(2)
Carefully check the exhaust gas temperature indicating system without dismantling to ensure that all connections are tight and that lines are secure.
B. Check Thermocouple Probes and Harness
(1)
Check for burned, broken or deformed probes.
(2)
Check for chafed or broken wiring.
(3)
Check continuity and insulation resistance of harness.
(4)
Check system integrity using operational test described in 77-21-01, A/T.
2. Electrical Check of EGT System On JT8D-15A, -17A, -17AR Engines Incorporating Boost Circuit During Every Basic Hot Section Inspection
A. Short Circuit Check
(1)
Disconnect terminals at locations A, B, C, D and E in Figure 601.
(2)
Set ohmmeter range switch to a range which has center scale value of approximately ten ohms.
(3)
Place one prod in good contact with well-cleaned alumel terminal A and other in good contact with well-cleaned chromel terminal B. Flex cable gently and observe instrument needle. If any deflection results (unless caused by accidental contact between other terminals of cable), reject part.
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B. Continuity Check
(1)
Set ohmmeter range switch to a range, which has center scale value of approximately ten ohms.
NOTE: Broken wires which come into contact intermittently due to flexing will cause needle to fluctuate. False intermittent indications will result if ohmmeter prods are not in firm contact with clean terminals; also, if ohmmeter prods, leads, or jacks are defective fluctuations can occur.
(2)
Perform continuity check for each of the following thermocouple lead circuits:
(a)
A to C
(b)
B to D*[1]
(c)
B to E
*[1] Resistance of this circuit should be higher than that of other two because of integral fixed resistor.
NOTE: While performing continuity check, gently flex lead.
P7WA 77-20 Page 602, 603, Apr 01/82
EGT System Boost Circuit Check 518
77-21-01 Figure 601 Sep 20/82
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
(d) Lead is unserviceable if high resistance (open circuit) is read on ohmmeter.
(3)
Perform continuity check at the junction box for each of the following junction box and harness and lead assembly circuits (with probes attached):
(a)
1 to J
(b)
6 to J
NOTE: While performing continuity check, gently flex cable assembly.
(c)
Junction box and harness and lead assembly, thermocouple probe No. 1 or 6 is unserviceable if high resistance (open circuit) is read on ohmmeter. Isolate problem per Section 77-22-01, I/C.
(4)
Reconnect terminals at locations A, B, C, D and E.
(5)
Perform continuity check from terminal A to B.
(6)
If a high resistance (opens circuit) is read on ohmmeter, system is unserviceable. If lead was serviceable per steps (2) thru (4), isolate the discontinuity by testing the thermocouple cable assembly per Section 77-22-01, I/C.
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C. Insulation Resistance Check See Fig. 601.
(1)
Ensure that the thermocouple cable and leads are properly installed and connected.
NOTE: Insulation resistance values slightly below the limits set forth below may not result in a deficient EGT system. It does, however, clearly indicate that the insulation is deteriorating and will result in eventual shorts or erroneous EGT readings. Therefore, EGT system components with insulation resistance readings below the limits set forth require attention to restore the insulation resistance to acceptable values and prevent further deterioration and possible EGT system failure.
(2)
Using low voltage ohmmeter, check insulation for short circuits, excessive leakage, and internal insulation chafing discrepancies. Check proper operation of instrument by touching test prods together and noting needle deflection to read 0 ohms.
NOTE: Low voltage ohmmeter utilizing less than 40 volts (dc) and maintained at an accuracy of 5 percent shall be used for determining insulation resistance. Megger or other high voltage test shall not be used under any circumstances.
(3)
Place one lead of ohmmeter in good contact with lug A. Place other lead in contact with wire braid.
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