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5.  Operation
A.  Each engine exhaust gas indicating system consists of eight thermocouple probes connected in parallel to an indicator. The airplane power is used to power the indicators and for integral lighting (Fig. 2).
B.  The difference in temperatures between the hot junction at the thermocouple probes and the cold junction at the indicator causes electrical signals to be generated in the thermocouple circuit. These electrical signals are proportional to the temperature difference between the two junctions and are measured by the indicator. Since the generated signals correspond only to the temperature difference, correction due to control cabin ambient temperature variation must be made to this measured temperature difference to obtain the actual temperature of the exhaust gases; the indicator does this correction automatically. The measurement circuit compensates for the cold junction temperature and converts the thermocouple millivolt input signal into a dc voltage suitable for amplification to drive the torquer. The pointer is driven upscale by the torquer and downscale by a return spring. The return spring provides a means for driving the pointer to a positive OFF position in event of power failure to the indicator.
C.  The thermocouple probes are connected in parallel to obtain the average temperature of the exhaust gases. Another advantage of the parallel connection is that if a probe burns out or is damaged, thermocouple circuit is not disrupted.
520 
77-21-01 Page 2  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 01/75 

500  Exhaust Gas Temperature Indicating System 
Feb 01/75  Figure 1  77-21-01 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

Exhaust Gas Temperature Indicating System Circuit  504 
77-21-01  Figure 2  Oct 20/86 
Page 4 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

EXHAUST GAS TEMPERATURE INDICATING SYSTEM (SERVOED) - TROUBLE SHOOTING

500 
Feb 01/95  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  77-21-01 Page 101 

EXHAUST GAS TEMPERATURE INDICATING SYSTEM (SERVOED) -ADJUSTMENT/TEST

1. General
A.  The exhaust gas temperature (EGT) indicating system is tested using a millivolt source. Tests include measurements of voltage.
B.  Three operational tests are provided which may be used optionally to establish integrity of exhaust gas temperature indicating system. None of the tests provide sufficient accuracy for system calibration, and are to be used only to indicate whether system operation is or is not satisfactory. Operational test No. 1 is preferred if conditions allow.
2.  Equipment and Materials
A.  Low voltage ohmmeter capable of measuring resistance of 100,000 ohms. Meter should utilize less than 40 volts (dc) and should have accuracy of 5%
NOTE:  Low-powered digital meters may give a misleading indication when attempting to measure extremely high or infinite resistances. Analog meters operate at higher voltages and are preferred to digital meters.
B.  DC power source variable from 0 to 50 millivolts - Model BH153, Howell Instruments, Fort Worth, Texas
C.  
DC millivolt meter capable of measuring to士 0.01 millivolt

D.  
Portable container of ice or accurate thermometer

E.  
Soldering iron, heat blower gun or other suitable heat source

500 
Feb 01/95  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  77-21-01 Page 501 

3.  Test Insulation Resistance
A. Check thermocouple leads, from indicator to engine terminal strip, for short to ground.
(1)  
Disconnect plug from exhaust gas temperature indicator on engine instrument panel.

(2)  
Open left engine cowl panel (Ref 71-11-11).

(3)  
Disconnect indicator leads (Alumel - green and Chromel -white) from terminal strip (Fig. 501).

(4)  
Connect one side of ohmmeter to one of the leads and the other side of ohmmeter to ground.

(5)  
Check lead for short to ground. The resistance shall not be less than 100,000 ohms.

(6)  
Repeat steps (4) and (5) for the other lead.

(7)  
Remove ohmmeter and connect leads to terminal strip.

(8)  
Connect plug to the indicator.

B.  Check thermocouple harness and leads for shorts to ground.
(1)  
Disconnect harness leads from the engine terminal strip (Fig. 501).

(2)  
Connect one side of ohmmeter to one of the harness leads and the other side of ohmmeter to ground.
　
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