure will be high. If
engine load is low, vacuum will be high and throttle pres-
sure will be low.
Governor Valve
The governor valve, or governor as it is often called,
senses output shaft speed (and, therefore, vehicle speed) to
help control shifting. It works with the throttle valve to
determine shift points, or the vehicle speeds at which the
shifts will occur. A typical assembly consists of a driven
gear, weights, springs, a hydraulic valve, and a shaft.
The governor, which is driven by the transmission out-
put shaft, takes line pressure at the valve and modifies it
according to vehicle speed. Greater vehicle speed increases
the governor valve output pressure, or governor pressure.
Lower vehicle speed decreases governor pressure. Governor
pressure opposes throttle pressure acting on the shift valves.
The higher of the two pressures—governor or throttle—
overrides and controls the shift.
As stated, the governor is driven by the transmission
output shaft. The assembly may be mounted on the output
shaft, or it may be mounted in the transmission case and
driven through gears similar to speedometer drive and
driven gears. Either type takes line pressure from the valve
body and modifies it according to vehicle speed.
Governors vary in design. Aside from where they are
mounted and how they are driven, they vary in other
aspects of operation. The most common type of governor
contains a valve that is moved by centrifugal force. The
governor contains at least one valve that is held in the low-
speed position by a spring or springs. The governor is also
equipped with weights that can overcome spring tension.
As the governor rotates, the weights are thrown outward by
centrifugal force. This action overcomes spring tension and
causes the valve to move. Movement of the valve partially
opens the governor port to line pressure. Governor pressure