(3)  
On airplanes with electronic Fenwal compartment overheat accessory unit, when any alarm/fault condition is detected by the compartment overheat accessory unit, the alarm/fault will be stored in the non-volatile memory of the unit for recall as required.

B.  Test Operation
(1)  A wheel well overheat detection system continuity test can be made by moving the overheat fire test switch on the fire protection module to the fire position. The wheel well overheat indicating components should come on and then go off when the switch is released. The wheel well test relay in the accessory unit grounds the wheel well sensing elements simulating an overheat condition and causing the warning components to operate. During the continuity test, should all overheat warning lights fail to illuminate and the alarm bell to sound, an open in the detector circuit is indicated.
500 
Feb 20/90  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  26-17-0 Page 7 

WHEEL WELL OVERHEAT DETECTION SYSTEM - TROUBLE SHOOTING
1. General
A.  The two basic troubles for the wheel well fire detection system are a false alarm or no alarm. If either the wheel well warning light, master fire warning lights or alarm bell do not operate, the trouble can be assumed to be in the associated circuit. These circuits should be checked before proceeding to the applicable trouble chart. The most frequent cause of trouble is breaking of the power wire in a flexible lead or sensing element.
B.  The adjustment/test procedures in 26-17-0, Wheel Well Overheat Detection System -Adjustment/Test, may be used to assist in trouble shooting this system. A false alarm is usually due to a direct short to ground in the power wire of a heat sensing loop. If trouble shooting per the chart isolates the trouble to the heat-sensing loop, test the insulation resistance of the sensing loop. If the resistance of the sensing element is okay, but a loose or improper connected electrical connection was noted during the test, moisture may have entered at the improper connection. Remove moisture by blowing out connectors with dry nitrogen.
D.  Prior to making any tests, equivalent test connectors (wire end fittings) should be installed at the receptacle. The test connector should have exposed contact points or wires to attach to or touch test probes. The test connector protects airplane wire from the possibility of arcing or other damage and provides position contact for attachment of test equipment.
NOTE:  If precautions are not taken, occasionally an oversized test probe may be forced into the sockets of a sensing element, this may spread the contacts and prevent the mating pin from making contact in the sleeve.
E.  Whenever the fire protection module (P8-1) is removed and replaced, a test must be performed on the wheel well overheat detection system (Ref Wheel Well Overheat Detection System -Adjustment/Test).
500 
Nov 01/74  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  26-17-0 Page 101 

H28878
500 
26-17-0 Page 102  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Aug 15/67 

500  Wheel Well Overheat Detection System Troubleshooting Chart (No Alarm) 
Aug 01/95  Figure 102 (Sheet 1)  26-17-0 
Page 103 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

DETECTOR LOOP  D742 PINS  LOOP CENTER CONDUCTOR RESISTANCE (75°F) 
MAIN WHEEL WELL  21 TO 24  15 ohm max 

MAIN WHEEL WELL CENTER CONDUCTOR RESISTANCE
TABLE A

Wheel Well Overheat Detection System Troubleshooting Chart (No Alarm)  500 
26-17-0  Figure 102 (Sheet 2)  Aug 01/95 
Page 104 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

WHEEL WELL OVERHEAT DETECTION SYSTEM - ADJUSTMENT/TEST
1. General
A.  The following procedure tests only the lights, components and airplane wiring associated with the wheel well overheat detection system. Other lights, components and wiring associated with the overheat detection system are checked when the wing and lower aft body overheat detection system is tested.
2.  Wheel Well Overheat Detection System Test
A.  Prepare Wheel Well Overheat Detection System for Test
(1)  
Connect external electrical power.

(2)  
Open the following circuit breakers under fire protection detection on circuit breaker panel P6:

(a) 
 Engine-1

(b) 
 Engine-2

(c)  
APU (fire protection detection)

(d)  
Cargo Smoke (Passenger/Cargo Convertible Airplanes only)
　
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