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B.  The engine fuel feed system will supply fuel to the engines by a direct tank-to-engine feed or from any tank to either or both engines through the crossfeed valve. To obtain direct tank-to-engine feed, the fuel crossfeed valve is closed, the engine fuel shutoff valves are opened and the boost pumps are started. To obtain a crossfeed, the fuel crossfeed valve and the engine fuel shutoff valves are opened (Fig. 5). 
C.  The center tank boost pumps override main tank boost pumps since pressure from main tank boost pumps has to overcome a boost pump check valve cracking pressure of 12 psi, while center tank boost pumps have only a 1.3-psi check valve cracking pressure to overcome.
D.  In case of an engine fire, control of the engine fuel shutoff valves is assumed by the fire switches on the aft electronics panel. When the fire switches are placed in the FIRE position, the respective engine fuel shutoff valve is closed (Fig. 6). Placing the engine start lever to cutoff position will also close the respective engine fuel shutoff valve.
E.  When the crossfeed valve is in the closed position, the crossfeed valve position indicating light is not illuminated (Fig. 7). When the engine fuel shutoff valves are in the open position, the engine fuel shutoff position indicating lights are not illuminated. When the crossfeed manifold valve or the engine fuel shutoff valves are traveling from open to close or from close to open, the applicable position indicating lights on the overhead panel are brightly illuminated. When the crossfeed valve is in the full open position, the applicable position indicating light is dimly illuminated. When the engine fuel shutoff valves are in the closed position, the applicable position indicating lights are dimly illuminated. The crossfeed or engine fuel shutoff valve indicating light will remain brightly illuminated if valve fails to reach position corresponding to panel switch position.
F.  The APU fuel shutoff valve is controlled by the APU master switch. The APU fuel valve is closed when the fire switches are energized.
5C8 
Aug 01/96  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  28-22-0 Page 11 

Engine Fuel Shutoff Valve Circuit  508 
28-22-0  Figure 6  Jun 20/86 
Page 12 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

540  Fuel Crossfeed Valve Circuit 
May 1/76  Figure 7 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  28-22-0 Page 13 

ENGINE FUEL FEED SYSTEM - TROUBLE SHOOTING
1.  General
A.  The following trouble shooting procedures are based on performance of system functional checks and are presented in tree-type format. The functional steps (heavy-line boxes) are connected by shaded arrows to show OK condition path through successful performance responses. When all steps in condition path indicate an OK response, fuel feed system is operable.
B.  When a functional check step does not indicate an OK response, follow the NOT OK line to the thin-line box containing applicable trouble symptom. Turn to procedure indicated and follow a single line from box to box by analyzing results of each step until required corrective action is determined. Perform specified corrective action and repeat step at which failure was encountered.
C.  All electrical type trouble shooting procedures are based on assumption that wiring is OK and that electrical power is available. If corrective action in trouble shooting procedure fails to correct problem, check wiring continuity per applicable diagram in Wiring Diagram Manual (WDM).
D.  Malfunctions of engine fuel shutoff valves or crossfeed manifold valve can be due to either mechanical or electrical trouble within valve. If valve fails to fully open or close, try completing valve opening or closing by using manual override handle. If normal pressure on handle is all that is necessary to complete valve travel, trouble is probably electrical and can be checked by substituting a known operable motor/actuator assembly on the valve (Ref 28-22-11 R/I). If a definite bind or jam is felt when attempting valve actuation with manual override, trouble is mechanical but still might be located within motor/actuator assembly. This should be checked out by unbolting motor/actuator assembly and trying override handle movement on the bench. If movement is normal, the mechanical trouble must be within gate assembly or port adapter, the latter of which can only be replaced by complete defueling of tank.
E.  References in trouble shooting boxes, refer to applicable chapter - section - subject of the 737 maintenance manual unless reference includes the abbreviation "WDM" in which case the 737 wiring diagram manual should be consulted.
538 
Feb 15/79  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  28-22-0 Page 101 
　
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