(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 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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B.  Test Operation
(1)  
A test of the wing and lower aft body overheat detection system may be made with the overheat test switch on the air conditioning module. The wing and lower aft body overheat indicating components should illuminate and then 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 section warning lights and to the master caution and annunciator lights. If the left or right overheat warning light fails to illuminate, there is either an open circuit in the associated detector circuit or the sensor loop resistance exceeds the alarm threshold of the M237 compartment overheat accessory unit. The Fenwall M237 unit has an alarm threshold of 115 ohms. The Boeing M237 unit has an alarm threshold between 25 and 50 ohms. If the lights burn continually, (test switch not operated) a ground fault (false alarm) in the sensing elements or associated wiring is indicated.

(2)  
Airplanes with Boeing M237 compartment overheat accessory unit, when the left overheat section fails to test or ground faults, the malfunction can be isolated to a specific area of the airplane by operation of the test switches on the compartment overheat accessory unit. When performing the isolation test, always start with test switch No. 1. The left overheat test light on the accessory unit should illuminate during the continuity check and remain off during the ground fault check.

(a)  
Move test switch No. 1 to the DOWN position. In the DOWN position, the associated sensing elements are grounded, thus checking continuity from the control unit to the test switch. The energized overheat test relay isolates the remaining sensing elements and airplane wiring between the test switch and the test relay. If the overheat test light on the control module illuminates, the circuit being checked is okay. Continue to the next test switch and repeat the above procedure. If the test light fails to illuminate, the malfunction is located in the sensing elements and airplane wiring associated with the test switch.

(b)  
When a ground fault occurs in the left overheat section, actuate the test switches to determine the location of the malfunction. Move test switch No. 1 to the UP position. In the UP position, the associated sensing element circuit is opened. The closed overheat test relay isolates the remaining sensing elements and airplane wiring between the test switch and the test relay. If the left overheat test light should go out, the circuit being checked is okay. Continue to the next test switch and repeat the above procedure. If the light remains on, the ground fault is located in the sensing elements or airplane wiring associated with the test switch.

(3)  
Airplanes with electronic Fenwal compartment overheat accessory unit, faults can be isolated by accessing the non-volatile memory of the compartment overheat accessory unit. The non-volatile memory stores each condition with a 2-digit code which is read by using the BITE pushbuttons on the cover plate (Ref 26-18-00 MP). The non-volatile memory is capable of holding ten alarm/fault codes. The MAINT ADV light illuminates when a condition is stored in the non-volatile memory and will remain so until all active codes have been cleared. The compartment overheat accessory unit contains a fault history memory which holds the last ten codes cleared from the active non-volatile memory. Active codes cannot be cleared until the alarm/fault condition is corrected.

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