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737-300/400/500MAINTENANCE MANUAL
 B.  The master caution and annunciator lights are controlled by the master caution lights control section of the air conditioning module. The air conditioning annunciator light is at the right side of the lightshield. A master caution light is located at both sides of the lightshield. The annunciator light panel may be pressed to recall the warning on the lightshield, if the fault still exists.
 6.  Sensing Elements (Fig. 1)________________
 A.  The sensing element is a thermistor type device with a resistance inversely proportional to the alarm temperature. As the element temperature rises above the alarm temperature, its resistance is lowered. The sensing element consists of a single nickel wire embedded in insulation, empregnated with a salt compound and sheathed in an Inconel tube. Elements actuate at approximately 255°F for aft cargo compartment, right and left A/C pack, forward and aft keelbeam, sec 56 and sec 58, left and right wing leading edge bleed airducts, and left and right wing inboards, and 310°F for left and right engine strut cavities, leading edge overheat detectors.
 B.  If any portion of the sensing element is heated above alarm temperature, the element resistance drops sharply from a relatively high to an extremely low level. The single wire embedded in the center of the core is the power lead and the outside tubing is grounded. The insulating resistance of the core material decreases abruptly at the alarm temperature. Current flows through the core material to ground when an alarm temperature is sensed.
 7.  Operation (Fig. 2)_________
 A.  Normal Operation
 (1)  
Overheat detection is automatic when power is applied to the airplane and the MASTER WARN & CONTROL, OVHT WHL WELL WNG BODY, and MASTER CAUTION AIR COND circuit breakers are closed.

 (2)  
AIRPLANES WITH BOEING M237 COMPARTMENT OVERHEAT ACCESSORY UNIT; Temperature rise of an element causes the resistance to decrease. At a resistance corresponding to the alarm temperature, the temperature control unit will trigger an overheat warning signal. The signal will activate the left or right overheat warning lights on the air conditioning module and the master caution and master caution air conditioning annunciator lights on P7. When the element cools, the temperature and sensing element resistance values return to normal and the lights will go out. The master caution lights may be reset by momentarily pressing either master caution light. The annunciator light may be pressed to recall the overheat warning to the master caution and the annunciator lights. The master caution and annunciator light will illuminate if the overheat still exists.

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 26-18-00
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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737-300/400/500MAINTENANCE MANUAL
 (3)  AIRPLANES WITH ELECTRONIC FENWAL M237 COMPARTMENT OVERHEAT ACCESSORY UNIT; Temperature rise of an element causes the resistance to decrease. At a resistance corresponding to the alarm temperature, the control card of the compartment overheat accessory unit will trigger an overheat warning signal. The signal will activate the left or right overheat warning lights on the air conditioning module, the master caution and master caution air conditioning annunciator lights on P7, and the MAINT ADV light on the M237 module. The condition is stored in the non-volatile memory of the M237 module for recall as required. When the element cools, the temperature and sensing element resistance values return to normal and the lights will go out, the MAINT ADV light will remain illuminated until the memory is read and cleared. The fault/alarm codes are listed on the front panel of the unit and in AMM 26-18-00/101. The master caution lights may be reset by momentarily pressing either master caution light. The annunciator light may be pressed to recall the overheat warning to the master caution and the annunciator lights. The master caution and annunciator light will illuminate if the overheat still exists.
 B. Test Operation
 (1)  
A test of the wing and lower aft body overheat detection system may be made using the overheat test switch on the air conditioning module. The wing and lower aft body overheat indicating components should illuminate when the switch is pressed and go off when the switch is released. The test switch provides power to the overheat test relay which grounds the overheat system sensing elements causing an overheat signal to the left and right overheat warning lights and to the master caution and annunciator lights. If the left or right overheat warning light do not illuminate, there is an open circuit in the sensing loop. If the lights illuminate when the test switch is not pressed and there is not a fire in the wing or lower aft body, then there is a ground fault in the sensing loop.
　
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