NOTE: Minor nicks in shield are not cause for rejection. Cracks at any location are cause for rejection. Do not attempt to repair minor bends in shield.
(3)
Check exhaust and inlet port openings for carbon buildup. If necessary, remove carbon with a straight piece of steel wire.
(4)
Check probe for looseness.
(5)
Check the terminal for looseness or thread damage. Deformation of 0.025 inch is allowable if terminals are not loose and thermocouple is electrically sound. Do not attempt to straighten or repair bent posts.
(6)
On thermocouples, after abrasive blasting, check area for loss of stud insulation.
B. Check Exhaust Gas Temperature Thermocouple Probe Electrically
NOTE: Before performing any of following checks, be sure that thermocouple has been satisfactorily cleaned according to instructions in Exhaust Gas Temperature Thermocouple Probe - Cleaning/Painting.
(1)
Equipment and Materials
(a)
Soldering iron or hot air gun with at least 500 watt capacity
(b)
Sensitive ohmmeter or Wheatstone Bridge
(c)
Low voltage ohmmeter capable of measuring resistance of 10,000 ohms. Meter should utilize less than 40 volts (dc) and should have an accuracy of five percent.
(d)
Temperature-controlled oven
(e)
Pyropotentiometer
(2)
Check Thermocouple Integrity
500
May 15/78 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 77-21-21 Page 601
NOTE: For EGT thermocouple probe access information, refer to Exhaust Gas
Temperature Thermocouple Probe - Removal/Installation
(a)
Place a hot soldering iron in contact with one of thermocouple probes.
WARNING: HEAT GUNS, SOLDERING GUNS AND SOLDERING IRONS ARE NOT EXPLOSION-PROOF. DO NOT USE WHERE FUEL OR FUEL VAPOR IS PRESENT (REF MM 20-10-21, MAINTENANCE PRACTICES).
(b)
On engine instrument panel, observe indicator pointer for a temperature rise indication, thereby proving continuity.
NOTE: Thermocouples are located approximately at center of the probes. Soldering iron should, therefore, be placed on probe at the center to assure adequate heat transfer to give an indication.
(c)
Allow sufficient period of time to elapse for indicator pointer to show a decrease in reading, then repeat the process with successive thermocouple probes until all of them have been checked.
NOTE: This is not intended as an adjustment check of thermocouple. If results of this check indicate a discontinuity, check probe as indicated in par. 2.C.
(3) Check Continuity and Resistance
NOTE: For EGT thermocouple probe access information, refer to Exhaust Gas
Temperature Thermocouple Probe - Removal/Installation.
(a)
Disconnect leads from EGT probe.
(b)
Using sensitive ohmmeter or Wheatstone Bridge, check continuity and internal resistance of each thermocouple. Internal resistance of thermocouple should not exceed 0.120 ohms slow response 0.200 ohms fast response.
NOTE: Test leads must be securely attached to thermocouple studs. For most accurate reading, secure leads with thermocouple nuts.
(c)
Using low voltage ohmmeter, check insulation resistance between each thermocouple probe terminal and body of probe for minimum of 25 seconds. Resistance shall be at least 10,000 ohms.
500
77-21-21 Page 602 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Jan 20/83
WARNING: TO REDUCE POSSIBILITY OF IGNITING FUEL, DO NOT USE A
MEGGER OR OTHER HIGH VOLTAGE OHMMETER.
(4) Perform Functional Check
(a)
Perform a functional check of thermocouple probe if continuity and resistance check indicates suspicion of trouble.
(b)
Remove thermocouple probe from engine. Refer to Exhaust Gas Temperature Thermocouple Probe - Removal/Installation.
(c)
Functional check consists of subjecting thermocouple probe to actual temperature variations and checking response voltages. A temperature-controlled oven will provide temperature elevation, and a pyropotentiometer will give thermocouple readings directly in degrees of temperature. Thermocouple response within 4°C (7°F) of laboratory standard accuracy at 500°C (932°F) is regarded as acceptable.
(d)
If thermocouple calibration is desired, heat soak thermocouple probe to be tested together with one of predetermined accuracy in same oven and identical physical environment for at least one hour at 600°C (1112°F). A comparison between readings of test probe and standard may then be made.
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