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737-300/400/500MAINTENANCE MANUAL
 FUEL FEED LOW PRESSURE INDICATING SYSTEM - DESCRIPTION AND OPERATION
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General (Fig. 1)
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 A.  The fuel feed low-pressure indicating system provides an indication when the output pressure of any fuel boost pump falls below a preset value. The system consists of six pressure actuated switches and six indicator lights, one for each fuel boost pump, together with connecting fuel lines. If the outlet fuel pressure of a fuel boost pump falls to the preset value, its fuel low pressure switch will be actuated; this will illuminate the appropriate indicator light on the overhead panel.
Fuel Feed Low Pressure Switches
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 A.  Fuel feed low pressure switches control the fuel low pressure indicating lights on the overhead panel. Each switch assembly consists of a cylindrical housing containing a diaphragm and a switch. Each switch has one inlet port for a pressure sensing line coming from a fuel boost pump. The switch is vented to atmosphere. The fuel feed low pressure switches are located on wing front spar (Fig. 1).
 B.  The fuel feed low pressure indicating lights for tanks No. 1 and No. 2 will illuminate when the boost pump output pressure drops to 5 psi. As the pressure is increased, the indicator light will extinguish at 7 psi.
 C.  The fuel feed low pressure indicating lights for the center tank will illuminate when the boost pump output pressure drops to 14 psi. As the pressure is increased, the indicator light will extinguish at 16 psi.
Operation
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 A.  Low pressure fuel from any boost pump in any tank will turn on a fuel feed low pressure light (Fig. 2). Low pressure from both fuel boost pumps in the same tank will turn on the applicable low pressure lights and, in addition, the fuel annunciator light and the master caution light will illuminate (Fig. 3). When the master caution light is pressed, the circuit is rearmed and the master caution and fuel annunciator lights are extinguished. Pressing the fuel annunciator lights provided the low pressure condition remains (Ref Chapter 33, Master Warning and Caution Lights).
 B.  When the fuel in the center tank is depleted, the center tank boost pump low pressure lights come on. The center tank boost pump switches must be monitored continuously and turned off as soon as the low pressure lights come on. The low pressure lights for the center tank boost pumps go off when the boost pumps are turned off. The low pressure lights for the No. 1 or 2 tank boost pumps come on when low fuel pressure exists or when the boost pumps are turned off.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

737-300/400/500
MAINTENANCE MANUAL

OVERHEAD PANEL SEE A
FUEL VALVE FUEL VALVE CLOSED
CLOSED
B
B FUEL
0
-20 TEMP +20
-40  +40 
` C  BOOST PUMP FUEL LOW PRESSURE SWITCHES 
FILTER  VALVE  FILTER  SEE  B 
BYPASS B  BOPEN  BBYPASS 
CROSS  FEED 

LOW
LOW PRESSUREPRESSURE
A A
FUEL PUMPS
OFF
LR
ON
CTR

FUEL LOW PRESSURE LOW LOW
LOW LOW
INDICATING LIGHTS FRONT SPAR
PRESSUREPRESSUREPRESSUREPRESSURE
AA A A
AFT FWD FWD AFT
OFF OFF
FUEL
PUMPS

ON ON 12
A
SEE D
FWD
SEE C
(LEFT WING SHOWN, RIGHT WING OPPOSITE)
B
Fuel Feed Low Pressure Indicating System
Component Location
Figure 1 (Sheet 1)
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL

D

Fuel Feed Low Pressure Indicating System
Component Location
　
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