TM 9-6115-664-13&P
Section III. PRINCIPLES OF OPERATION
1-13. PRINCIPLES OF OPERATION.
The theory behind the operation of the APU is described in the following paragraphs. Each APU system is
dependant upon the other for efficient operation. The information contained here will assist operators, as well as unit and
direct support maintenance personnel in understanding how the APU functions. This knowledge will assist in isolating
components which have failed. Refer to the Electrical Schematic (Figure FO-1) found at the end of this manual.
1-14. ELECTRICAL SYSTEM.
a.
The APU operates on a 28 volt direct current (DC) electrical system. The DC system provides power to APU
control circuitry, relay logic, and engine cranking, as well as producing voltage for load application. There are 3 test points
and 5 LED troubleshooting lights located on the local control panel. There are 4 LED lights located on the remote control
panel. There are 2 LED lights located on the preheat module box. These test points and LEDs provide malfunction
isolation for critical components of the APU.
b.
The DC system is powered by a 24 VDC battery located in the vehicle. The battery is charged by the APU
output when the APU is running and the APU ON switch (A1-S4) is placed in the ON position. The APU is grounded
through connection to the vehicle by being bolted to the chassis as well as through the NATO connector.
c.
Engine cranking is initiated by placing the START / PRIME RUN / OFF switch (A1-S4) in the PRIME RUN
position. In this position, switch A1-S4 applies voltage to the following components and circuits:
Fuel pump, which primes the fuel system
Preheat module
d.
Switch A1-S4 is placed in the PRIME RUN position for a few seconds to allow the APU fuel system to prime.
Once the fuel system has primed, the operator moves switch A1-S4 to the START position. Voltage is applied to the
preheat module via relay A1-K3. This supplies power to the starter solenoid, oil snap switch (located on the oil filter
adapter), and relay A2-A1-K2, which applies voltage to the pull coil for one second. Relay A1-K3 also overrides relay A1-
K2, which controls the engine malfunction safety shutdowns (low oil pressure and high engine temperature).
e.
The action on relay A1-K2 allows the engine start solenoid to stay latched during the start cycle. When the
START / PRIME RUN / OFF switch is released to the PRIME RUN position, A1-K2 remains energized (providing no
malfunctions exist).
f.
When the engine reaches rated speed, the operator must momentarily move the APU ON switch (A1S3) to the
ON position, then release the switch. This energizes latching relay A1-K1. This sends a signal to turn on the alternator
and close the contactor. At this time the starter lockout circuit is also energized.
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