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B.  A sequencing oil pressure switch allows fuel and ignition to be supplied to the engine at the correct time of the starting cycle. The electronic speed switch terminates starter motor operation and ignition in correct sequence. In addition, the electronic speed switch prevents bleed air being extracted until the unit has reached service speed, and it also provides protection against overspeeding of the unit. The low oil pressure switch and the oil temperature switch shut down the APU because of engine low oil pressure or high oil temperature.
2.  APU Control Unit (Fig. 1)
A.  The APU control unit controls the APU starting, running, and shutdown. The control unit is located in the lower 43 section on E3-3 electrical and electronic equipment rack. The control unit consists of eight relays and five printed circuit boards. The relays are: start and stop relay K1, pneumatic solenoid arming relay K1-3, overspeed shutdown relay K2, overspeed anticycle relay K3, fire shutdown relay K4, low oil pressure shutdown relay K5, starter lockout relay K6, and relay to prevent voltage drop K7. The printed circuit boards, contain transistors, silicon-controlled rectifiers, diodes, capacitors, and resistors. A circuit breaker and overspeed reset switch are located on the face of the APU control unit.
B.  One printed circuit board, A4, consists of micro circuits that are sensitive to, and may be damaged by, electrostatic discharges.
CAUTION:  STATIC SENSITIVE. DO NOT HANDLE BEFORE READING PROCEDURE FOR HANDLING ELECTROSTATIC DISCHARGE SENSITIVE DEVICES (20-40-12). CONTAINS DEVICES THAT CAN BE DAMAGED BY STATIC DISCHARGE.
540 
Oct 20/83  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  49-61-0 Page 1 

G01965G02033
APU Control Component Location  500 
49-61-0  Figure 1 (Sheet 1)  Oct 20/83 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

540  APU Control Component Location 
Feb 01/74  Figure 1 (Sheet 2)  49-61-0 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

3.  Electronic Speed Switch (Fig. 1)
A.  The electronic speed switch controls the starting cycle sequence, normal running and overspeed protection for the APU. The switch is located in the aft passenger compartment in the lowered ceiling area. Frequency input to the switch from a tachometer generator located on the engine accessory drive provides the signal that determines the position of the 50 percent starter cutout the 95 percent load relay and the 110 percent overspeed relay.
4.  Sequencing Oil Pressure Switch (Fig. 1)
A.  The sequencing oil pressure switch ensures that combustion cannot be initiated during an APU start until a minimum lubricating oil pressure has been built up by the oil pump. The switch is located on the engine directly below the starter motor. The switch consists of a housing containing a diaphragm operated snap-action switch and having connections for oil pressure line and an electrical connector. When the lubricating oil pressure rises to approximately 4.5 psig, the switch actuates and closes the circuits for the fuel solenoid valve and the ignition unit.
5.  Low Oil Pressure Switch (Fig. 1)
A.  The low oil pressure switch energizes the low oil pressure relay K5 when the engine oil pressure drops to approximately 45 psig. The switch is located on the engine adjacent to the oil pressure sequencing switch. The components comprising the low oil pressure switch are the same as that for the sequencing oil pressure switch, except for a difference in the actuation pressure setting.
6.  Oil Temperature Switch
A.  The oil temperature switch trips the start and stop relay K1 to shut down the APU when the oil temperature in the high pressure oil supply reaches approximately 250 ° F (121 ° C). The switch is installed into the oil pump housing (Fig. 1).
OIL TEMPERATURE SWITCH 
SWITCH PART NO.  ACTUATION POINT 
375036 305080-1  250° + 10°F (121° + 6°C) 285° + 10°F (140° + 6°C) 

591 
49-61-0 Page 4  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 20/86 

7.  Operation
A.  The APU engine controls automatically initiate fuel injection and ignition, terminate starter operation, and terminate ignition unit operation in correct sequence. In addition, the engine controls provide protection against the unit reaching an excessive and dangerous speed, and against engine operation with low oil pressure and high oil temperature.
B.  After the master switch is positioned to START, the air inlet door and APU fuel valve open and the engine starts rotating. As the engine accelerates, the lubricating oil pressure rises. At approximately 3.5 psig, the sequencing oil pressure switch actuates. This completes the circuits to the fuel solenoid valve and the ignition unit, and combustion is started. When the unit accelerates to approximately 19,000 to 21,000 rpm the 50 percent switch in the electronic speed switch opens. This breaks the circuit to the start relay R5 to terminate starter motor operation. The unit continues to accelerate. At approximately 55 psig, the low oil pressure switch actuates, extinguishing the low oil pressure light. At approximately 37,300 to 38,900 rpm, the 95 percent switch in the electronic speed switch actuates. This breaks the circuit to the ignition unit to terminate ignition system operation, arms the circuit for the bleed air valve solenoid and the low oil pressure shutdown relay K5, and completes the circuit to the elapsed time indicator. For APU control component operation electrically in the system, refer to 49-11-0.
　
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