Opel 1900 1973, Service Manual, Page: 415 / 625

54 1973 OPEL SERVICE MANUAL
Figure 1 2 Shift Valve
To prevent a “hunting” condition of the shift valve,
modulator pressure supply to the second spool of
control valve is cut off as the shift valve opens line
pressure to the second clutch. The oil in this pocket
is exhausted out through the detent passage. An ad-
ditional force keeping the valve in an “upshifted”
position is obtained by line pressure acting on the
larger diameter second spool of the shift valve. Be-
cause of this, even though the governor pressure
might be maintained at a constant pressure after the
valve upshifts, a higher modulator pressure is re-
quired to cause the valve to downshift.
If the accelerator is depressed to the point where the
detent spring force is felt, the vacuum will drop and
the modulator pressure will increase. If the spring
force plus the modulator pressure acting against the
end spool of the shift control valve is great enough
to overcome the governor and line pressure acting on
the shift valve, a “part throttle” forced downshift
will occur. If not, the transmission will remain in the
higher gear.
If the accelerator is depressed through the detent, the
detent valve supplies detent regulator pressure to all
three spools of the shift control valve, a higher down-
shifting force is obtained as compared to the part
throttle condition. Because of this, a “through
detent” forced downshift can be obtained at a speed
higher than for the “part throttle” condition. How-
ever, there is still a limiting speed at which a
“through detent” forced downshift will occur.
If the selector lever is placed in “Manual Low”, line
pressure is supplied directly to the spring pocket
between the valves. Since line pressure can never be
less than governor pressure, the force established by
line pressure on the shift valve plus the spring force
will move the shift valve to a downshifted position
regardless of car speed.
2 3 Valve and 3 2 Control Valve
The function and operation of the 2 3 Shift and
Shift Control Valves is the same as for the 2 valve
except as described below. See Figure
The downshifted position establishes “second” gear,
and the upshifted position establishes “third” or
“high” gear.
Figure 2 3 Shift Valve and 3 2 Control Valve
Modulator pressure is supplied to the end spool of
the 2 3 control valve through the 3 2 control valve.
When the shift valve moves to the upshifted position,
line pressure is introduced to the third clutch circuit.
The third clutch circuit also directs pressure to the
end spool of the 3 2 control valve.
At light throttle conditions, third clutch pressure
acting on the end of the 3 2 control valve moves the
valve against the spring and the force established by
the modulator pressure. This exhausts the modulator
pressure from behind the end spool of the 2 3
control valve and the spring is the only remaining
force acting on the shift valve to produce a down-
shift. In this condition, it is not possible to obtain a
“part throttle” forced downshift.
If the accelerator is depressed far enough to cause a
substantial drop in vacuum, the increased modulator
pressure on the 3 2 control valve plus the spring will
the force of the third clutch pressure. This
feeds modulator pressure back to the 2 3 control
valve and a “part throttle” forced downshift will
occur. As with the 1 2 shift valve, there is a limiting
speed at which this can occur.
When the selector lever is placed in “Second”, line
pressure is directed to the spring pocket between the
2 3 Shift and Shift Control Valves and the shift