GMC 1976 ZEO 6083 Maintenance Manual page 452

7- 4 TRANSMISSION
Figure 3-Vacuum Modulator Assembly
VACUUM MODULATOR ASSEMBLY
The engine vacuum signal is received by the
vacuum modulator, Figure 3, which consists of an
evacuated metal bellows, a diaphragm and a spring .
These are so arranged that the bellows and spring
apply a force that acts on the modulator valve so that
it increases modulator pressure .
Engine vacuum and the enclosed spring oppose
the bellows and spring to control modulator pres-
sure .
To reduce the effect of altitude on shift points,
the effective area of the diaphragm is different than
that of the bellows. Atmospheric pressure acts on the
resulting differential area to reduce modulator pres-
sure .
GOVERNOR ASSEMBLY
The vehicle speed signal to the transmission is
supplied by the governor, which is driven by the
output flange . The governor consists of flyweights
and a regulator valve. Centrifugal force of the fly-
weights is imposed on the regulator valve, causing it
to regulate a pressure signal that increases with
speed.
Governor pressure acts on the modulator valve
to cause modulator pressure to decrease as vehicle
speed increases.
FUNCTIONS OF VALVES AND
HYDRAULIC CONTROL UNITS
Line Pressure Regulator
Regulates line pressure to satisfy engine torque .
Manual Valve
Establishes the range of transmission operation,
PARK, R, N, D, S, or L, as selected by the vehicle
operator through the manual selector lever.
Governor Assembly
Generates a speed sensitive oil pressure that in-
creases with output shaft or vehicle speed. Governor
pressure is used to control the shift points and
modulator pressure regulation .
Vacuum Modulator Valve
Provides modulator pressure that senses engine
torque and vehicle speed. The vacuum modulator is
used to vary the shift points according to throttle
opening and to raise line pressure proportional to
input torque to the transmission .
1-2 Shift Valve
Activates the 1-2 and 2-1 shifts .
1-2 Regulator Valve
Controls the flow of modulator pressure to the
1-2 shift valve to regulate the minimum shift point.
1-2 Detent Valve
Senses regulated modulator pressure tending to
hold 1-2 shift valve downshifted and provides an area
for detent pressure for detent 2-1 shifts .
2-3 Shift Valve
Activates the 2-3 and 3-2 shifts .
2-3 Modulator Valve
Senses modulator pressure to apply a variable
force that tends to hold the 2-3 shift valve down-
shifted .
3-2 Valve
Shuts off modulator pressure from acting on the
shift valves after the direct clutch has been applied.
This allows fairly heavy throttle operation in third
speed without downshifting. In third speed, detent
pressure or modulator pressure above 87 psi can be
directed to the shift valves to provide the downshift
forces .