Pontiac FIREBIRD 1972 Service Manual page 1002

TWO SPEED AUTOMATIC (M-35) TRANSMISSION
7F- 15
blind cavity at the left end of the valve. Oil pressure in this
area tends to oppose spring pressure and tries to move the
modulator valve to the right. At the same time, mainline
pressure flows from the area between spools 1 and 2 of the
modulator valve into the booster valve passage where this
mainline pressure is directed to the right end of the booster
valve. Mainline pressure applies force to the left on the
small spool rear face of the booster valve. This modulator
pressure force, combined with the normal spring force on
the pressure regulator valve, results in a higher mainline
pressure.
Higher vacuum moves the modulator valve further to the
right which shuts off the source of mainline pressure to the
booster valve. Continued motion to the right uncovers a
modulator exhaust passage for the booster valve oil. The
exhaust port of this passage is located in the manual valve
bore of the valve body. This will be covered in detail later.
The modulator valve is supplied with governor pressure in
the area between spools 2 and 3. Since spool 3 is larger
than spool 2, governor pressure tends to move the modula-
tor valve to the right, reducing modulator pressure to the
booster valve. This application of governor pressure re-
sults in smoother part-throttle upshifts by reducing main-
line pressure.
The action of the vacuum modulator helps tailor mainline
pressure to meet changing requirements. It does this by
varying booster pressure in relation to modulator spring
pressure, engine vacuum, governor pressure and mainline
pressure itself.
GOVERNOR CIRCUIT
With the vehicle moving and increasing in speed, mainline
pressure is directed to the governor where it is re-
regulated and delivered to the low-drive shift valve, the
vacuum modulator valve and the rear face of the down-
shift timing valve (Fig. 7F-24). As the governor rotates
with the output shaft, the inner and outer weights are
thrown outward due to centrifugal force. The governor
valve, which is connected by a small through-shaft to the
weights, is pulled inward to open the mainline delivery
port. Mainline pressure enters the open port and becomes
I
GOVERNOR
I
Fig.
7F-24 Governor Circuitry
EXHAUST
I
n
Fig. 7 F - 2 5 Throttle Valve Circuitry
governor pressure. This pressure, while acting on the vari-
ous control valves, applies an outward force on the flat
area of the large spool of the governor valve. When this
governor pressure overcomes the centrifugal force of the
weights, the governor weight is moved outward and closes
the mainline feed to the governor circuit.
At lower speeds, when centrifugal force is reduced, gover-
nor pressure moves the governor valve further outward,
opposing the pull of the weights, to open this governor
pressure to exhaust. As governor pressure drops, the pull
of the weights returns the valve inward and regulates gov-
ernor pressure in direct relation to road speed.
THROTTLE VALVE CIRCUIT
With the selector lever in Drive range, throttle valve (TV)
pressure is used to provide a wide range of automatic shift
points to match the transmission ratio to varying driving
conditions. Throttle valve (TV) pressure aids spring pres-
sure to oppose governor pressure acting on the low-drive
shift valve (Fig. 7F-25). If there was no throttle pressure
acting on the low-drive shift valve, the upshift would al-
ways occur at the same road speed, that is, when governor
pressure overcomes spring pressure on the low-drive shift
valve.
Because the throttle (TV) valve and the detent valve are
connected by linkage to the accelerator, the effect of dif-
ferent accelerator positions directly controls TV and de-
tent pressure. Depressing the accelerator pedal causes the
throttle valve plunger to increase the spring force on the
TV valve, moving it to the right. This opens mainline
pressure to the area between the spools of the TV valve.
At this point, the pressure enters the TV pressure passage
and also passes through a small hole drilled in the TV
valve and fills the blind cavity at the right end of the valve.
The pressure in'the TV passage is delivered to the detent
valve and to the low-drive shift valve. TV pressure aids
spring pressure to oppose governor pressure acting on the
low-drive shift valve. As hydraulic force builds in the TV
circuit, the pressure acting on the right end of the TV
valve equals or overcomes spring pressure and the TV
valve moves to the left, closing the mainline pressure port.