Opel 1900 1973, Service Manual

Page: 418 / 625

AUTOMATIC TRANSMISSION
ACCUMULATOR
Figure 1-2 Accumulator
l-2
Accumulator
In order to obtain a smooth 1-2 upshift, it is neces-
sary to control the rate of pressure build-up in the
second clutch. This is accomplished by introducing
a hydraulic modulator in the clutch circuit. Prior to
the second clutch being applied, the spring holds the
accumulator piston in an upward position. When
line pressure is introduced to the second clutch cir-
cuit by way of the 1-2 shift valve, the pressure in the
second clutch will be permitted to build up rapidly
until the clutch pressure acting on the accumulator
piston is sufficient to overcome the accumulator
spring. As the clutch pressure increases on the top
side of the accumulator piston, the piston will start
to move against the spring force until the piston is
forced all the way down. This provides a time delay
for the apply of the second clutch before the pressure
reaches its maximum value. When upshifts are made
at heavier throttle, it is necessary to increase the
pressure at which this time delay occurs. This is
accomplished by introducing a vacuum sensitive
pressure from the 1-2 accumulator valve to the bot-
tom side of the accumulator piston. This pressure
assists the spring and the clutch pressure will have to
build up to a higher value before the accumulator
piston will move. See Figure
SERVO
Figure Low Servo
Low Servo
The low servo provides a dual function. See Figure
It is the means by which the band is applied
and it provides an accumulator action for the third
clutch during a 2-3 upshift.
In first and second gear, servo apply pressure acting
on the bottom side of the low servo piston moves the
piston against the spring force and applies the band.
During a 2-3 upshift, third clutch pressure is intro-
duced to the top side of the servo piston. When third
clutch pressure, acting on the top side of the piston
and assisted by the servo release spring, is sufficient
to
the servo apply force, the servo will
downward. This removes the apply force from
the band and the band will release while the clutch
is being applied. During the downward movement of
the piston, a time interval occurs which cushions the
apply of the third clutch in the same manner that the
1-2 accumulator cushions the apply of the second
clutch. It is because of this accumulator action that
the band adjustment is so critical. The main function
of the band adjustment is to control the servo release
spring load rather than to control band clearance.