TM-9-2520-234-35
POWER TRAIN ASSEMBLY; ALLISON MODEL XTG411-2A
TECHNICAL MANUAL; FIELD AND DEPOT MAINTENANCE MANUAL
JULY 1962
TM-9-2520-234-35 - Page 53 of 323
PAR 28-3 0
OPERATION OF HYDRAULIC SYSTEM S
CHAP 2, SEC I V
Section IV. OPERATION OF HYDRAULIC SYSTEM
28. PRELIMINARY INSTRUCTIONS
a. Hydraulic Schematic s
—
(figs. 6 through 19, and fig. 372-
fold-out 1)
(1) The hydraulic system, because o f
its importance to the operation of the power
train, should be carefully studied and under-
stood. There are 14 partial hydraulic sche-
matics in color (figs. 6 through 19) and a fold-
out hydraulic schematic (fig. 372). The color-
ed schematics show positions of valves and
moving components, and the various hydraulic
circuits as indicated by the captions.
The
fold-out schematic, in black and white, shows
the entire hydraulic system.
(2) Hydraulic schematic views (figs. 6
through 18) parallel the torque path schematic
views (figs. 22 through 34) in Section III of
this chapter. The hydraulic schematic views
illustrate what happens hydraulically, while
the torque path schematic views illustrate what
happens mechanically.
b . Explanations of Hydraulic System
(1) In the schematic views, a particu-
lar color or color variation in figs. 6 through
18, is continued in the system until it passes
through a regulator or an orifice which changes
the pressure of the oil.
(2) To avoid needless repetition, refer-
ences are made to initial explanations. To
link the hydraulic action with the correspond-
ing mechanical action, references are made
to the paragraphs explaining the mechanical
actions.
29. GENERAL INFORMATIO N
a. Torque Converter and Lockup Drive.
Regardless of gear or steer condition, drive
from the vehicle engine to the range gearing
is either hydraulic (torque converter) or me-
chanical (lockup clutch engaged).
b . Transmissio n Oi l Level s
(1) The hydraulic system is designed so
that after the vehicle engine has been operat-
ing a short time, two distinct oil levels are
established in the transmission. The oil level
in the transmission rear housing is compara-
tively low (fig. 19). The other is in the trans-
mission sump, which surrounds (but is sepa-
rated from) the range gear section in the main
transmission housing. The two levels guaran-
tee a sufficient oil reserve, while maintaining
the most efficient oil level in the operating
sections of the transmission.
(2) Two oil levels exist because ai r
pumped by the scavenge pump is discharged
into the transmission rear housing. Thi s
creates a constant air pressure on the oil in
this housing. The air pressure forces the oil
level down, pushing the oil into the reservoir,
until air can escape into the reservoir. The
upper areas of the input transfer housing and
the main transmission housing are connected.
A breather, located near the top of the input
transfer housing, allows atmospheric pressure
to be present in these housings.
30. HYDRAULIC SYSTEM FUNCTION –
NEUTRAL RANGE (fig. 6)
Note . Refer to par. 46 and fig. 22
for the torque path through the power
train.
a . In neutral range, main pressure i s
supplied only by the input pressure pump (pump
not shown in fig. 6; see fig. 372, fold-out 1).
This oil is directed to four points at all times.
These points are: the main-pressure regulator
valve, the lockup cutoff valve, the throttle and
throttle regulator valves, and the steer pres-
sure regulator valve. This part of the hydrau-
lic circuit always is charged when the power
train is operating.
b . At the main-pressure regulator valve,
oil enters the valve bore at two points. The
upper connection directs the oil between two
of the valve lands. It surrounds the valve
stem and flows into holes drilled in the stem,
which carry the oil through a spring-loaded
ball check valve to the cavity above the large
2 5
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