Opel 1900 1973, Service Manual

Page: 416 / 625

AUTOMATIC TRANSMISSION 55
valve will be held in downshifted, or second gear,
condition regardless of car speed.
Manual Low and Reverse Control Valve
As described in the text on “Mechanical Operation”,
the third clutch is applied in manual “Low” and in
“Reverse” to prevent a free wheeling condition. In
“Drive” range third gear, third clutch pressure is
also directed to the release side of the low servo (to
be covered later). This is the pressure which causes
the low band to release during a 2-3
How-
ever, in manual low, the band must remain applied
even though the third clutch is on.
The above conditions are achieved by routing third
clutch pressure to the release side of the low servo
through the manual low and reverse control valve.
See Figure In “Drive” range, the spring holds
the valve in its “bottomed” position and permits the
third clutch pressure to be directed to the servo re-
lease circuit.
SERVO RELEA
LOW CONTROL
Figure Manual L
OW
When the selector lever is placed in manual “Low”,
line pressure is introduced between the manual low
and reverse control valves. This forces the low con-
trol valve over against the spring. In this position,
third clutch pressure is cut off from servo release and
servo release is opened to exhaust. The third clutch
exhaust passage is
open to detent regulator pres-
sure which applies the third clutch since the shift
valve is in the “downshifted” position. Because the
servo release passage is open to exhaust, the low band
will remain applied.
When the selector lever is placed in “Reverse,” line
pressure acts on the end of the reverse control valve
and forces the low control valve into the same posi-
tion as in manual “Low”. This causes the third
clutch to be applied.
Boost Control Valve
To obtain the required pressure increase previously
described for “Second” “Low” and “Reverse”, line
pressure is introduced to the stepped area between
the two spools of the pressure regulator boost valve.
In “Second” or “Low”, some means has to be prov-
ided to prevent the pressure to the boost valve from
being exhausted through the reverse passage. It is
also necessary to prevent reverse pressure from being
exhausted through the intermediate passage when in
“Reverse”.
The boost control valve consists of a steel ball in a
and pressure sensitive chamber. See Figure
41. Where the reverse passage is pressurized. the
pressure and flow seat the ball against the intermedi-
ate passage and the pressure is directed to the boost
passage. In “Second” or manual “Low”, the ball is
seated against the reverse passage and the pressure is
directed to the boost passage.
Figure Boost Control Valve
Low Speed Downshift Timing Valve
When the vehicle is coasting to a stop, and a
downshift takes place, it is necessary to delay the
application of the band while the third clutch is
being released. This is accomplished by directing
the servo release pressure through the coast down-
shift timing valve. See Figure