FAULT ISOLATION:
NO

1 DO A CHECK FOR VOLTAGE AT
51 REPAIR THE CIRCUITS FROM
THE BATTERY CHARGER, M5, THE BATTERY CHARGER, M5,
CONNECTOR D42, BETWEEN PINS 10 CONNECTOR D42, PINS 4,7,10, TO
AND 7, PINS 7 AND 4 AND THE 115V AC GROUND SERVICE BUS
PINS 10 AND 4 (WDM 24-31-21). ON THE P6-4 LOAD CONTROL
IS THERE 110-120V AC CENTER (WDM 24-31-21). BETWEEN THE PINS?

YES NO

2 DO A CHECK FOR VOLTAGE AT
THE BATTERY CHARGER, M5,
28V DC OUTPUT TERMINALS

(WDM 24-31-21).
IS THERE 26 VOLTS OR MORE AT THE 28V DC OUTPUT TERMI-NALS?
YESNO

3 DO A CHECK FOR VOLTAGE AT
53 REPAIR THE CIRCUIT FROM
THE "BATTERY CHARGER" CIRCUIT THE BATTERY CHARGER, M5,
BREAKER ON THE J9 BATTERY 28V DC OUTPUT, POSITIVE TERMI-
SHIELD (WDM 24-31-21). NAL TO THE "BATTERY CHARGER"
IS THERE 26 VOLTS OR MORE CIRCUIT BREAKER ON THE J9 AT THE CIRCUIT BREAKER?BATTERY SHIELD (WDM 24-31-21).
YES SEE SHEET 2
(BLOCK 4)

Battery Voltage is Less Than 26 Volts with AC Power on the Charger
Figure 105 (Sheet 1)

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FROM SHEET 1
(BLOCK 3)
YES NO

4 DO A CHECK FOR VOLTAGE AT
54 REPAIR THE CIRCUIT FROM THE BATTERY, M6, CONNECTOR THE "BATTERY CHARGER" CIRCUIT
D44, POSITIVE AND NEGATIVE BREAKER ON THE J9 BATTERY
TERMINALS (WDM 24-31-21). SHIELD TO THE BATTERY, M6,
IS THERE 26 VOLTS OR MORE CONNECTOR D44, POSITIVE AT THE TERMINALS?TERMINAL (WDM 24-31-21).
YES

Battery Voltage is Less Than 26 Volts with AC Power on the Charger
Figure 105 (Sheet 2)

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A13706
////////////////////////A / GTCP85-129 SERIES /737-300/400/500/ ENGINES /MAINTENANCE MANUAL ////////////////////////
"LOW OIL PRESSURE"  PREREQUISITES 
LIGHT DOES NOT COME  MAKE SURE THESE CIRCUIT BREAKERS ARE CLOSED: 
ON WHEN THE APU  P6-5 - C33 
MASTER SWITCH MOVES  APU CONTROL UNIT 
TO THE "ON" POSITION  MAKE SURE THE AIRPLANE IS IN THIS CONFIGURATION: 
FROM THE "OFF"  ELECTRICAL POWER IS ON (AMM 24-22-00/201) 
POSITION 

POSSIBLE CAUSES:
1.
APU CONTROL UNIT, M280, PROBLEM (AMM 49-61-11/401)

2.
LOW OIL PRESSURE SWITCH PROBLEM (AMM 49-61-51/401)

3.
APU MASTER SWITCH, S248, PROBLEM

4.
P5-4 MODULE FOR THE APU AND AC SYSTEM GENERATOR PROBLEM.

FAULT ISOLATION:

TO THE "ON" POSITION.APU FUEL VALVE DOES NOT FULLY DID THE APU FUEL VALVE OPEN (FIG. 107).
FULLY OPEN?
YES NO NO

2 MOVE THE APU MASTER SWITCH
21 MOVE THE APU MASTER SWITCH
TO THE "OFF" POSITION. TO THE "OFF" POSITION.

DISCONNECT THE LOW OIL EXAMINE THE CIRCUIT FROMPRESSURE SWITCH, CONNECTORTHE LOW OIL PRESSURE SWITCH,D1254. CONNECTOR D1254, PIN 3, TO
MOVE THE APU MASTER SWITCH THE APU CONTROL UNIT, M280, TO THE "ON" POSITION.CONNECTOR D50B, PIN 7
DO A CHECK FOR 28V DC AT (WDM 49-41-01).THE LOW OIL PRESSURE SWITCH, IS THE CIRCUIT OK? CONNECTOR D1254, PIN 3
YES
(WDM 49-41-01). IS THERE 28V DC AT PIN 3?
YES NO

3 MOVE THE APU MASTER SWITCH
TO THE "OFF" POSITION.

MEASURE THE RESISTANCE AT THE LOW OIL PRESSURE SWITCH, CONNECTOR D1254, FROMPIN 2 TO PIN 3 (WDM 49-41-01).
IS THE RESISTANCE 0-1 OHMS (MAXIMUM)?
YES

SEE SHEET 2 (BLOCK 4)
LOW OIL PRESSURE Light Does Not Come On When the APU Master Switch
Moves to the ON Position from the OFF Position
Figure 106 (Sheet 1)

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