I might not understand you situation, but Artex 345 ELT does not have integrated GPS data. You have to get it from somewhere. I have been surprised to learn the ELT I bought needs to be reprogrammed to NMEA 0183 baud rate 4800. I've even talked to one of the avionics shops that Artex points to and they couldn't talk specifics of my ARTEX saying they didn't really know. They just simply said 'We could do that" for $200. I'm frustrated that I was ignorant and wasn't informed about my purchase and installation before I bought it.
I’m no wiring expert, but do you have a wire from D-Sub connector pin 9 at the Artex 345 wired to the G3X connector? That’s a place GPS data is available. I understand you would assign that in preliminary setup of the G3X. I have not got that far yet so it is a mystery to me...that’s just what I’ve been told.
An EAA friend who did all his own wiring of 3 G3Xs in his RV10 helped me wire for GPS data. This is what he said would work. I have not tested it yet however. This photo is from my CubCrafters Garmin G3X manual page 4 of 5. It shows penciled in the GDU 465 P4602 connector where I used the free RS-232 tx 5 or pin 41 for that data.
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