Troubleshooting Aircraft Tachometer  System EGT Circuit Error Resistance of Circuit Out of Tolerance Shorts to Ground/Shorts Between Leads This error is found by using the EGT and comparing the  reading of the aircraft EGT indicator with the analyzer  temperature reading.  [Figure 10-78]  The analyzer and  aircraft temperature readings are then compared. The engine thermocouple circuit resistance is a very  important adjustment since a high-resistance condition gives  a low indication on the aircraft EGT indicator. This condition  is dangerous, because the engine is operating with excess  temperature, but the high resistance makes the indicator  read low. It is important to check and correct this condition. These errors are found by doing the insulation check using  an ohmmeter. Resistance values from zero to 550,000 ohms  can be read on the insulation check ohmmeter by selecting  the proper range. A function of the rpm check is troubleshooting the aircraft  tachometer system. The rpm check circuit in the analyzer  is used to read engine speed during engine run-up with an  accuracy of ±0.1 percent. The connections for the rpm check  are the instrument cable and aircraft tachometer system lead  to the tachometer indicator. After the connections have been  made between the analyzer rpm check circuit and the aircraft  tachometer circuit, the two circuits, now classed as one, are  a parallel circuit. The engine is then run-up as prescribed in  applicable technical instructions. Both systems can be read  simultaneously. If the difference between the readings of the aircraft  tachometer indicator and the analyzer rpm check circuit  exceeds the tolerance prescribed in applicable technical  instructions, the engine must be stopped, and the trouble  located and corrected.  10-66