and the temperature reading on the temperature analyzer.  When the temperature difference exceeds the allowable  tolerance, troubleshoot the aircraft system.  The EGT indicator is tested after being removed from the  aircraft instrument panel and disconnected from the aircraft  EGT circuit leads. Attach the instrument cable and EGT  indicator adapter leads to the indicator terminals and place  the indicator in its normal operating position. Adjust the  analyzer switches to the proper settings. The indicator reading  should correspond to the readings of the analyzer within the  allowable limits of the EGT indicator. Correction for ambient temperature is not required for this  test, as both the EGT indicator and analyzer are temperature  compensated. The temperature registered on the aircraft  EGT indicator should be within the specified tolerance  of the aircraft system and the temperature reading on the  analyzer readout. When the temperature difference exceeds  the allowable tolerance, troubleshoot the aircraft system.  The thermocouple harness continuity is checked while the  EGT system is being checked functionally. The resistance  of the thermocouple harness is held to very close tolerances,  since a change in resistance changes the amount of current  flow in the circuit. A change of resistance gives erroneous  temperature readings. The resistance and insulation check  circuits make it possible to analyze and isolate any error in  the aircraft system. How the resistance and insulation circuits  are used is discussed with troubleshooting procedures. To read engine speed with an accuracy of ±0.1 percent during  engine run, the frequency of the tachometer-generator (older  style) is measured by the rpm check analyzer. The scale of the  rpm check circuit is calibrated in percent rpm to correspond to  the aircraft tachometer indicator, which also reads in percent  rpm. The aircraft tachometer and the rpm check circuit are  connected in parallel, and both are indicating during engine  run-up. The rpm check circuit readings can be compared  with the readings of the aircraft tachometer to determine the  accuracy of the aircraft instrument. Many newer engines use a magnetic pickup that counts  passing gear teeth edges, which are seen electrically as  pulses of electrical power as they pass by the pickup.  [Figure 10-79]  By counting the amount of pulses, the rpm  of the shaft is obtained. This type of system requires little  maintenance, other than setting the clearance between the  gear teeth and the magnetic pickup.  When thermocouples are subjected for a period of time  to oxidizing atmospheres, such as encountered in turbine  engines, they drift appreciably from their original calibration.  On engine parallel harnesses, when individual thermocouples  can be removed, these thermocouples can be bench-checked,  using one heater probe. The temperature reading obtained  from the thermocouples should be within manufacturer’s  tolerances. This error is found in the regular testing of aircraft  thermocouples with a hot heater probe and is a broken lead  wire in the parallel harness, or a short to ground in the harness.  In the latter case, the current from the grounded thermocouple  can leak off and never be shown on the indicator. However,  this grounded condition can be found by using the insulation  resistance check. An appropriate analyzer is used to test and troubleshoot the  aircraft thermocouple system at the first indication of trouble,  or during periodic maintenance checks. The test circuits of the analyzer make it possible to isolate  the troubles listed below. Following the list is a discussion  of each trouble mentioned. 1.  One or more inoperative thermocouples in engine  parallel harness 2.  Engine thermocouples out of calibration 3.  EGT indicator error 4.  Resistance of circuit out of tolerance 5.  Shorts to ground 6.  Shorts between leads EGT Indicator Check Resistance and Insulation Check Tachometer Check Troubleshooting EGT System One or More Inoperative Thermocouples in  Engine Parallel Harness Engine Thermocouples Out of Calibration Figure 10-79.  Magnetic pickup and gear.  10-65