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A. The bleed air overtemperature switch is installed on the pneumatic duct on the strut right side between the precooler and the PRSOV (Fig. 1).
B. The overtemperature switch senses the temperature of the air in the bleed duct downstream of the precooler. The switch gives an electrical ground to the ASCTU when the temperature is below 470 °F (243 °C). When the temperature goes above 470 °F, the switch gives an open circuit to the ACSTU.
Operation
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A. Functional Description
(1) When the bleed air overtemperature switch opens, the control logic in ASCTU will close the HPSOV and PRV. When this occurs, the circuit that controls the HPSOV and PRV latches the valves closed. The valves will not automatically open when the temperature decreases below 470 °F. To release the latched closed condition, it is necessary to operate the ENGINE BLEED switch from OFF to On and then back to OFF again. The HPSOV and PRV will then open if the bleed air temperature is below 470 °F.
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MAINTENANCE MANUAL
Temperature Sensor Installation
Figure 1
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MAINTENANCE MANUAL
WING ISOLATION VALVE SWITCH
ENGINE BLEED
SWITCH
B
Engine Bleed Air Temperature Indication
Figure 2
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E21960
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MAINTENANCE MANUAL
NOTE: SYSTEM 1 IS SHOWN.
SYSTEM 2,3 AND 4 ARE SIMILAR.
Overtemperature Switch and Temperature Sensor Circuit Simplified Schematic
Figure 3
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747-400 MAINTENANCE MANUAL
BLEED AIR OVERTEMPERATURE SWITCH - REMOVAL/INSTALLATION
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1. General_______
A. This procedure contains two tasks. The first task removes the bleed air overtemperature switch. The second task installs the bleed air overtemperature switch. The second task also does a test of the installation of the switch.
B. The bleed air overtemperature switch is installed on the right side of the lower forward strut. It is necessary to remove the access door for the mid-lower forward strut to get access to the bleed air overtemperature switch.
C. The installation of the bleed air overtemperature switch is the same for each strut.
TASK 36-22-01-004-001
2. Remove the Bleed Air Overtemperature Switch (Fig. 401)___________________________________________
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