TM 5-3805-258-24-1
S Y S T E M S O P E R A T I O N
POWER TRAIN
TRANSMISSION HYDRAULIC CONTROLS
When the transmission selector lever is in NEU-
TRAL, direction selection spool (17) closes the pas-
sage between the larger orifice in valve spool (14)
and the reservoir. This lets the pressure, in the
chamber at the left end of valve spool (14), increase.
The increase in pressure moves the valve to the right
against the force of the springs. The movement of the
valve spool closes a passage from the area behind the
right end of load piston (11) and the reservoir. At this
time, pressure differential valve (14) is in the posi-
tion shown in the schematic. This lets the pressure in
the system increase.
As the pressure in the chamber at the left end of
pressure differential valve (14) increases, the valve
moves more to the right. This opens the direction
clutch circuit to pump oil. It also closes the right end
of valve (14) to the reservoir. The pressure in the
direction clutch circuit increases. The increase is felt
in the spring chamber of valve (14). When the pres-
sure in the direction clutch circuit is at its maximum,
the pressure in the spring chambers, plus the force of
the springs, moves valve (14) to the left. The valve
moves to the left until the flow of pump oil to the
direction clutch circuit is stopped. At this time, the
movement of the valve spool stops. Now the valve
spool moves to the right and then to the left, (meters)
to keep a constant pressure in the direction clutch
circuit.
Oil, from the pump, also goes to modulation relief
valve (13). It fills the chamber around the modula-
tion relief valve. The oil goes through an orifice in the
valve spool and opens the poppet valve at the left end
of the valve spool. This lets oil fill the slug chamber at
the left end of the valve spool.
When the No. 3 clutch is full of oil, the pressure in
the speed clutch circuit starts to increase. The in-
crease is felt in the slug chamber at the left end of
modulation relief valve (13). When the pressure in
the speed clutch circuit is at the initial setting of the
modulation relief valve, the modulation relief valve
moves to the right. This lets extra oil go to the torque
converter.
At the same time, pump oil also goes through an
orifice to the area between the right end of load
piston (11) and the cover on the selector and pressure
control valve body. This area is closed to the reservoir
by the position of differential valve (14). The amount
of flow to the area behind load piston (11) is restrict-
ed by the orifice. The pressure felt by the modulation
relief valve, because of the increase in pressure in the
speed clutch circuit, is also felt behind load piston
(11). The orifice in the supply passage causes the oil
to go to the area behind the load piston at a specific
rate. As the modulation relief valve moves to the
right, the load piston moves to the left. This causes
the pressure in No. 3 clutch (19) to increase
gradually.
This gradual increase in pressure is known as
modulation. The load piston moves more to the left,
against the force of its springs, until the area behind
the load piston is open to a drain passage. At this
time, modulation stops. As the oil goes out the drain
passage, oil continues to fill the area behind the load
piston. This keeps the load piston in a position with-
out any further movement.
The operation of the load piston and the modula-
tion relief valve keeps the system pressure at a con-
stant rate.
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