AUXILIARY POWER UNIT OVERVIEW AND DIAGRAMS (HATZ).
THIS WORK PACKAGE COVERS:
APU Power Unit Overview and Diagrams (HATZ)
The auxiliary power unit (APU) provides the power source that drives the starter/generator and auxiliary hydraulic
pump. The APU starter/generator supplies electrical power (+28 V dc) to charge vehicle batteries and supply elec-
trical power to operate other vehicle electrical components when the main engine is not running. The auxiliary hy-
draulic pump under normal operation provides hydraulic pressure to operate the hydraulic impact wrench and the
fuel transfer pump. In an emergency situation, the auxiliary hydraulic pump can provide hydraulic pressure to
raise/lower the spade or boom or to retrieve main, hoist, or auxiliary winch cables under no load conditions. The
APU consists of a diesel engine, starter system, fuel system solenoid, monitoring system, generator output system,
relay box, door fan, door fan temperature relay, sump fan, sump fan temperature relay, hour meter, and fuel pumps
number one and two.
The APU diesel engine is a 2--cylinder, 4--cycle, air cooled, add firing, engine that develops 13.5 hp at an engine
speed of 2000 rpm (at 500 foot elevation with outside air temperature of 70_F).
The APU starter system consists of the generator/starter, a preheater relay, two 15 ampere circuit breakers, a pre-
heater switch (PREHEAT OFF/MOM ON), the APU generator switch (APU GEN ON/OFF), a starter relay, glow
plug, and associated wiring. With the MASTER switch ON, operating voltage (+24 V dc) to the starter is supplied
from the vehicle batteries through the starter relay. The starter relay is energized when the APU GEN switch is set
to the OFF position, and both the PREHEAT and START switches are held in the MOM ON position. In addition,
with the PREHEAT switch held in the MOM ON position, operating voltage (+24 V dc) is supplied to the glow plug
to aid the APU diesel engine starting process.
The APU fuel system solenoid circuit consists of the fuel solenoid, 15--ampere circuit breaker, the FUEL SHUTOFF
switch, relay box, wiring harness 3W730, and other related electrical wiring. When the MASTER switch is ON, the
APU fuel solenoid is energized when the FUEL SHUTOFF switch is set to the ON position. 24 V dc is applied to
relay box relays K1 and K2, through timing capacitor C1, then to ground at COMMON GROUND E3. The initial
surge of voltage charges the PULL--IN and HOLD--IN circuits at the same time, which pulls in the FUEL solenoid
arm. With the FUEL solenoid arm in, fuel is allowed to flow to the APU engine. As timing capacitor C1 fully
charges, the voltage to the PULL--IN circuit is dropped off, leaving only the HOLD--IN circuit to keep the FUEL sole-
noid arm in. Loss of vehicle battery power, or positioning the FUEL SHUTOFF switch and/or the MASTER switch to
OFF will remove the voltage to relay box relays K1 and K2, which in turn will remove electrical power from the
FUEL solenoid. A mechanical spring returns the FUEL solenoid arm to its original position, thereby shutting off the
fuel supply to the APU engine.
When relay K1 is energized, 24 V dc is supplied to both APU fuel pumps and to the hourmeter. Fuel pump number
1 (lower) draws fuel from the vehicle fuel tank and supplies it through two external fuel filters and a fuel/water sepa-
rator located on the APU to fuel pump number 2 (upper). Fuel pump number 2 (upper) draws fuel from the fuel/wa-
ter separator, then supplies it to the APU fuel injector pump. Excess fuel is returned to the vehicle fuel tank.
The APU monitoring system consists of the low oil pressure circuits, high air temperature circuits, oil pressure
gauge circuits, two oil temperature sensing thermostatic switches (160 to 180_F and 180 to 200_F) (71.1 to
82.2_C and 82.2 to 93.3_C), a door fan, and a sump vaneaxial fan. The low pressure circuit includes the LOW OIL
PRESS indicator and the low oil pressure switch. The high air temperature circuits include the HIGH AIR TEMP
indicator and the high air temperature switch. The oil pressure gauge circuits include the ENGINE OIL PRESSURE
gauge and the Oil Pressure Sending Unit (transmitter). The indicators are located on the APU control box while the
switches and sending unit are part of the APU diesel engine.