AUXILIARY POWER UNIT - TROUBLE SHOOTING
1.  General
A.  This section provides trouble shooting tips, indications and/or symptoms for detecting and isolating APU malfunctions. To find and correct auxiliary power unit troubles, first study the symptoms carefully; then check each possible cause, beginning with the most probable, until the exact cause of the trouble is determined. The charts list troubles in the sequence most likely to occur before starting, during starting, after starting and during shutdown. The charts do not list all possible short circuits; wire breaks or loose connections, as normal maintenance and inspection procedures should eliminate most of these troubles. In the event of an electrical failure, Chapter 49 of the wiring Diagram Manual should be consulted.
WARNING:  THE CURRENT INVOLVED IN THE IGNITION UNIT IS OF A VERY HIGH VOLTAGE AND CAN BE FATAL. WHEN TROUBLE SHOOTING THE IGNITION SYSTEM, BE SURE THAT POWER IS REMOVED FROM THE SYSTEM FOR A MINIMUM OF 4 MINUTES BEFORE MAKING ANY DISCONNECTIONS. AFTER DISCONNECTING THE HIGH TENSION LEAD, ENSURE COMPLETE DISCHARGE OF CAPACITORS BY IMMEDIATELY SHORTING IGNITION UNIT TERMINAL TO GROUND. DO NOT OPERATE THE IGNITION SYSTEM UNLESS THE IGNITION PLUG IS PROPERLY GROUNDED.
CAUTION:  TO AVOID DAMAGING APU, OBSERVE OPERATING LIMITS (REF 49-11-0, MP, OPERATE APU).
NEVER EXCEED STARTER DUTY CYCLE IN THE PROCESS OF ATTEMPTING TO DETECT APU PROBLEMS. HIGH ENERGY STARTERS ARE EASILY OVERHEATED AND CONSEQUENTLY DAMAGED.
DO NOT OPERATE APU UNLESS FUEL IS PRESENT AT THE FUEL CONTROL UNIT. DAMAGE FROM LACK OF LUBRICATION WILL OCCUR.
DO NOT OPERATE APU UNLESS OIL IS AT ITS PROPER LEVEL; DAMAGE FROM LACK OF LUBRICATION WILL OCCUR.
B.  For equipment location, refer to Description and Operation (APU component location).
C.  APU trouble shooting is divided into four parts; malfunctions before start, malfunctions during start, malfunctions after start, and malfunctions related to APU shutdown. Each malfunction identifies a procedure.
500 
Aug 15/80  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  49-00 Page 101 

2.  Equipment and Materials
A.  Multimeter - Simpson 260
B.  Pressure gage - 0 to 100 PSIG; 0 to 200 PSIG
C.  RPM Indicator and Tachometer - Generator Test Set - F72891-20
3.  Trouble Shooting Tips
A.  General Conditions
(1)  
Many external signs of trouble may be evident during APU operation. An observer can be used in trouble shooting to aid the APU operator. Some conditions to watch for are:

(a)  
Smoke in the ventilation system.

(b)  
Strange noises in the APU compartment.

(c)  
Fire or smoke in the exhaust ducting.

(d)  
No air pressure in APU drain system while APU is operating.

(e)  
Fluids draining from airplanes APU drain system.

(2)  
The APU must reach governed speed during a normal starting operation (Ref Maintenance Practices).

(3)  
The APU engine shuts down automatically in event of low oil pressure, high oil temperature, turbine overspeed and automatic fire detection (Ref Chapter 26, Fire Protection).

B.  Indications and/or Symptoms
(1)  
APU Battery DC Voltmeter (Ref Chapter 24, Electrical).

(a)  
On a normal start, (initial starter engagement) battery voltage will drop 8-10 volts and slowly rise 3 to 5 volts during first 10 seconds.

(b)  
Excessive battery voltage drop (in excess of 12 volts) indicates battery is weak and should be replaced.

(c)  
Excessive battery voltage drop followed by steady state could indicate sufficient voltage but failure of APU to rotate.

(d)  
A small battery voltage drop of 1-2 volts could be caused by starter rotation without engagement.

CAUTION:  STARTER WILL OVERSPEED AND DESTRUCT.

(e)  
Voltage rises suddenly to normal (24-28 volts) at 50% to indicate release of start relay (R5) and starter assembly drop out.

(2)  
DC Ammeter (AMM 24-31-0/001)

(a) Reads current of source selected by the DC selector for TR1, 2, 3, and BAT.

(3)  
Freq. Meter (AMM 24-21-0/001)

(a)  The frequency meter will indicate the frequency of the APU generator, if the APU generator is selected. Under normal conditions, "no load", and sea level operation, the APU frequency meter should read 405-410 Hz.

(4)  
Duct Pressure Indicator (AMM 36-21-0/001)

(a)  With the bleed valve switch to the OPEN position, duct pressure should stabilize. Duct pressure (nominal) should read 45 + 5 psig at sea level with no pneumatic load.
　
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