(2)
The control card contains a microprocessor and memory which performs fault detection and BITE functions. If a fault is detected, a record is made in memory, to be recalled as required.
(3)
The relay card contains 3 relays which control the warning
indications and test circuit.Alarm Bell (Fig. 1)
__________
A. The alarm bell warns of overheat in the wheel well area. The bell is controlled by the compartment overheat accessory unit. The alarm bell is in the aural warning device box.
Master Fire Warning Lights (Fig. 1)
__________________________
A. The master fire warning lights are on the lightshield module P7. When overheat occurs, the fire warning lights illuminate.
B. The master fire warning lights are controlled by the temperature control unit. Either light may be pressed for bell cutout and for extinguishing the master fire warning lights.
Sensing Element (Fig. 1)
_______________
A. A sensing element provides overheat detection in the main wheel well. The sensing element is attached to structure in the ceiling of the wheel well. The element is attached by quick-release fasteners.
B. The sensing element is a thermistor type with resistance inversely proportional to temperature. As the element temperature rises, its resistance is lowered. The sensing element consists of a single nickel wire embedded in insulation, which is impregnated with a salt compound and is sheathed in an Inconel tube.
C. 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 alarm temperature is sensed.
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26-17-00
ALL ú ú 01 Page 5 ú Jul 12/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
8. Operation (Fig. 2)
_________
A. Normal Operation
(1)
Overheat detection is automatic when power is applied to the airplane and the overheat detection and aural warning circuit breakers are closed.
(2)
If a fire occurs in the wheel well area, the sensing element temperature rises. The temperature rise causes the resistance value of the element to decrease. At a resistance value corresponding to alarm temperature, the compartment overheat accessory unit will trigger an overheat warning signal. The signal will activate the fire alarm bell, turn on the wheel well overheat light on the engine and APU fire control module, and illuminate the fire warning lights on P7. When the overheat condition is removed, the temperature and sensing element resistance values return to normal. The overheat signal will be removed and the wheel well overheat indicating components will go off. The alarm bell can be silenced when either the bell cutout switch or one of the master fire warning lights are momentarily pressed.
(3)
AIRPLANES WITH FENWAL M237 COMPARTMENT OVERHEAT ACCESSORY UNIT; if an alarm/fault condition is detected by the unit, the alarm/fault will be stored in the non-volatile memory of the unit.
(a) A procedure to clear the memory of the unit can be found in AMM 26-18-00/201.
B. Test Operation
(1) A wheel well overheat detection system continuity test can be made using the TEST switch on the engine and APU fire control module. The wheel well overheat indicating components come on when the test switch is set to OVHT/FIRE and then go off when the switch is released. The wheel well test relay grounds the wheel well sensing elements, simulating an overheat and causing the warning components to operate.
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