Overheat Detector (OVHT DET) Switch
NORMAL – detection loop A and loop B are active.
A – detection loop A is active.
B – detection loop B is active.

Fire Warning BELL CUTOUT Switch
Push –
.
extinguishes both master FIRE WARN lights

.
silences the fire warning bell

.
silences the remote APU fire warning horn (on the ground only)

.
resets the system for additional warnings.

Extinguisher (EXT) TEST Switch
(spring–loaded to center)
1 or 2 – tests bottle discharge circuits for all three extinguisher bottles.

Fault/Inoperative (FAULT/INOP) and
Overheat/Fire (OVHT/FIRE) TEST Switch

(spring–loaded to center)
FAULT/INOP – tests fault detection circuits for both engines and the APU.

Copyright . The Boeing Company. See title page for details.
Fire Protection -Controls and Indicators

737 Flight Crew Operations Manual
OVHT/FIRE – tests overheat and fire detection loops on both engines and APU, and wheel well fire detector
Note: See Fire and Overheat Detection System Fault Test in Section 20.

APU Fire Warning Switch
Illuminated (red) –
.
indicates fire in APU

.
unlocks APU fire warning switch. Note: Master FIRE WARN lights illuminate, fire warning bell sounds, and in the main wheel well the APU fire warning horn sounds (on ground only), and

APU fire warning light flashes. In – normal position, mechanically locked if no fire signal. Up –
.
arms APU extinguisher circuit

.
closes fuel shutoff valve, APU bleed air valve, and APU inlet door

.
trips generator control relay and breaker

.
allows APU fire warning switch to rotate. Rotate (left or right) –

.
discharges APU fire bottle.

Engine Fire Warning Switch
Illuminated (red) –
.
indicates fire in related engine

.
unlocks related engine fire warning switch. Note: Master FIRE WARN lights illuminate and fire warning bell sounds. In – normal position, mechanically locked if no fire signal. Up –

.
arms one discharge squib on each engine fire extinguisher

.
closes fuel, hydraulic shutoff and engine bleed air valves

.
disables thrust reverser

.
trips generator control relay and breaker

.
deactivates engine driven hydraulic pump LOW PRESSURE light

.
allows engine fire warning switch to rotate. Rotate (left or right) – discharges related fire bottle.

Copyright . The Boeing Company. See title page for details.
8.10.2  April 28, 2000
Fire Protection -
Fire Warning Switch Override

Fire Warning Switch Override
Push – unlocks fire warning switch.
Overheat/Fire Protection Panel Lights

Engine (ENG) OVERHEAT Light
Illuminated (amber) – indicates overheat in related engine.
Note: MASTER CAUTION and OVHT/DET system annunciator lights illuminate.

FAULT Light
Illuminated (amber) – with the overheat detector switch in NORMAL - indicates

both detector loops for an engine have failed.
Illuminated (amber) – with the overheat detector switch in A or B – indicates the
selected loop for an engine has failed.

Note:  MASTER CAUTION and OVHT/DET system annunciator lights do not illuminate.
Copyright . The Boeing Company. See title page for details.
Fire Protection -Controls and Indicators 737 Flight Crew Operations Manual

WHEEL WELL Fire Warning Light
Illuminated (red) – indicates fire in main gear wheel well
Note: Master FIRE WARN lights illuminate and fire warning bell sounds.

Engine BOTTLE DISCHARGE Light
Illuminated (amber) – indicates related fire extinguisher bottle has discharged.

APU Detector Inoperative (DET INOP) Light
Illuminated (amber) – indicates APU detector loop has failed.
Note:  MASTER CAUTION and OVHT/DET system annunciator lights illuminate.

APU BOTTLE DISCHARGE Light
Illuminated (amber) – indicates APU extinguisher bottle has discharged.

Extinguisher Test (EXT TEST) Lights
Illuminated (green) – EXT TEST switch is positioned to 1 or 2 and circuit continuity is normal.
Cargo Fire Panel

Extinguisher (EXT) Test Lights
Illuminated (green) - Cargo Fire TEST switch is pushed and fire bottle discharge squib circuit continuity is normal.
Copyright . The Boeing Company. See title page for details.
8.10.4   April 28, 2000
　
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