DECREASE TOO MUCH WHEN THE PNEUMATIC LOAD WAS APPLIED TO THE APU?
NO ENGINE SPEED DECREASES TOO MUCH WHEN A BLEED LOAD IS APPLIED (FIG. 121).

Bleed Air for Main Engine Start Is Not Sufficient
Figure 123A (Sheet 2)

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BLEED AIR VALVE  PREREQUISITES 
OPERATES OPEN AND  MAKE SURE THIS SYSTEM WILL OPERATE: 
CLOSED AGAIN AND  APU (AMM 49-11-00/201) 
AGAIN 

POSSIBLE CAUSES:
1.
BLEED AIR VALVE, V56, PROBLEM (AMM 49-52-11/401)

2.
CLOGGED FILTER FOR THE DIFFERENTIAL PRESSURE REGULATOR (AMM 49-52-21/301)

3.
DIFFERENTIAL PRESSURE REGULATOR PROBLEM (AMM 49-52-21/401)

4.
APU CONTROL THERMOSTAT PROBLEM (AMM 49-52-32/401).

FAULT ISOLATION:

Bleed Air Valve Operates Open and Closed Again and Again
Figure 124

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PREREQUISITES
MAKE SURE THIS SYSTEM WILL OPERATE: APU (AMM 49-11-00/201)
BLEED AIR VALVE OPERATES OPEN AND CLOSED AGAIN AND AGAIN

Bleed Air Valve Operates Open and Closed Again and Again
Figure 124A

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PREREQUISITES
MAKE SURE THE AIRPLANE IS IN THIS CONFIGURATION: ELECTRICAL POWER IS ON (AMM 24-22-00/201)
APU ENGINE SPEED DECREASES

POSSIBLE CAUSES:
1.
INLET DOOR SWITCH, S284, PROBLEM OR ADJUSTMENT (AMM 49-15-00/501)

2.  
INTERMITTENT ELECTRICAL SIGNAL TO THESE COMPONENTS: . FUEL SOLENOID VALVE (AMM 49-31-31/401) . SEQUENCING SWITCH FOR THE OIL PRESSURE (AMM 49-61-41/401) . ELECTRONIC SPEED SWITCH, S634, (AMM 49-61-22/401).

3.
APU CONTROL UNIT, M280, PROBLEM (AMM 49-61-11/401).

FAULT ISOLATION:
NO

1 DO A CHECK OF THE ADJUST-
MENT OF THE INLET DOOR SWITCH,
S284 (AMM 49-15-00/501).
IS THE INLET DOOR SWITCH ADJUSTED CORRECTLY?

YES YES

2 REMOVE THE INLET DOOR  
52 REPAIR THE DAMAGE THAT YOU
SWITCH, S284, CONNECTOR D1246. FIND.
IS THERE DAMAGE TO THE  IF NECESSARY, REPLACE THE
CONNECTOR?  INLET DOOR SWITCH, S284.
NO
NO

YES NO4 EXAMINE THE CIRCUIT FROM
THE INLET DOOR SWITCH, S284,
CONNECTOR D1246, PIN 4, TO THE

APU CONTROL UNIT, M280, CON-
NECTOR D50B, PIN 46
(WDM 49-31-01).
 IS THE CIRCUIT OK?
YES SEE SHEET 2
(BLOCK 21)

APU Engine Speed Decreases
Figure 125 (Sheet 1)

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FROM SHEET 1 (BLOCK 4) YES
NO

21 MEASURE THE RESISTANCE AT THE ELECTRONIC SPEED SWITCH, S634, CONNECTOR D1245, FROM PINS C TO D, J TO M, K TO L (WDM 49-31-01).
　
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