TRANSMISSION HYDRAULIC CONTROLS
NEUTRAL WITH ENGINE RUNNING
When the engine is started and the selector valve
is in NEUTRAL, oil from the pump goes to main
relief valve (4). around relief valve (20), to the
selector valve group. Relief valve (20) lets some oil
go to cooler (5). Oil from the cooler goes behind
the cooler bypass valve (24) and into a cavity for
clutch and planetary gear lubrication.
At the selector valve group, the flow of oil
divides into two separate circuits: signal pressure
circuit and system pressure circuit.
Signal Pressure Circuit
As the engine is started, spring force holds the
priority reducing valve (29) so the signal pressure
passage to the pilot selector valve (16) is the first
circuit to get pressure. As the pressure becomes
higher than the spring force, the valve moves to the
right and opens the system pressure passage. The
valve begins to move when the signal pressure
becomes 83 psi (570 kPa). When the signal pressure
becomes 96 psi (660 kPa), the valve has moved to
the right far enough to not let any more pressure
into the signal pressure passage to the pilot selector
The pilot selector valve (16) has seven positions.
When the control lever is in the NEUTRAL
position, the valve sends signal pressure to passages
(A) and (F).
The function of the signal pressure circuit is to
control the movement of the speed selector spools.
The signal pressure in passages (A) and (F) moves
speed selector spool (35) to the left and speed
selector spool (36) to the right.
System Pressure Circuit
After the pressure becomes 83 psi (570 kPa) in
the signal pressure circuit, priority reducing valve
(29) opens the passage to the modulating reducing
valve (31). The system pressure moves the modu-
lating reducing valve (31) to the left until speed
clutch pressure (P1) is 90 psi (620 kPa). Since pilot
pressure is holding spool (35) and (36) in the
system pressure goes to the
passage for the No. 7 clutch (37) and to the
passage for the No. 3 clutch (38).
With the speed selector spool (35) held in the
position shown by the signal pressure in passage
(A), a passage (27) is opened by the speed selector
spool (35). This open passage lets pressure oil on
the left end of the differential, check, and safety
spool (15) go to the reservoir and prevents the
spool (15) from moving to the right. Because the
differential, check, and safety spool (15) can not
move to the right, the direction clutches have no
pressure oil and will not engage.
If the engine is started in any speed except
NEUTRAL, system pressure is sent from the speed
selector valve (35) through the orifices in the
differential, check, and safety valve (15) to the
spring side of the valve (15) holding the valve
against its stop. In this position, the direction
clutches can not get pressure oil. When the selector
lever is moved to the NEUTRAL position, the oil
in the spring side of the valve is free to go to the
transmission reservoir. The oil pressure on the left
side of the valve then works against the spring
force and the differential, check, and safety valve
(15) moves to the NEUTRAL position. In this
position, the direction clutches have no pressure
and will not engage.