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 C.  The insulation resistance of the core changes abruptly when 400°F alarm temperature is reached. At this temperature, the core resistance decreases, allowing current flow through the core to ground.
Wheel Well Overheat Detection Card (Fig. 1)
__________________________________
 A.  The detection loop has a corresponding detection control card. These cards constantly monitor and process signals produced by each loop.
 B.  The detection control cards are in the M826 fire detection card file. The card file is in the electrical/electronic equipment compartment (AMM 26-10-01/401).
 Card slot number: 24 Card nomenclature: WHEEL WELL OVHT DETFire/Overheat Test Module (Fig. 1)
_________________________
 A.  The FIRE/OVHT TEST switch is located on M7317 miscellaneous switching panel (P5). The switch initiates tests for the various fire detection system. The operational test for each applicable section contains the FIRE/OVHT test. By pressing the switch, the switch sends a signal to a relay that simulates a fire/overheat signal. The test results in cockpit fire and overheat indicators, and appropriate EICAS messages.
 B.  To replace the module, refer to 26-10-02/401, Miscellaneous Switching
 Module.Operation (Fig. 2)
_________
 A.  Functional Description (Fig. 2)
 (1)  The system receives 115-volts AC from circuit breaker W/W FIRE on P6 main power distribution panel.
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MISCELLANEOUS SWITCHING MODULE, M7317
SEE A
A
747-400
MAINTENANCE MANUAL

EICAS
CARD FILE, M826 FIRE DETECTION
E/E EQUIPMENT COMPARTMENT CENTER
FIRE/OVHT
FIRE/OVHT
TEST
TEST SWITCH
A
Wheel Well Overheat Detection System Component Location
Figure 1 (Sheet 1)

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A

747-400
MAINTENANCE MANUAL

WING LANDING GEAR WELL
BODY LANDING GEAR WELL
SEE B
WHEEL WELL OVERHEAT CONTINUOUS LOOP SENSORS
FWD
B Wheel Well Overheat Detection System Component Location
Figure 1 (Sheet 2)

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A
747-400
MAINTENANCE MANUAL

Wheel Well Overheat Detection System Simplified Schematic
Figure 2

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A
747-400
MAINTENANCE MANUAL

 (2)  
When a segment of an overheat detector reaches a temperature greater than approximately 400°F, an alarm signal (low resistance) is provided by the detection circuit on the wheel well fire card. A signal is sent to EIU and then to EICAS creating a caution EICAS message.

 (3)  
The fire alarm signals are cancelled when the fire detector cools to approximately 360°F.

 (4)  
The circuit continuity can be checked by fire/overheat test module and Central Maintenance Computer.

 B.  Control
 (1)  The wheel well overheat detection system is operating when
 electrical power is applied.

 C.  The CMC can initiate the fire detection system ground test if either of the ground tests switches is enabled and the CMC test relays are closed. The CMC initiates a ground test to determine the integrity of the fire detection card file and its inputs/outputs using the LRU's built-in test equipment.
　
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