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EGT probe failure mode?

A

alcladrv

Well Known Member
After powering up my G3X and BEFORE the engine is started, I have an Alcor EGT probe where the temperature just keeps climbing from ambient into the 1600 - 1700 F range if I just let it keep going. Is that a typical failure mode or is it indicative of of a connection issue? The other 3 probes stay steady at ambient temp.

Thanks
 
They do all sorts of goofy stuff and in my case wasn?t consistent or duplicable (is that a word?). Probes aren?t cheap but try a new one.
 
If you haven't already tried it, you might want to disconnect/swap probes between the offending one and another one that is working, just to verify it's the probe and not the receiving end.
 
Also simply disconnect the wires at the connection from the probe to the harness.

Hold them apart and then short them together. You will probably see full scale high and full scale low. How fast it moves in the full scale high error test might give you an indication of the actual probe failure.

Neither open or shorted will hurt your instrument.
 
Thermocouples produce a voltage output proportionate to temperature. You can take a good digital voltmeter and connect it to the thermocouple probe and measure its output directly. Note this requires the probe to be disconnected from the aircraft wiring harness. This same test can be made right at the engine monitor connector - disconnect the connector from the engine monitor and probe the + and - sides of the thermocouple. At rest in ambient room temperature the voltage produced should be quite small (at 70F it should be producing about 0.9 millivolts). As you heat the device its output voltage rises at a rate of about 41 microvolts per degree Celsius. That means at an EGT of 1500F it should be making about 33 millivolts.

Troubleshooting is fairly simple: stick your GOOD digital volt meter, set to millivolt range, across the output of the thermocouple, then heat it with a heat gun and see if its output voltage responds in a fairly linear fashion with rising temperature.

Good background reading here:
https://en.wikipedia.org/wiki/Thermocouple
 
well dang, this works! thanks. I just tested my EGT probe as you describe. VAF is wonderful!

Canadian_JOY said:
Thermocouples produce a voltage output proportionate to temperature. You can take a good digital voltmeter and connect it to the thermocouple probe and measure its output directly. Note this requires the probe to be disconnected from the aircraft wiring harness. This same test can be made right at the engine monitor connector - disconnect the connector from the engine monitor and probe the + and - sides of the thermocouple. At rest in ambient room temperature the voltage produced should be quite small (at 70F it should be producing about 0.9 millivolts). As you heat the device its output voltage rises at a rate of about 41 microvolts per degree Celsius. That means at an EGT of 1500F it should be making about 33 millivolts.

Troubleshooting is fairly simple: stick your GOOD digital volt meter, set to millivolt range, across the output of the thermocouple, then heat it with a heat gun and see if its output voltage responds in a fairly linear fashion with rising temperature.

Good background reading here:
https://en.wikipedia.org/wiki/Thermocouple
Click to expand...
 
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