AUXILIARY POWER UNIT (ONAN) OVERVIEW AND DIAGRAMS
THIS WORK PACKAGE COVERS:
Auxiliary Power Unit (ONAN) Overview And Diagrams
The auxiliary power unit (APU) provides the power source that drives the auxiliary hydraulic pump. The auxiliary
hydraulic pump charges a back--up hydraulic system that permits operation of all hydraulic components in a no load
condition. In addition, the auxiliary hydraulic pump is the sole source of hydraulic power to operate the fuel transfer
pump or impact wrench. If necessary, the APU can also supply electrical power (+28 V dc) to keep vehicle batter-
ies charged and supply power to operate other vehicle electrical components when the main engine is not running.
The APU consists of a diesel engine, starter system, fuel system solenoid, monitoring system, and generator out-
The APU diesel engine is a 2--cylinder, 4--cycle, air cooled, add firing, engine that develops 10.8 hp at an engine
speed of 200 rpm (at 500 foot elevation with outside air temperature of 70 degrees 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
plugs, preheaters and associated electrical 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 sup-
plied to the glow plugs and preheaters 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 SHUT
OFF switch, and related electrical wiring. When the MASTER switch is ON, the APU fuel solenoid is energized
when the FUEL SHUT OFF switch is set to the ON position.
The APU monitoring system consists of the low oil pressure circuits, high air temperature circuits, and oil pressure
gauge circuits. The low oil pressure circuits include 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 (transmit-
ter). The indicators are located on the APU control box while the switches and sending unit are part of the APU
diesel engine. When the MASTER switch is ON, the APU LOW OIL PRESS light comes on when the FUEL SHUT
OFF switch is turned ON. The LOW OIL PRESS light remains lit until the APU engine is started and engine oil
pressure reaches the safe operating limit. If the LOW OIL PRESS light comes on while the APU engine is running,
the engine should be shut down and the malfunction corrected. The HIGH AIR TEMP indicator is a warning light
that indicates that the APU engine temperature has exceeded the safe operating limit. With the MASTER switch
ON, the ENGINE OIL PRESSURE gauge indicates APU engine oil pressure.
The APU generator output system consists of the APU generator/starter, the APU voltage regulator, the APU gen-
erator switch (APU GEN ON/OFF), a hydraulic pressure switch, and related electrical wiring. The APU generator
output system functions as a secondary or back--up system power source (+28 V dc) when the main engine is not
running. The hydraulic pressure switch prevents simultaneous operation of the generator and the auxiliary hydrau-
lic system in order to avoid loading or stalling the APU.
A diagram is included to show the relationship of the various APU components.