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C.  The APU is shut down by positioning the master switch to OFF (normal shutdown). The overspeed oscillator in the electronic speed switch is energized and in turn simulates an engine overspeed. The 110 percent switch actuates and breaks the bleed air valve and fuel solenoid valve circuits to close the valves, and energizes the trip coil of the start and stop relay K1.
D.  The APU shuts down automatically in event of an engine overspeed, a fire in APU compartment, low oil pressure, and high oil temperature. When the engine overspeeds, the 110% switch in the electronic speed switch actuates and energizes the overspeed shutdown relay K2. On airplanes having an electronic speed switch with starter motor overspeed protection, an automatic shutdown will occur approximately 11 to 15 seconds following starter engagement if the electronic speed switch does not receive an electrical signal from the APU tachometer-generator. When fire is sensed by the fire detection system, the fire shutdown relay is energized. When the engine oil pressure drops to approximately 45 psig, the low oil pressure shutdown relay K5 is energized. If any one of the three relays is energized, the trip coil of the start and stop relay K1 is energized, and the APU shuts down. When the engine oil temperature rises to approximately 250°F (121°C) or 285°F (140°C), power is supplied to the base of transistor Q1-1, and again the trip coil of the start and stop relay K1 is energized, and the APU shuts down.
590 
Jun 20/84  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  49-61-0 Page 5 

APU CONTROL UNIT - REMOVAL/INSTALLATION
1. General
A.  The APU control unit is installed on the E3-3 electronic shelf. The E3-3 electronic shelf is found in the lower side of section 43.
2.  APU Control Unit Removal (Fig. 401)
A.  Prepare to Remove the APU Control Unit.
(1)  
Set these switches:

(a)  
Set the APU master switch on the P5 forward overhead panel to OFF and attach a DO-NOT-OPERATE tag.

(b)  
Set the BATTERY switch to OFF and attach a DO-NOT-OPERATE tag.

 

(2)  
Open these circuit breakers on the P6 circuit breaker panel and attach DO-NOT-CLOSE tags:

(a)  
APU CONT

(b)  
APU GENERATOR CONTROL

 

(3)  
Open the access door to the electronic equipment compartment.

B.  Remove the APU Control Unit
(1)  Do these tasks to remove the APU control unit:
(a)  
"Electrical/Electronic Black Box Removal" (AMM 20-10-111/201).

(b)  
"Metal-Encased ESDS LRUs Removal" (AMM 20-40-12/201).

CAUTION:  YOU MUST CAREFULLY DO THE STEPS IN THE TASKS TO REMOVE THE APU CONTROL UNIT. A FAILURE TO DO THE STEPS CORRECTLY CAN CAUSE DAMAGE TO THE EQUIPMENT.
3.  APU Control Unit Installation (Fig. 401)
A.  Install the APU Control Unit
(1)  Do these tasks to install the APU control unit:
(a)  
"Electrical/Electronic Black box Installation" (AMM 20-10-111/201).

(b)  
"Metal-Encased ESDS LRUs Installation" (AMM 20-40-12/201).

CAUTION:  YOU MUST CAREFULLY DO THE STEPS IN THE TASKS TO INSTALL THE APU CONTROL UNIT. A FAILURE TO DO THE STEPS CORRECTLY CAN CAUSE DAMAGE TO THE EQUIPMENT.
501 
Feb 01/95  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  49-61-11 Page 401 

E82234
APU Control Unit Installation  501 
49-61-11Page 402   Figure 401 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 01/95 

B.  Restore the Airplane to Normal Configuration
(1)  
Remove the DO-NOT-CLOSE tags and close these circuit breakers on the P6 circuit breaker panel:

(a)  
APU GENERATOR CONTROL

(b)  
APU CONT

 

(2)  
Remove the DO-NOT-OPERATE tag from the APU master switch on the P5 forward overhead panel.

(3)  
Remove the DO-NOT-OPERATE tag from the BATTERY switch.

(4)  
Do the APU operation test (AMM 49-11-0/501):

(a)  Do the APU shutdown using the fire switch (AMM 49-11-0/201).
NOTE:  Use one of the two fire handles to do a test of the circuit through the APU control unit.
(5)  Close the access door to the electronic equipment compartment.
501 
Feb 01/95  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  49-61-11 Page 403 

ELECTRONIC SPEED SWITCH - REMOVAL/INSTALLATION
1. General
A.  The electronic speed switch is installed in a mounting tray above aft lowered ceiling panel No. 2 in the passenger compartment. The tray is shock mounted, lessening effect of aircraft vibrations on the electronic speed switch.
　
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