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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
FWD
FWDIGNITER CABLE IGNITION UNITIGNITER
IGNITION UNIT
CABLE IGNITER PLUG
IGNITER PLUG
B1 B2
FWD
STARTER MOTOR
1 AIRPLANES WITHOUT THE ETC
C 2
AIRPLANES WITH THE ETC
APU Ignition and Starting System Component Location
Figure 1 (Sheet 2)

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////////////////////////A / GTCP85-129 SERIES /737-300/400/500/ ENGINES/MAINTENANCE MANUAL ////////////////////////
B.  When the ignition unit is energized, the vibrator is supplied with 28 Vdc. The vibrator changes the input current to a pulse. The pulse is transmitted to the transformer. The transformer increases the voltage and sends it through the rectifier into the storage capacitors. When the storage capacitors are fully charged, they will let out a surge of high intensity current to the primary winding of the booster coil. The secondary winding of the booster coil supplies a surge current to the igniter cable. The igniter cable transmits the current to the center electrode of the igniter plug.
 5.  Igniter Plug (Fig. 1)
____________
 A.  The igniter plug supplies the high energy spark for the ignition of the fuel-air mixture in the combustion chamber. The igniter plug is installed on the forward side of the combustion chamber. The igniter plug has the outer casing, center electrode, and ceramic insulator. The energy from the spark is sufficiently high to break down the air distance between the center electrode and the outer casing. This permits the spark to cause the ignition of the fuel-air mixture in the combustion chamber.
 6.  Igniter Cable (Fig. 1)
_____________
 A.  The igniter cable supplies a low-resistance path between the ignition unit and the igniter plug. The igniter cable has an insulated electrical conductor that is closed in a metal braided conduit. At each end of the igniter cable, there are threaded connectors that are insulated. The igniter cable and connectors are shielded to prevent radio interference.
 7.  Operation
_________
 A.  During the APU operation, the APU ignition and start system is automatically controlled. When the APU master switch is moved to the START position, the start relay R5 is energized to supply electrical power to the starter motor. The starter motor begins to turn the APU engine. As the APU engine speed increases, the oil pressure increases. When the oil pressure is approximately 4.0 psig (26.7 kPa), the sequencing switch for the oil pressure closes. This permits electrical power to energize the ignition unit and fuel solenoid valve.
 B.  When the APU engine speed is approximately 19,000 to 21,000 rpm, the 50% contact in the electronic speed switch opens. The electrical power is removed from the start relay R5. The starter motor stops its operation. The APU engine continues to increase its speed. When the APU engine speed is approximately 37,300 to 38,900 rpm, the 95% contact in the electronic speed switch is energized. This removes the electrical power to the ignition unit. Ignition stops. The APU engine continues to increase its speed to 41,000 rpm. If the APU does not start in less than 90 seconds after the air inlet door opens, the APU will automatically stop. The time delay in the APU control unit will stop the APU. The time limit of the time delay prevents an overheat of the starter motor and limits the electrical load on the battery.
 NOTE: If the APU master switch is not set to the START position before
____ the air inlet door fully opens, the APU control unit will not permit the APU to start.
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STARTER MOTOR - REMOVAL/INSTALLATION
____________________________________
 1.  General_______
 A.  This procedure gives the tasks to remove and to install the starter
 motor. The starter motor is installed on the gearbox assembly.
　
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