(3)  
The right sensing loop elements are at engine 2 strut cavity, right wing bleed air duct, right wing leading edge, and right air conditioning packs. The left sensing loop elements are at engine 1 strut cavity, left wing bleed air duct, left wing leading edge, left air conditioning packs, keel beam, aft cargo compartment, and APU cavity.

(4)  
The wing and lower aft body overheat detection system can be tested by an overheat test switch on the air conditioning module. The overheat test switch tests continuity through the sensing loops by simulating an overheat condition.

NOTE:  With the Fenwal compartment overheat accessory unit, the alarm indications require approximately 4 seconds to appear during wing and aft body overheat test.
2.  Bleed Air Duct Overheat Test and Indication Section Components
A.  The bleed air duct overheat test and indication section components are in the air conditioning module (P5-10).
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B.  Overheat Warning Lights
(1)  
Two amber overheat warning lights are provided for overheat indication in the wing and lower aft body area. An overheat condition will cause the steady illumination of either the left or right overheat warning light and the master caution lights.

(2)  
The warning lights may be tested individually by a press-to-test feature or tested when the master caution test switch is closed.

C.  Test Switch
(1)  A switch is provided for testing the wing and lower aft body overheat detection system. The switch may be used either while the airplane is in flight or on the ground (Fig. 1, Sheet 2). Pressing the test switch grounds the sensing elements. If all circuits are operating correctly, the overheat lights will illuminate.
NOTE:  On airplanes with the Fenwal M237 compartment overheat accessory unit, the overheat warning indications require a holding the test switch for approximately 4 seconds.
D.  Master Caution Lights Control Section
(1)  
The master caution lights control section provides control of the master caution lights and air condition annunciator light on the lightshield module (P7). The control section is a solid-state switching circuit.

(2)  
When an overheat warning occurs, the applicable overheat control unit provides a ground to the control section. The ground completes switching circuits which allow the master caution lights and master caution air conditioning annunciator light on P7 to illuminate. Either master caution light may be depressed to reset the master caution lights and annunciator light.

Compartment Overheat Accessory Unit Components
A.  (Airplanes with Boeing M237 overheat accessory unit)
(1)  The compartment overheat accessory unit (control module) components described in the following paragraphs will be only those associated with the wing and lower aft body overheat detection system. The compartment overheat accessory unit is located on electronic equipment rack E3 in the electronic compartment. Refer to 26-10-0 for additional information on the compartment overheat detection control module.
B.  Overheat Control Unit
(1)  The overheat control units plug in to the accessory unit and are held in place by screws. monitor the sensing element resistance and provide an alarm signal when the element resistance drops to the predetermined trip level (Fig 2).When the overheat condition has passed, the system returns to its standby condition.
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C.  Left Overheat Test Light
(1)  The overheat test light is provided to facilitate testing the left overheat section to determine whether a malfunction exists. The test light controlled by the left and APU overheat control unit. An overheat or a simulated overheat condition in the left overheat section will energize the output relay of the left and APU overheat control unit and provide a signal for the steady illumination of the test light. The light has a press-to-test feature. The test light will also illuminate whenever the left overheat light on the air conditioning module illuminates.
D.  Test Switches
(1)  
Three switches in the left sensing loop facilitate isolating and locating a malfunction. Each switch has three positions and is spring loaded to a normal position.

(2)  
The test switches are in series with the sensing elements (Fig 3). Each test switch may be actuated in turn to determine which section of the detection system has malfunctioned. When a test switch is pushed up, the applicable part of the detector circuit is tested for a pushed down, the applicable part of the detector circuit is checked for continuity.
　
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