GMC 1976 ZEO 6083 Maintenance Manual page 451

downshift at speeds below approximately 70 miles
per hour . At lower speeds, downshifts will occur at
lesser throttle openings without use of the switch .
The oil cooler is located in the right hand tank of
the radiator. The transmission is cooled by directing
oil from the converter to the radiator. Oil returning
from the radiator feeds the transmission lubrication
system .
The oil system incorporates an intake pipe and
filter assembly . The transmission fluid and filter as-
sembly should be replaced after each 12,000 miles or
12 months . In addition, in the event of a transmission
malfunction that resulted in metal shavings or clutch
plate material, the filter and fluid should be changed;
and the oil cooler and cooler lines should be flushed.
The transmission quadrant has six selector posi-
tions that enable the driver to control the operation
of the transmission under various driving conditions .
The six selector positions appear on the quadrant in
the following sequence, from left to right ; PARK-
park, R-reverse, N-neutral, D-drive, S-super,
-lo.
PARK-Park position positively locks the out-
put flange to the transmission case by means of a
locking pawl and prevents the vehicle from rolling
either forward or backward . For this reason, it is
recommended that the engine be started with trans-
mission selector lever in Park position . If it is neces-
sary to re-start the engine with vehicle rolling, place
selector lever in Neutral and start the engine.
R-Reverse enables the vehicle to be operated in
a reverse direction.
N-Neutral position enables the engine to be
started and run without driving the vehicle. It is
recommended that Neutral be used to start the en
gine only if it is necessary to re-start the engine with
the vehicle rolling. At all other times use Park .
D-Drive is used for all normal driving condi-
tions _and maximum economy. Drive has three gear
ratios from starting to direct drive. Downshifts are
available for safe passing by depressing the accelera-
tor pedal.
S-Super adds performance for congested traffic
or engine braking in hilly terrain . The Super range
has the same starting gear ratio as Drive, but pre
vents the transmission from shifting above second
speed to retain acceleration when extra performance
is desired .
L-Lo range permits operation at a lower gear
ratio, and should be used where maximum engine
braking is desired, such as in descending a steep
grade. When selector lever is moved from Drive to
PRESSURE CONTROL
Figure 2-Pressure Control
TRANSMISSION 7- 3
Lo range at normal highway speeds, the transmission
will shift to second gear and remain in second gear
until vehicle speed is reduced to approximately 45
mph . The transmission will then shift to first gear
and remain in first gear regardless of vehicle or en-
gine speed, until selector lever is moved back into
either Super or Drive position .
HYDRAULIC SYSTEM
The transmission is controlled automatically by
a hydraulic system, Figure 2. Hydraulic pressure is
supplied by the transmission oil pump, which is en
gine driven . Main line pressure is controlled by a
pressure regulator valve train located in the trans-
mission case and by the vacuum modulator which is
connected to engine vacuum . The pressure regulator
controls 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 openings and vehicle speeds .
To control line pressure properly, modulator
pressure is used which varies in the same manner as
torque input to the transmission . Since the torque
input to the clutches is the product of engine torque
and converter ratio, modulator pressure must com-
pensate for changes in either or both of these.
To meet these requirements, modulator pressure
is regulated by engine vacuum, which is an indicator
of engine torque, (Governor pressure also controls
modulator pressure). It will decrease with an in-
crease in vehicle speed to compensate for the chang-
ing converter torque ratio, by virture of the governor
pressure influence .