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B.  APU CONTROL UNIT WITH THE "TEST PASS" AND "TEST FAIL" LIGHTS; There is an overspeed reset/test switch on the face of the APU control unit to do a check for internal BITE problems.
 3.  Electronic Speed Switch (Fig. 1)
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 A.  The electronic speed switch controls the APU start, APU usual operation and overspeed protection. The electronic speed switch is found in the aft passenger compartment in the ceiling area. The tachometer generator sends the signal to the electronic speed switch for the APU engine speed. The electronic speed switch has the 50% starter cutout, 95% load relay and 110% overspeed relay.
 4.  Sequencing Oil Pressure Switch (Fig. 1)
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 A.  The sequencing switch for the oil pressure makes sure the combustion does not start until the APU engine has 4.0 psig (27.6 kPa) oil pressure. The sequencing switch is found on the APU engine directly below the starter motor. The sequencing switch has an electrical connector, a connector for the oil pressure line and 2 microswitches. When the oil pressure increases to approximately 4.0 psig (27.6 kPa), the two microswitches close. One microswitch completes the circuit to the ignition coil, and the other microswitch completes the circuit to the fuel solenoid valve. When both microswitches are closed this permits initial combustion to occur. When the APU rpm reaches 95%, the circuit to the ignition coil is opened by the speed switch. Above 95% rpm, the microswitch in the ignition coil circuit has no effect on APU operation. Above 95% rpm, the microswitch in the fuel soleniod valve circuit keeps the fuel solenoid valve energized as long as oil pressure remains above 4 psig (27.6 kPa).
 5.  Low Oil Pressure Switch (Fig. 1)
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 A.  The oil pressure switch operates when the oil pressure in the APU engine goes below approximately 45 psig (310 kPa). The oil pressure switch is found on the APU engine adjacent to the sequencing switch. The oil pressure switch has an electrical connector, a connector for the oil pressure line and a switch.
 6.  Oil Temperature Switch (Fig. 1)
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 A.  The oil temperature switch operates when the oil temperature is greater than approximately 285°F (140°C). The oil temperature switch is installed on the oil pump housing.
 7.  AIRPLANES WITH THE ETC; Compressor Inlet Temperature Sensor
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 A.  The temperature sensor for the compressor inlet gives the compressor inlet temperatures to the ETC. The temperature sensor T2 is a resistive transmitter which sends a signal to the ETC when the APU speed is above 95%. This signal is used by the ETC to control the EGT during acceleration and loaded conditions. The temperature sensor is installed on the compressor plenum.
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////////////////////////A / GTCP85-129 SERIES /737-300/400/500/ ENGINES /MAINTENANCE MANUAL ////////////////////////

ETC ADJUSTMENT NUT
CONTROL (ETC)

Electronic Temperature Control Installation
Figure 2

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8.  AIRPLANES WITH THE ETC; Electronic Temperature Control (Fig. 2)
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 A.  The electronic temperature control (ETC) does a check of the exhaust gas temperature directly from the two ETC thermocouple probes. The ETC thermocouple probes are installed in the turbine exhaust port. The ETC also supplies the overload protection for the APU. When an overload condition occurs, the ETC sends a signal to the proportional control valve to close the bleed air valve. This will permit the EGT to decrease to the correct limit. After the EGT decreases to the correct limit, the ETC sends a signal to the proportional control valve to open the bleed air valve. During the APU operation, the ETC sends a signal to the EGT indicator on the P5 forward overhead panel. On some airplanes, the ETC also sends the signal to the EGT indicator after the APU stops.
　
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