Pontiac FIREBIRD 1972 Service Manual page 1054

7G- 16 1972 PONTIAC SERVICE MANUAL
FUNCTIONS OF VALVES AND
HYDRAULIC CONTROL UNITS
PRESSURE CONTROL
The transmission is controlled automatically by a hydrau-
lic system. Hydraulic pressure is supplied by the transmis-
sion oil pump, (Fig. 7G-13) which is engine driven. Main
line pressure is controlled by a pressure regulator valve
train and by the vacuum modulator which is connected to
engine vacuum. The pressure regulator valve train con-
trols line pressure automatically, in response to a pressure
signal from a modulator valve, in such a way that the
torque requirements of the transmission clutches are met
and proper shift spacing is obtained at all throttle open-
ings. To control line pressure properly, a modulator pres-
sure is used which varies in the same manner as torque
input to the transmission. Modulator pressure is regulated
by engine vacuum which is an indicator of engine torque
and carburetor opening.
VACUUM MODULATOR ASSEMBLY
The engine vacuum signal is provided by the vacuum
modulator, which consists of an evacuated metal bellows,
a diaphragm and twcj springs (Fig. 7G-14). These are so
arranged that, when installed, the bellows and its external
spring apply a force which acts on the modulator valve.
This force acts on the modulator valve so that it increases
modulator pressure. Engine vacuum and the other spring
acts in the opposite direction to decrease modulator. That
is, low engine vacuum creates high modulator pressure;
high engine vacuum, low modulator pressure. To reduce
the effect of engine power loss at high altitudes on shift
points, the effective area of the diaphragm is made some-
what larger than that of the bellows. Atmospheric pres-
sure then acts on the resulting differential area to reduce
modulator pressure.
GOVERNOR ASSEMBLY
The vehicle speed signal to the modulator valve is supplied
by the transmission governor, which is driven by the out-
put shaft. The governor consists of a pair of dual weights
and a regulator valve (Fig. 7G- 15).
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Fig. 7G-13 Transmission Oil Pump
Fig. 7G-14 Vacuum Modulator Assembly
As the car begins to move, the weight assemblies move
outward to provide a regulating force against the valve
through the springs between the primary and secondary
weights. As car speed is further increased, regulating force
against the valve is provided by the secondary weights
moving outward. At approximately 22 MPH, the primary
weights have reached the limit of their travel and the force
against the valve is then entirely through the secondary
weights.
Thus, governor valve pressure is determined at very low
speeds by the primary and secondary weights and at
higher speeds by the secondary weights plus the force of
the springs between the weights. In this manner, governor
pressure is increased rapidly but smoother from very low
speeds to approximately 22 MPH, where it increases at a
slower rate.
PRESSURE REGULATOR VALVE
The pressure regulator valve regulates line pressure ac-
cording to a fixed spring force and forces controlled by
SCREEN
DRIVE
+
- - -
GOVERNOR
SCREEN
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Fig. 7G-15 Governor Assembly