(c)  
With ac power lost or unavailable and the L FWD PUMP switchlight, on the P5 panel, in either the on or off position, and the APU selector ON, the dc pump starts.

 (3)  
DC Pump Operation - In Air

 (a)  
With the APU selector ON, the operating boost pumps supply fuel to the APU. The dc fuel pump starts upon loss of ac power.

 (4)  
DC Pump Pressure Indication

 (a)  
A dc pump pressure switch provides a ground for the Engine Indicating and Crew Alerting System (EICAS) for a pressure in the pump greater than 4 psi. A DC FUEL PUMP ON maintenance message results from the pressure in the pump. This message is available for recall on the lower EICAS display screen on the ground.

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757
MAINTENANCE MANUAL

 (5)  
APU Shutoff Valve Control

 (a)  
The APU command and the APU fire switch position determine shutoff valve position. With the APU fire switch, on the pilot's aft control stand P8, in NORMAL and the APU control selector ON or START, the APU shutoff valve opens. Placing the APU control selector to ON or START energizes the APU fuel control relay and the valve motor receives 28 vdc power to open. With the APU control switch OFF, the APU fuel control relay relaxes and 28 vdc power closes the valve. Pulling the APU fire switch closes the APU fuel shutoff valve by bypassing the 28 vdc power around the APU fuel control relay.

 (6)  
APU Shutoff Valve Indication

 (a)  
The Engine Indication and Crew Alerting System (EICAS) provides indication of faulty valve operation. If the shutoff valve limit switch disagrees with the valve commanded position the APU FAULT light on the pilot's overhead panel P5 comes on and the EICAS level C caution message APU FUEL VALVE appears on the upper display screen. The caution message and the FAULT light go out when the valve position agrees with commanded position.

 (7)  
Left Forward Main Boost Pump Ground Operation

 (a)  
With the airplane on the ground and the APU control selector ON the L FWD boost pump starts. If the boost pump generates sufficient pressure, the DC fuel pump remains off. The L FWD boost pump remains on regardless of its switch position as long as there is 115 vac power, the APU control selector is ON, and the boost pump is not faulty. Pulling the 115 vac circuit breaker for the L FWD boost pump shuts the pump off. The L FWD pump will also shut off if the APU control selector is positioned to OFF. The L FWD boost pump has an overheat switch in each of its 3 windings which trips at a maximum temperature of 400°F. The L FWD boost pump operation is tied into the air/ground system. If the air mode is simulated, for maintenance purposes while the airplane is on the ground, and the L FWD PUMP switch is in the off position, the dc fuel pump will fuel feed the APU. If the L FWD PUMP switch is in the on position, the L FWD boost pump will fuel feed the APU.
　
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