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 3.  Bleed Air Duct
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 A.  The bleed air duct is found in the APU compartment between the upper shroud and the bleed air valve. The bleed air duct is attached to the upper shroud and the bleed air valve with support clamps. The pneumatic line filter for the cooling air shutoff valve and the check valve for the fuel sensing flow control are installed on the bleed air duct.
 4.  Miscellaneous Switching Module
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 A.  The miscellaneous switching module (M278) is an accessory unit with solid state circuits. The miscellaneous switching module is found on the E3-2 electronic shelf. During a main engine start, the miscellaneous switching module keeps the APU bleed valve in the open position. With a constant flow of APU bleed air, the main engine starter will not engage again and again. For a more detailed description of the starter protection circuit, refer to AMM 80-11-00/001.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
////////////////////////A / GTCP85-129 SERIES /737-300/400/500/ ENGINES/MAINTENANCE MANUAL ////////////////////////
5.  APU Control Thermostat (Airplanes without the ETC) (Fig. 1)
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 A.  The APU control thermostat is installed in the turbine exhaust port. The APU control thermostat has a steel housing, spring-loaded ball valve, thermostatic core, and pneumatic line connection. The APU control thermostat is part of a single thermostat control system. The single thermostat control system also has the 3-way solenoid valve. The APU control thermostat operates as an acceleration control thermostat and a load control thermostat.
 B.  When the APU engine speed is less than 95%, the APU control thermostat is an acceleration control thermostat. With the 95% contact in the speed switch open, no electrical power is supplied to the bleed air valve or the 3-way solenoid valve. When the APU speed is more than 95%, the APU control thermostat is a load control thermostat. Electrical power is supplied to the 3-way solenoid valve. When the 3-way solenoid valve closes, the APU control thermostat operates in the load control mode. In this mode, the APU control thermostat is an overload protection for the APU.
 C.  The 3-way solenoid valve is found below the surge bleed valve on the lower right side of the APU. During the APU start and operation, the APU control thermostat controls the acceleration limiter valve on the fuel control unit. The APU control thermostat bleeds the compressor air through the 3-way solenoid valve. The 3-way solenoid valve is usually in the open position. When the APU engine speed is 95%, the 3-way solenoid valve stops the compressed air from the acceleration limiter valve. The EGT is controlled by the APU control thermostat. The APU control thermostat bleeds the compressed air from the bleed air valve to close the bleed air valve if the EGT is too high.
 6.  Proportional Control Valve (Airplanes with the ETC) (Fig. 2)
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 A.  The proportional control valve is installed on the bottom of the APU near the turbine exhaust port. The proportional control valve is a part of the electronic temperature control system. The electronic temperature control uses the proportional control valve for overload protection of the APU. During an overload condition, the electronic temperature control sends a signal to the proportional control valve to move the bleed air valve to the closed position. This will remove the pneumatic load from the APU and permit the EGT to decrease. When the EGT decreases, the electronic temperature control sends a signal to the proportional control valve to open the surge bleed valve. For a more detailed description of the electronic temperature control, refer to AMM 49-61-00/001.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
CONTROL AIR CONNECTION
ACCELERATION LIMITER VALVE HIGH-PRESSURE FUEL PORT
FUEL
FILTER

FUEL GOVERNOR
FUEL
CONTROL UNIT

FUEL PRESSURE DRAIN PORT FOR PORT THE FUEL
INLET
LEGEND
CONTROL AIR
DRAIN
　
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