THEORY OF OPERATION
The M915A3 Tractor Truck consists of the following functional systems: drive train, fuel system, exhaust system, cool-
ing system, electrical system, air system, brake system, steering system, traction control system, suspension system, air
conditioning system, and collision warning system.
This work package explains how the components and systems of the M915A3 work together. A functional description is
provided for each major component and system.
The drive train of the M915A3 consists of a Detroit Diesel, DDEC IV engine and an Allison 6-speed automatic transmission
connected to RT 40-145P rear tandem axles.
Fuel to power the engine is pumped out of the fuel tank by an engine-mounted fuel pump. The engine fuel system con-
sists of one electronic unit injector per cylinder, a transfer pump, low-pressure fuel lines, and primary and secondary
The engine is governed by an electronic control system. The system controls idle speed and limits engine maximum
speed. The driver controls engine speed through the position of the electronic throttle position sensor (foot pedal).
Fuel filters are spin-on types. The primary fuel filter has a hand fuel primer pump and a water drain.
Fuel may be drained from the tank through the drain port located on the bottom of the tank.
There is a computer-controlled ether quick-start system for use in cold weather.