JoeM Posted August 15, 2021 Report Posted August 15, 2021 Wondering if there is a way to test components of the OEM CHT with spark plug probe on #3 cyl? It recently failed (pegged left) while troubleshooting Cyl #3 running hot after left mag OH. I also have an EI UBG16. I have confirmed wire continuity from probe to connector behind panel. Thanks for any input!
Hank Posted August 15, 2021 Report Posted August 15, 2021 Sounds to me like it's grounded out. Check for something else rubbing, or some missing insulation on the CHT probe.
markgrue Posted August 15, 2021 Report Posted August 15, 2021 You can check the thermocouple with a voltmeter on the leads. Heat the cht probe with a heat gun and you will see a corresponding rise in the voltage. It is in the milivolt range so you will need a sensitive voltmeter. Mark
Vance Harral Posted August 15, 2021 Report Posted August 15, 2021 1 hour ago, markgrue said: You can check the thermocouple with a voltmeter on the leads. Heat the cht probe with a heat gun and you will see a corresponding rise in the voltage. It is in the milivolt range so you will need a sensitive voltmeter. The OEM CHT probe on vintage Mooneys is a thermistor, not a thermocouple like modern engine monitors use. It does not generate a voltage when heated, but rather varies its resistance in response to heat. You can still check it with a multi-meter, but you want to measure resistance as you heat the probe, not voltage. The usual failure mode is that the insulated positive lead at the top of the unit twists, and breaks the resistive connection internal to the probe. Sometimes this can happen from vibration, but it's equally common for the little nut that connects the wire to be overtorqued by ham-fisted repairmen, who wind up twisting the lead instead of just snugging up the connection. Aircraft Spruce carries the probe, but it is irritatingly expensive for what it is, and frequently out of stock: https://www.aircraftspruce.com/catalog/inpages/rochcht.php
carusoam Posted August 15, 2021 Report Posted August 15, 2021 Let’s see…. If I had a problem like a bad probe… And wanted to know everything I could about that bad probe… I could start a new thread… and wait several days… to collect answers… Or find an existing thread that was a half week old already with answers from the top mechanics available on MS already… See if this one helps… -a-
Recommended Posts