TRANSMISSION HYDRAULIC CONTROLS
SYSTEMS OPERATION
TM 5-3805-263-14&P-2
FIRST SPEED FORWARD
When the selector lever is moved from NEU-
TRAL to FIRST FORWARD, the pilot selector
valve (16) lets the signal pressure in passages (A)
and (F) go to the reservoir and sends signal
pressure to passages (B) and (G).
When the signal pressure in passage (F) goes to
the reservoir, the spring force on the speed selector
spool (36) moves the spool to the left and lets the
pressure oil in the passage for the No. 3 clutch (38)
go to the reservoir. The signal pressure in passage
(B) moves the speed selector spool (35) to the right
and lets the pressure oil in the passage for the No.
7 clutch (37) go to the reservoir. The No. 3 and
No. 7 clutches are now not engaged.
When the speed selector spools (18) and (35) are
m o v e d t o t h e p o s i t i o n s s h o w n b y t h e s i g n a l
pressure in passages (G) and (B), the passages to
clutch No. 5 (23) and No. 6 (30) are open to
system pressure.
With the speed selector spool (35) in the
position shown, the passage (27) from the left end
of the differential, check, and safety spool (15)
(open to the reservoir in NEUTRAL) has pressure
oil. This pressure oil moves the differential, check,
and safety valve (15) to the right and lets pressure
oil go to the direction clutches. Since the direction
selector valve (28) is in the position shown,
pressure oil will go to the passage (33) for the No.
1 clutch.
When the shift is made from NEUTRAL to
FIRST FORWARD and the No. 5 and No. 6
clutches are being filled, the spring pressure moves
the modulating reducing valve (31) to the right. As
the clutches fill, the system pressure has an
increase. The increase in pressure works against the
slug in the modulating reducing valve (31) and the
valve moves to the left to stop the increase in
pressure. With this higher pressure, the load piston
(32) moves to the right and the spring force
between the modulating reducing valve and the
load piston has an increase. The increase in spring
force moves the modulating reducing valve (31) to
the right and this lets the system pressure have an
increase. This cycle takes place again and again
until the system pressure is at a maximum. This
gradual increase in pressure is modulation. The
modulating reducing valve (31) gives modulation
for all the clutches (speed and direction).
The differential, check and safety valve (15)
causes a pressure difference of 60 psi (415 kPa)
between the P1 pressure and the P2 pressure. The
P2 pressure to the direction clutches is always 60
psi (415 kPa) less than the P1 pressure to the speed
clutches. This pressure difference lets the speed
clutches engage first. The direction clutches
engage last and take the load. When P1 pressure is
at a maximum of approximately 340 psi (2350 kPa)
(controlled by the main relief valve), the P2 pres-
sure is at approximately 280 psi (1950 kPa) with
the engine at HIGH IDLE. When the engine is at
LOW IDLE, P1 pressure is approximately 310 psi
(2120 kPa) and P2 pressure is approximately 250
psi (1720 kPa). The shift is now complete.
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