Buick LeSabre 1965 Service Manual page 233

SUPER TURBINE "300'
HYDRAULIC CONTROLS 5 - 1 9
2. Downshift from drive to low
When detent pressure against the
shift regulator valve in combina-
tion with the spring reaches a
value sufficient to overcome gov-
ernor valve pressure against the
first land of the shift valve, both
valves move to the shift valve end
of the bore and the transmission
is downshifted by exhausting oil
under pressure to the high clutch
and spring side of the low servo
piston. See Figure 5-20.
3. Manual Low
With the manual shift control
valve positioned in low (L) range
oil under pressure is directed to
the space between the shift valve
and the shift regulator valve. Oil
under pressure in this space
moves the shift valve to the end
of its bore. With the shift valve
thus positioned no oil under pres-
sure is directed to the high clutch
piston or spring side of the low
servo piston, thus the low band
is applied and the transmission
is in low range. See Figure 5-22.
d. Vacuum Modulator and
Valve
The vacuum modulator and valve
assembly is a device to translate
load (engine manifold vacuum),
barometric pressure (altitude) and
speed (governor valve oil pres-
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Figure 5-22—Manual Low
sure) into modulated oil pres-
sures to regulate main line oil
pressure at an efficient value.
Main line oil enters the v a l v e
between the first and second lands
of the valve, flows through the
drilled ports to the space between
the first land and the valve body.
Here, the oil when it reaches
sufficient pressure moves the
valve against its spring to regu-
late the exit oil (called modulator
oil).
1. Manifold vacuum effect
Figure 5-21-Up-Shifted
The modulator valve spring is
housed in a sealed container in
such a way that engine manifold
vacuum may act upon it to reduce
the force of the spring against the
valve and thus affect modulator
oil pressure. Conditions of load
or grade that lower manifold vac-
uum increase modulator oil pres-
sure, while high manifold vacuum
decreases modulator pressure.
See Figure 5-23.
2. Altitude or barometric
pressure effect
If the car is operated at high
altitudes where barometric pres-
sure is reduced the aneroid de-
vice in t h e vacuum modulator
housing expands and acts against
the valve spring to reduce modu-
Figure 5-23—Vacuum Modulator
and Valve
lator oil pressure in proportion
to the barometric pressure.
At high altitudes engine output is
reduced. Comparable reduction in
transmission main line oil pres-
sure is necessary to accomplish
s m o o t h s h i f t s u n d e r t h e s e
conditions.
3. Governor effect
As car speed increases governor
valve oil pressure increases (up
to the limit of the valve as de-
scribed in subpar. e below).
Oil at governor valve pressure
bearing on the fourth land of the
vacuum modulator valve has the
effect of reducing the spring
pressure against the valve, there-
by reducing modulator oil pres-
sure as governor pressure (car
speed) increases.
4. Modulator boost effect
With the manual shift control
valve positioned in Low (L) range,
oil at main line pressure bears
against the second land of the
modulator valve which separates
the two pieces of the valve and
tends to move the valve to the
bottom of its bore independent of
the valve spring. Thus, modulator
oil under pressure is directed to
the main line pressure regulator
valve to provide an increase in
main line oil pressure in low
range, regardless of engine vac-
uum. If driving conditions result
in low engine vacuum however,
the valve spring will move the
two sections of the valve back