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 9.  Operation
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 A.  After the APU master switch moves to the START position, the APU fuel valve opens, the air inlet door opens and the APU engine turns. When the oil pressure increases to approximately 4.0 psig (27.6 KPa), the sequencing switch for the oil pressure closes. This permits the electrical power to energize the fuel solenoid valve and the ignition unit. Combustion starts and the APU engine increases its speed. When the APU engine speed is approximately 19,000 to 21,000 rpm, the 50% contact in the electronic speed switch opens. This removes the electrical power from the start relay R5 and stops the starter motor. The APU engine continues to increase its speed. When the oil pressure increases to approximately 55 psig (379 kPa), the oil pressure switch opens and the LOW OIL PRESSURE light goes off. When the APU engine speed increases to approximately 37,300 to 38,900 rpm, the 95% contact in the electronic speed switch closes. This removes the electrical power from the ignition unit and sends electrical power to the solenoid for the bleed air valve. Also, this sends electrical power to the elapsed time indicator and the APU control unit.
 B.  AIRPLANES WITH THE ETC; The proportional control valve, ETC and ETC thermocouple probes supply the overload protection for the APU. While the EGT increases to a specified value, the ETC thermocouple probes send signals to the ETC. The ETC compares the two signals and sends a signal to the proportional control valve to open. This will bleed the compressed air from the bleed air valve and move the bleed air valve to the closed position. With the bleed air valve in the closed position, the pneumatic load on the APU and the EGT decrease. When the EGT decreases to the correct limit, the ETC sends a signal to the proportional control valve to permit the bleed air valve to open. This will permit the airplane to use the bleed air from the APU.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
////////////////////////A / GTCP85-129 SERIES /737-300/400/500/ ENGINES/MAINTENANCE MANUAL ////////////////////////
C.  The APU stops when the APU master switch is moved to the OFF position. The 110% contact in the electronic speed switch is energized. This is the same as an APU engine overspeed. When the 110% contact closes, electrical power is removed from the bleed air valve and the fuel solenoid valve. The valves close and the APU stops.
 D.  The APU stops automatically for an APU engine overspeed, APU fire, low oil pressure and high oil temperature. If there is an APU engine overspeed, the 110% contact in the electronic speed switch closes and the APU stops. If there is an APU fire or when the oil pressure decreases to approximately 45 psig (310 kPa), the APU stops. When the oil temperature increases to approximately 285°F (140°C), electrical power is supplied to the APU control unit to stop the APU.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
APU CONTROL UNIT - REMOVAL/INSTALLATION
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 1.  General
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 A.  This section gives the steps to remove and to install the APU control unit. The APU control unit is installed on the E3-3 electrical shelf. The E3-3 electrical shelf is found in the lower right side of section 43.
 TASK 49-61-11-004-001
 2.  APU Control Unit Removal (Fig. 401)
________________________
 A.  References
 (1)  
AMM 20-10-07/201, E/E Rack Mounted Components

 (2)  
AMM 20-40-12/201, Electrostatic Discharge Sensitive Devices

 B.  Access
 (1)  
Location Zones
 101 Control Cabin (Left)
 205 Electronic Compartment

 (2)  
Access Panels
 1201 Electronic Equipment Access Door

 

 C.  Procedure
 S 864-002
 (1)  Set the switch and the circuit breaker:
 (a)  
Set the APU master switch on the P5 forward overhead panel to the OFF position and attach a DO-NOT-OPERATE tag.

　
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