(2)
AIRPLANES WITH BOEING M237 COMPARTMENT OVERHEAT ACCESSORY UNIT; Faults in the left sensing loop can be isolated by operation of the test switches on the compartment overheat accessory unit. The overheat test light on the accessory unit illuminates during the continuity check and remains off during the ground fault check.
(a) Move test switch SW1 to the down position. In the down position, the associated sensing elements are grounded, 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 operable. 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/or airplane wiring associated with the test switch.
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737-300/400/500MAINTENANCE MANUAL
(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 SW1 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 overheat test light extinguishes, the circuit being checked is operable. 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 and/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 (AMM 26-18-00/201). The nonvolatile 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.
C. AIRPLANES WITH FENWAL COMPARTMENT OVERHEAT ACCESSORY UNIT; BITE Operation
(1) The system has both hardware and software built in test features. A description of BITE follows, and a the BITE operating instructions are posted on the cover plate of the M237 module.
(a)
Pushing the OVHT TEST switch on the P5-10 panel, tests all detectors for continuity and faults. All flight compartment displays of compartment overheat will display if each loop is operating correctly. While the test is in progress, the display indicates code 92. Any faults detected during the test will be stored in the non-volatile memory for recall as required.
(b)
At the M237 compartment overheat accessory unit, pushing the DISP TEST switch initiates a self test of the display and associated circuits. Code 88 is displayed as long as the switch is held if there is no failed dc power source.
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A 737-300/400/500MAINTENANCE MANUAL
(c)
Momentarily pushing the LOC TEST switch initiates a self test of the control circuits and sensing elements. While the local test is being performed, the display indicates code 90. If a fault condition is detected during the test sequence, the code for that fault condition is indicated by the display for approximately two minutes. Another momentary push of the LOC TEST switch during the two minute display interval causes the test sequence to continue. If not continued, the test sequence terminates after approximately two minutes. When the entire test sequence has been performed, code 99 is displayed for approximately one minute. The test sequence initiated by the LOC TEST switch can be terminated at any time by pushing the DISP TEST switch.
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