552 
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E.  The fuel atomizer injects atomized fuel into the combustion chamber in a uniform pattern. The pattern is unaffected by changes in the fuel pump delivery pressure. Fuel is directed to the primary (small) orifice plate to provide thorough atomization of fuel at low fuel pressure. As fuel pressure increases with increasing unit speed, the flow divider valve opens and allows fuel to flow to both the primary and secondary orifices for correct fuel atomization at high fuel pressures.
F.  When the unit reaches service speed, the governor regulates the fuel flow to match power demands maintaining the engine speed nearly constant. When the engine exceeds the service speed, the governor flyweights move outward to open the sleeve-type bypass valve to bypass fuel and so decrease the engine speed (Fig. 2). When the engine speed drops as a load is applied, the governor flyweights move inward to close the bypass valve to permit greater fuel flow and so increase the engine speed.
G.  When shutdown is initiated, power is supplied to the overspeed oscillator of the electronic speed switch. This in turn, excites the normally closed 110% switch, causing it to open. This simulates an engine overspeed. The 110% switch actuates and breaks the fuel solenoid valve circuit. The valve closes. For fuel system component operation electrically, refer to 49-11-0.
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FUEL CONTROL UNIT - REMOVAL/INSTALLATION
1.  Equipment and Materials
A.  Lubricant - MIL-G-21164 (Ref 20-30-21)
2.  Prepare for Fuel Control Unit Removal
A.  Position BATTERY SWITCH and APU START SWITCH to OFF and placard that APU maintenance is "in work."
B.  Open the circuit breaker on M280 module-APU accessory unit on E3-3 electronic shelf.
C.  Open APU compartment access door latches and open access door. Install door support rods.
D.  Support lower shroud, open shroud latches, and remove shroud.
CAUTION:  EXERCISE EXTREME CARE NOT TO DAMAGE SHROUD DRAIN LINES WHEN HANDLING SHROUD.
3.  Remove the Fuel Control Unit (Fig. 401)
A.  Disconnect fuel hose from elbow on low pressure fuel filter. Catch dripping fuel in suitable container.
B.  Disconnect high pressure fuel line from elbow on fuel solenoid valve. Catch dripping fuel in suitable container.
C.  Disconnect electrical plug from fuel solenoid valve.
D.  Disconnect control air line (thermostat) from Y fitting on fuel control unit.
E.  Remove control air line (compressor discharge air) from Y fitting on fuel control unit and from T fitting located adjacent to fuel control unit base.
F.  Remove fuel control unit mounting nuts and washers.
G.  Carefully withdraw fuel control unit in a straight line to avoid damage to drive shaft splines and mounting studs.
H.  If removal of fuel solenoid valve is required, remove per 49-31-31.
I.  If removal of low pressure fuel filter is required, remove as follows:
(1)  
Remove nut, washer, and bolt attaching low pressure fuel filter to bracket.

(2)  
Remove nut attaching bracket to seal drain line fitting and remove bracket.

Aug 15/76 49-31-11 Page 401 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

A72292 A72315
Fuel Control Unit Installation  597 
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597  Fuel Control Unit installation 
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(3)  
Loosen locknut on fitting connecting filter to fuel control unit.

(4)  
Unscrew filter from fitting.

(5)  
Remove old packing from fitting and discard.

4.  Install Fuel Control Unit (Fig. 401)
A.  If high pressure fuel filter element was removed from fuel control unit, service fuel control unit (AMM 49-31-81/301).
B.  If fuel solenoid valve was removed, install fuel solenoid valve (AMM 49-31-31/401).
C.  If low pressure fuel filter was removed from fuel control unit, install filter (AMM 49-31-21/401).
D.  Carefully position fuel control unit on mounting studs. Make sure that fuel control unit drive shaft engages with accessory drive gear shaft.
NOTE:  Apply light coat of lubricant to square drive flats and splined teeth of fuel control unit drive shaft.
　
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