With the access plate removed, proceed to troubleshoot the APU fuel system
by disconnecting the fuel return line (Item 1, Figure 11) from the engine
quick-disconnect coupling (Item 3, Figure 11). After disconnecting the fuel
return line, set the MASTER switch and the APU FUEL SHUT OFF switch to the
ON position. Place a container under the engine quick-disconnect coupling
to collect any fuel that may flow from the system.
Using a screwdriver,
push open the quick-disconnect valve and observe if fuel flow is present.
When a fuel flow is present, you can proceed to check the operation of the
fuel solenoid plunger. If fuel flow is not present, then the APU control
box electrical system must be checked. Refer to Figure 11 for the location
and an illustrative breakdown of the fuel return line and components.
Figure 11.
APU Fuel Return Line Location and Components.
Assuming there was fuel flow at the fuel return connector, you then check
the operation of the fuel solenoid plunger. Check the fuel solenoid plunger
by turning the APU FUEL SHUT OFF switch ON and OFF while you observe the
action of the fuel solenoid plunger.
The plunger should move upward when
the switch is set to the ON position and downward when the switch is set to
the OFF position.
Various circuit tests must be made when troubleshooting the APU fuel system.
Figure 12, provides a schematic diagram for the APU electrical system, and
always refer to it when testing the APU electrical circuitry.
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