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A
737-300/400/500MAINTENANCE MANUAL
APU FIRE DETECTION SYSTEM - TROUBLE SHOOTING
____________________________________________
1. General_______
A. Trouble with the fire detection system will usually be indicated by absence of fire warning lights or alarm bell when the test switch is operated on the Engine and APU Fire Control panel, or by a false alarm. If a short circuit has occurred on a sensing line, the APU detector inoperative light should be illuminated. If an open circuit has occurred, the APU warning lights will not be illuminated when the system is tested. Lights on the Engine and APU Fire Control panel, and master fire warning lights should be tested by operating the master lights test circuits (AMM 33-15-00) before assuming the detection system is faulty.
B. False alarms are usually intermittent and rarely remain for any length of time. If a false alarm should persist, isolation of the sensor, connector or interconnecting wiring will be required. Isolation of the faulty component can be accomplished with the help of the trouble shooting chart.
C. The sensor and interconnecting wiring can be checked with an ohmmeter. An open sensor can be found by making a continuity check between center pins or sockets on connectors on the ends of the sensor. Shorts can be found by looking for continuity between the pins or sockets and the shell of the connector. If moisture in a connector is the cause of a false alarm or APU INOP indication, remove moisture by blowing out connectors with dry nitrogen.
D. Prior to making any tests, equivalent connectors should be installed at the plug or receptacle of the sensor or airplane wiring. The test connector should have exposed contact points to attach or touch test probes. Use of wire end fittings reduce the possibility of damage to the electrical contact surface and provide a positive attach point for test equipment.
E. Whenever the Engine and APU Fire Control panel P8-1 is removed and replaced, a test must be performed on the APU fire detection system (AMM 26-15-00, A/T).
2. Prepare for Troubleshooting___________________________
A. Provide electrical power (AMM 24-22-00, MP).
B. Ensure the following circuit breakers are closed on the P6 panel.
(1)
FIRE PROTECTION DETECTION, APU
(2)
MASTER WARNING AND CONTROL
(3)
LANDING GEAR LIGHTS
C. Open the following circuit breakers on P6 panel.
(1)
FIRE PROTECTION DETECTION ENG NO. 1
(2)
FIRE PROTECTION DETECTION ENG NO. 2
(3)
OV HT WHL WELL WNG BDY
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737-300/400/500MAINTENANCE MANUAL
3. _______________________________________________APU Fire Detection System Troubleshooting Chart
Table I. IF -to or greater than the values shown in minimum insulation resistance is equal of element connector. Make sure the across center conductor and outer shell short. Connect a 100V DC megohmmeter detectors. Check each detector for a Disconnect both ends of all the RESISTANCE LESS THAN MINIMUM VALUE -
RESISTANCE EXCEEDS MINIMUM VALUE -Wiring is shorted. Repair and/or replace airplane wiring.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
the values in Table II. IF -measured value is equal to or less than the detector element. Make sure the to the center conductor at each end of detector element. Connect an ohmmeter airplane wiring at each end of the element for open. Disconnect the CONTINUITY NOT OK - Check detector
RESISTANCE LESS THAN MINIMUM VALUE -Wiring is open. Repair and/or replace airplane wiring. ___
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