Pontiac FIREBIRD 1972 Service Manual page 905

TURBO HYDRA-MATIC (M-40) TRANSMISSION
7E- 15
a ' ' - .
Fig. 7E-7 Typical Selector Indicator
flood, hold the accelerator pedal to the floor forcibly while
starting the engine.
RANGE SELECTION
The selector quadrant has six selector positions: P, R, N,
D, S, L (Fig. 7E-7).
Park position positively locks the output shaft to the
transmission case by means of a locking pawl to pre-
vent the vehicle from rolling in either direction. This
position should be selected whenever the driver
leaves the vehicle. The engine may be started in Park
position.
Reverse enables the vehicle to be operated in a reverse
direction.
Neutral position enables the engine to be started and
run without driving the vehicle.
Drive Range is used for all normal driving conditions
and maximum economy. Drive Range has three gear
ratios, from the starting ratio to direct drive. Detent
downshifts are available for safe passing by depress-
ing the accelerator to the floor.
Super Range adds new performance for congested
traffic or hilly terrain. Super Range has the same
starting ratio as Drive Range, but prevents the trans-
mission from shifting above second speed to retain
second speed acceleration when extra performance is
desired. Super Range can also be used for engine
braking. Super Range can be selected at any vehicle
speed and the transmission will shift to second gear
and remain in second until the vehicle speed or the
throttle are changed to obtain first gear operation in
the same manner as in Drive Range.
Lo Range can be selected at any vehicle speed and the
transmission will shift to second gear and remain in
second until vehicle speed is reduced to approxi-
mately 45 mph, depending on axle ratio. Lo Range
position prevents the transmission from shifting out
of first gear. This is particularly beneficial for main-
taining maximum engine braking when continuous
first gear operation is desirable.
HYDRAULIC SYSTEM DESCRIPTION
PRESSURE CONTROL
The transmission is automatically controlled by a hydrau-
lic system. Hydraulic pressure is supplied by the transmis-
sion gear type oil pump, which is engine driven. Main line
pressure is controlled by a pressure regulator valve train
located in the pump. This regulator controls line pressure
automatically in response to a pressure signal from a
modulator valve, in such a way that the torque require-
ments of the transmission are met and smooth shifts are
obtained at all throttle openings.
To control line pressure properly, a modulator pressure is
used which varies in the same manner as torque input to
the transmission. Since the converter torque output is the
product of engine torque and converter ratio, modulator
pressure must compensate for changes in either or both of
these.
To meet these requirements, modulator pressure is regu-
lated by engine vacuum, which is an indicator of engine
torque and carburetor opening. It is decreased by gover-
nor pressure with an increase in vehicle speed because
converter torque ratio does the same.
VACUUM MODULATOR
The engine vacuum signal is provided by the vacuum
modulator which consists of an evacuated metal bellows,
a diaphragm and springs. These are so arranged that,
when installed, the bellows and one 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 act in the opposite
direction to decrease modulator pressure; or low engine
vacuum, high modulator pressure; high engine vacuum,
low modulator pressure.
GOVERNOR
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 two sets of flyweights,
two springs and a regulator valve. Centrifugal force on the
flyweights is imposed on the regulator valve, causing it to
regulate a pressure signal that increases with increasing
speed.
Centrifugal force is proportional to the square of vehicle
speed. This means that a given change in vehicle speed