THEORY OF OPERATION - TM-9-2320-312-100035

TM 9-2320-312-10

THEORY OF OPERATION

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INTRODUCTION

The M878A2 Yard Tractor consists of the following functional systems:

drive train;

fuel system;

exhaust system;

cooling system;

electrical system;

air system;

brake system;

steering system;

fifth wheel system;

air conditioning system;

cab tilt system; and

optional auxiliary arctic heater system (Refer to Additional Authorization List, WP 0018 00).

This work package explains how the components and systems of the M878A2 work together. A functional description is

provided for each major system.

DRIVE TRAIN

The drive train of the M878A2 consists of a Caterpillar 7.2L electronic engine and an Allison MD 3560P transmission,

connected to a Spicer 1710 propshaft and Dana D90-14-2S double reduction planetary rear axle.

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.

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 tempera-

tures exceeds 127F (35C).

An automatic ether cold start system assists in cold-weather starting below 32F (0C).

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

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 sys-

tem cannot be mechanically adjusted.

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:

fuel tank with a capacity of 50 gal (189.3 L);

primary and secondary fuel filters;

fuel transfer pump; and

fuel pressure regulator--controls fuel flow back to the fuel tank.

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