The generator, which is coupled to the engine by a drive chain, also serves the dual role as a starter. When the preheat switch
is held in the closed (ON) position, and the generator switch is open (OFF), one side of the preheat relay energizing coil on the
APU engine is grounded through the preheat switch and a set of APU generator switching relay contacts. This causes the preheat
relay contacts to close since the opposite side of the energizing coil is connected directly to the vehicle bus voltage. Power is
then supplied through the closed preheat relay contacts to the preheaters, the APU generator/starter field winding, and to one
side of the starter switch circuit. When the starter switch is closed, it completes the APU starter relay coil circuit, and bus power
is supplied to the APU generator/starter armature circuit through these relay contacts, thus motorizing the generator/starter to
start the engine. Therefore both the preheat and starter switches must be closed (ON), and the generator switch must be open
(OFF) in order to start the APU. Since the preheat relay energizing coil ground circuit is passed through the set of APU switching
relay contacts (as mentioned above), the preheat relay cannot activate if the generator switch is on. The reason for this circuit
arrangement is to prevent vehicle bus voltage from being applied to the voltage regulator field through the preheat relay contacts,
thus damaging the regulator when the generator switch is on (field circuit from generator to regulator closed) and the preheat
switch is activated (bus voltage applies to generator field winding).
A gear-type pump draws oil from the crankcase and delivers it through a replaceable oil filter to the engine. Normal oil pressure
should be 25 psi (172 kPa) or higher under normal operating conditions. An oil pressure switch will activate a low oil pressure
warning light on the auxiliary power unit control box of the oil pressure drops below 14 2 psi (96.5 13.8 kPa). A crankcase
drain valve is provided with a handle so that the lubricating oil can be drained without removing the unit from the vehicle.
A constant speed governor is set to maintain engine speed at 2000 100 rpm. The governor uses a ball and cup mechanism on the
camshaft gear as the sensing device. A yoke resting on the cup connects to an arm and spring mechanism controlling the throttle
lever. Any change in engine speed is transmitted from the cup to the yoke and to the throttle. An adjusting screw is provided on
the front of the unit to adjust the speed by changing the spring tension.
The fuel system consists basically of a primary and secondary fuel filter, fuel transfer pump, fuel injection pump and two fuel
injection nozzles. The transfer pump operates off the camshaft, drawing fuel from the supply tank and delivering it through the
two filters to the injection pump. The injection pump meters and delivers the fuel at high pressure to the nozzles.
The generator/starter is mounted to the chain case and engine housing and driven by the engine through a roller chain and drive
sprocket. The output is controlled automatically by a voltage regulator and current limiting device.
The pump is mounted to the chain case and driven by the engine. It supplies the pressure for the auxiliary hydraulic system. This
system powers the refuel pump and impact wrench and also supplies power for emergency operation of the boom, spade and
END OF WORK PACKAGE
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