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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL

Engine Fire Detetion System Component Location
Figure 1

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 4.  Engine and APU Fire Control Panel (Fig. 2)
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 A.  Engine fire detection system components of the engine and APU fire control panel are two fire/overheat detector selector switches, test switch, engine 1 and 2 overheat lights, engine fault light, and engine 1 and 2 fire switch handles. The panel is on the aft electronics panel of the control stand.
 B.  There is one fire/overheat detector selector switch for each engine. Setting the switch to NORMAL enables both loops to operate in AND -logic. Setting the switch to A or B enables the indicated loop and disables the other.
 C.  Setting the TEST switch to OVHT/FIRE checks detector continuity, alarm circuit continuity, and integrity of the sensing and warning circuits at a loop resistance equivalent to the alarm temperature level. The engine fire switch handle lights, OVERHEAT lights, master FIRE WARN lights, and bell activate.
 D.  Setting the TEST switch to FAULT/INOP checks the fault discrimination circuit. The FAULT and MASTER CAUTION lights illuminate.
 E.  The fire, overheat, and fault lights illuminate when the appropriate signals from the accessory unit are sent.
 5.  Operation
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 A.  With 28-volt dc power applied to the system, current is sent from each control card to the detectors in the corresponding loop. Each detector element has a resistance value, and is in series with the other elements in its loop.
 B.  When temperature increases, the resistance of the fire detector element drops, and the detector card senses the voltage change, triggering a fire and/or overheat indication. If the detector is shorted, there is a sudden drop in detection loop resistance, to a level below the fire/overheat range, triggering a fault. If the detector loop is open circuited, the system will continue to function normally, but will fail the FIRE/OVHT test. The detector card interprets these changes as faults, and provides a fault indication.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
1 NC
ENG NO.1
FIRE
DETECTION

A
198 AA
28V DCNORMBAT BUS
B 16 5
31
13 6
5

MASTER WARN
B
AND CONTROL S5 OVERHEAT DET SWITCH
A

12515  B
P6-2 CIRCUIT BREAKER3OVHT/FIRE PANEL
A
294 C4
OFF1818 D2 FAULT/INOP E
S4 FIRE TEST
L15 FAULT LIGHT SWITCH

F+V
LOOP
LOOP BA
1
INOP LTR
POWERENABLE
11
LOOP A-ENABLE
INOP LTROVHT
2411
DELAY LOGIC
V+G
REF
OAOA
R
2
MD&T DISABLE
OVHT10
OVHT
'AND'28
GATES UNLOCK -ENABLEINOP
HANDLELOGIC (FAULT)
V+
27 REF I O
AB
FIRE
H
-
S8 ENG 1 FIRE SWITCH
J
FIRE
V+
REF FA
2
ENGINE 1 OVERHEAT - FIRE CONTROL CARD A
4MD&T
KA
87 L3
A
1MENABLE
FA N
L5 ENG 1 OVERHEAT LIGHT FIRE
-ENABLE
LOOP
'AND'REF
DELAY LOGIC
OVHT
B
V+
GATES P8-1 ENGINE AND APU FIRE CONTROL PANEL POWER
OB DISABLE OVHTINOP-ENABLE(FAULT)
LOGIC
V+I
REF
B FB
POA
O
DELAY R FIRE
BFA -
FIREFB
V+
REF FB
ENGINE 2 OVERHEAT - FIRE CONTROL CARD B
M279 FIRE DETECTION ACCESSORY UNIT (E3-3)
Engine Fire Detection System Simplified Schematic
Figure 2 (Sheet 1)

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