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heedh
02-24-2019, 04:59 PM
I've just installed two MGL type-K EGT sensors on cylinders 2 & 4 of my Rotax 914UL. The airplane is a SeaRey (high-mounted pusher prop), so there is a significantly long run (18 feet) of k-type thermocouple wire between the sensor and the back of my AF-5500. I used overlap-type (solderless barrel) connectors to splice from the sensor to the extension wire, then crimped D-Sub pins on the thermocouple wire for the EGT sensor 25-pin connector. When I fired up the engine, both EGTs were off-the-scale high within 20-30 seconds at idle throttle. For fun, I reconnected the sensors using 22-AWG wire for the long run from the sensor to the EFIS - same result.

Do I need to account for the long length of wire somehow? The AD_VALUEs were identical (I believe > 4000, but did not note it before leaving the hangar), leading me to believe that I was somehow maxing out the resistance sensor.

Any advice on how to troubleshoot?

Shawn McGinnis
02-25-2019, 12:34 PM
Advanced Flight Systems internal EMS systems use ungrounded (http://advanced-flight-systems.com/forum/threads/1401-Type-J-or-K-CHT-probes?p=6523&viewfull=1#post6523) thermocouples. To verify this use your multimeter between one of the output leads and the casing of the probe. You should measure in the megohms. If not give us a call to order the proper probes.

heedh
02-25-2019, 02:48 PM
Thanks Shawn - measured 21 ohms between the pin at the EFIS and the probe casing, so calling to order new probes now. For my benefit/education, can you explain what the difference between grounded and ungrounded probes are? I assume ungrounded thermocouples isolate the probe case from the engine body? Or something else?

Shawn McGinnis
02-25-2019, 03:49 PM
Yes basically. Grounded probes will react faster but ungrounded probes are more accurate and less susceptible to noise.