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 TM 9-2320-312-24-1
THEORY OF OPERATION
INTRODUCTION
1.
The M878A2 Yard Tractor consists of the following functional systems:
a.
drive train;
b.
fuel system;
c.
exhaust system;
d.
cooling system;
e.
electrical system;
f.
air system;
g.
brake system;
h.
steering system;
i.
fifth wheel system;
j.
air conditioning system;
k.
cab tilt system; and
l.
optional auxiliary arctic heater system (Refer to Additional Authorization List, TM 9-2320-312-10).
2.
This work package explains how the components and systems of the M878A2 work together. A functional descrip-
tion is provided for each major system.
DRIVE TRAIN
1.
The drive train of the M878A2 consists of a Caterpillar 7.2L electronic engine and an Allison MD 3560P transmis-
sion, connected to a Spicer 1710 propshaft and Dana D90-14-2S double reduction planetary rear axle.
2.
The engine is diesel-fueled and turbocharged, with six in-line cylinders. It generates 275 horsepower at a governed
speed of 2400 rpm. Coupled with the Allison automatic transmission, it provides power to handle all requirements
of the yard tractor's mission, to include pulling up steep ramps with heavy loads in roll-on/roll-off (RO/RO) naval
operations.
3.
The engine is equipped with an air inlet manifold heater which assists in cold-weather starting. Heater turns on
when sum of coolant temperature and air inlet temperature is less than 109F (25C). Heater shuts off when sum of
temperatures exceeds 127F (35C).
4.
An automatic ether cold start system assists in cold-weather starting below 32F (0C).
5.
The transmission is a torque converter-type with five (5) forward speeds, a N (Neutral), and one R (Reverse) speed. A
power take-off, mounted on the transmission, and a hydraulic pump provide hydraulic pressure to operate the hydraulic
system for the fifth wheel lift.
FUEL SYSTEM
1.
The fuel system uses a Hydraulic Electronic Unit Injector (HEUI) at each cylinder to deliver high-pressure fuel into
the combustion chamber. The engine Electronic Control Module (ECM) controls the HEUI fuel system. The HEUI
fuel system cannot be mechanically adjusted.
2.
Fuel for combustion is supplied by the low-pressure fuel system. The low-pressure fuel system also supplies excess
fuel flow to cool the HEUI injectors and to remove air from the system. Components of the low-pressure system
are:
a.
fuel tank with a capacity of 50 gal (189.3 L);
b.
primary and secondary fuel filters;
c.
fuel transfer pump; and
d.
fuel pressure regulator--controls fuel flow back to the fuel tank.
Change 1
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