(3)  
Test switch No. 1 tests the detector circuit from the control unit through the left wing sensing elements to the test switch. Test switch No. 2 tests the detector circuit from test switch No. 1 through the air conditioning pack sensing elements to test switch No. 2. Test switch No. 3 tests the detector circuit from test switch No. 2 through the forward and aft keel beam sensing elements to test switch No. 3. If the malfunction still exists, it is assumed that it is located somewhere in the aft cargo sensors, section 58 or section 56 sensors (Fig. 2 and 3).

E.  Overheat Test Relay
(1)  A test relay is used in the wing and lower aft body test circuit. The test relay completes grounding circuits which enable checking the continuity at the overheat detection system.
4.  Compartment Overheat Accessory Unit Components (Fig. 2) (Airplanes with Fenwal M237 compartment overheat accessory unit)
A.  The compartment overheat accessory unit is on electronic equipment rack E3 in the electronic compartment.
B.  The cover plate of the M237 compartment overheat accessory unit contains an LED display, a MAINT ADV light, and four BITE pushbutton controls as follows:
(1)  
MEM READ - Accesses the non-volatile memory.

(2)  
MEM CLEAR - Clears the non-volatile memory.

(3)  
LOC TEST - Initiates self-test of the control circuits and sensing elements.

(4)  
DISP TEST - Initiates self-test of the control circuits and the LED display.

500 
Feb 20/90  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  26-18-0 Page 7 

5.  Master Caution and Annunciator Lights
A.  The master caution and annunciator lights are located on the lightshield module (P7). When an overheat condition occurs, the master caution and annunciator lights will illuminate.
B.  The master caution lights and master caution air conditioning annunciator light will alert the crew to an overheat warning light on the air conditioning module (P5-10). The master caution annunciator lights are controlled by the master caution lights control section of the air conditioning module (Fig. 2). The air conditioning annunciator light is located at the right side of the lightshield. A master caution light is located at either side of the lightshield for the captain and first officer, respectively. The annunciator light panel may be pressed to recall the warning on the lightshield if the fault still exists (Fig. 2). For information, refer to Chapter 33, Master Caution and Warning Lights.
EFFECTIVITY
AIRPLANES WITH BOEING M237 COMPARTMENT
OVERHEAT ACCESSORY UNIT

G42450
Left Overheat Section Test Circuit  500 
26-18-0  Figure 3  Feb 20/90 
Page 8 

BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

6.  Sensing Element
A.  The sensing element is a thermistor type device which has a resistance inversely proportional to the alarm temperature, that is, 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, which is impregnated with a selected salt compound and sheathed in an Inconel tube. The elements are all set to actuate at approximately 255°F.
B.  If any portion of the sensing element is subjected to an overheat condition, 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 the alarm temperature is sensed.
7.  Operation
A.  Normal Operation
(1)  
Normal overheat detection is automatic when power is applied to the airplane and the overheat detection and aural, master warning and control circuit breakers are closed (Fig 2).

(2)  
Airplanes with Boeing M237 compartment overheat accessory unit, the sensing element is subjected to a temperature rise causing the resistance value of the element to decrease. At a resistance value corresponding to the alarm temperature, the overheat control unit will trigger an overheat warning signal. The signal will activate the respective 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 overheat condition is removed, lights will go out. The master caution lights may be reset by momentarily depressing either master caution light. The annunciator light may be depressed to "recall" the overheat warning to the master caution lights and the air conditioning annunciator light. The master caution and annunciator light will illuminate if the overheat condition still exists.
　
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