REMOVE THE COMBUSTOR UNIT (AMM 49-31-42/401). EXAMINE THE COMBUSTOR FOR TOO MUCH CARBON. DOES THE COMBUSTOR UNIT HAVE TOO MUCH CARBON?
NO 
SEE SHEET 2
(BLOCK 3)
APU Engine Has an Explosive PUFF, BANG or Flame During an APU Start 
Figure 116 (Sheet 1)
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FROM SHEET 1 (BLOCK 2)
NO NO NO
3 DO A TEST FOR THE OPENING 
23 ADJUST THE OPENING PRES-PRESSURE OF THE ACCELERATION SURE FOR THE ACCELERATION 
LIMITER VALVE (AMM 49-11-00/ LIMITER VALVE 
501).(AMM 49-11-00/501). 
IS THE OPENING PRESSURE ____ IF THE OPENING PRESSURENOTE:OK? 
CANNOT BE ADJUSTED, YES 
REPLACE THE FUEL CON-TROL UNIT (AMM 49-31-11/401). 
START AND OPERATE THE APU
(AMM 49-11-00/201). 
DOES THE PROBLEM CONTINUE? 
YES 
YES
4 DO AN INSPECTION OF THE
FUEL DRAIN ON THE APU
(AMM 49-31-71//601). 
IS THE FUEL DRAIN CLOGGED?
NO 
YES
5 EXAMINE THE APU SHROUD
DRAINS. 
ARE THE APU SHROUD DRAINS CLOGGED?
NO 
YES
6 REMOVE CONNECTOR D1264
FROM THE FUEL SOLENOID VALVE. 
DISCONNECT THE FUEL LINE
TO THE FUEL ATOMIZER FROM THEFUEL SOLENOID VALVE. 
DO THE APU DRY MOTORING(AMM 49-11-00/201).
 DID FUEL FLOW FROM THE FUEL SOLENOID VALVE?
APU Engine Has an Explosive PUFF, BANG or Flame During an APU Start 
Figure 116 (Sheet 2)
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A13879
////////////////////////A / GTCP85-129 SERIES /737-300/400/500/ ENGINES /MAINTENANCE MANUAL //////////////////////// 
"APU GEN OFF BUS"  PREREQUISITES  
LIGHT DOES NOT COME  MAKE SURE THIS SYSTEM WILL OPERATE:  
ON WHEN THE APU  APU (AMM 49-11-00/201)  
ENGINE SPEED GETS  MAKE SURE THE AIRPLANE IS IN THIS CONFIGURATION:  
TO 95%  ELECTRICAL POWER IS ON (AMM 24-22-00/201) 
POSSIBLE CAUSES:
1. 
APU CONTROL UNIT, M280, PROBLEM (AMM 49-61-11/401)
2. 
ELECTRONIC SPEED SWITCH, S634, PROBLEM (AMM 49-61-22/401)
3. 
TACHOMETER GENERATOR PROBLEM (AMM 49-61-61/401)
4. 
P5-4 MODULE FOR THE APU AND AC SYSTEM GENERATOR PROBLEM.
FAULT ISOLATION: 
NO
1 START THE APU 
(AMM 49-11-00/201). 
PRESS THE LENS OF THE "APU 
GEN OFF BUS" LIGHT ON THE P5
21 DO THE APU USUAL SHUTDOWN 
(AMM 49-11-00/201). 
REPLACE THE BULBS. 
DOES THE PROBLEM CONTINUE?
NO
FORWARD OVERHEAD PANEL. 
YESDID THE LIGHT COME ON? 
YES NO
2 MOVE THE APU MASTER SWITCH
TO THE "OFF" POSITION. 
MEASURE THE RESISTANCE AT THE TACHOMETER GENERATOR,CONNECTOR D1262, FROM PIN A TO PIN B (WDM 49-41-01). 
IS THE RESISTANCE 15-35 OHMS?
YES NO
3 MEASURE THE RESISTANCE AT 
THE ELECTRONIC SPEED SWITCH, 
S634, CONNECTOR D1245, FROM 
PINS C TO D, J TO M, K TO L
52 REPLACE THE P5-4 MODULE FOR THE APU AND AC SYSTEM GENERATOR (WDM 24-27-11; 49-41-01). 
(WDM 49-31-01). ARE THE RESISTANCES 0-1 OHMS (MAXIMUM)?
YES 
SEE SHEET 2 (BLOCK 4) 
APU GEN OFF BUS Light Does Not Come on When the APU Engine Speed Gets to 95% 
Figure 117 (Sheet 1)
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A14015
FROM SHEET 1 (BLOCK 3)
YES 
NO
4 MEASURE THE RESISTANCE AT THE ELECTRONIC SPEED SWITCH,
 
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