AUXILIARY POWER UNIT OVERVIEW AND DIAGRAMS (HATZ) --
When the MASTER switch is ON, the APU LOW OIL PRESS light comes on when the FUEL SHUTOFF 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 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 PRES-
SURE gauge indicates APU engine oil pressure.
If the engine oil temperature reaches 160 to 180_F (71.1 to 82.2_C), the door fan oil temperature thermostatic
switch will close, applying 24 V dc through relay box relay K4, which turns on the door fan. There is no electrical
indication that the door fan is operating. If the engine oil temperature should reach 180 to 200_F (82.2 to 93.3_C),
then the sump fan oil thermostatic switch will close applying 24 V dc to relay box relay K3, which applies 24 V dc to
the sump vaneaxial fan. There is no electrical indication that the sump fan is operating.
The hour meter indicates the amount of time (in operating hours) that the APU engine has been in operation. The
hour meter is used to determine when PMCS and other maintenace tasks are to be performed.
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 (vehicle) en-
gine is not running. The hydraulic pressure switch prevents simultaneous operation of the generator and the auxil-
iary hydraulic system in order to avoid loading or stalling the APU.
A diagram is included to show the relationship of the various APU components.