Andy,
You originally stated you have an early GRT Sport. It likely has a built in GRT GPS, usually connected to Serial port 3. Yes, and it is
Like Bob, I also didn't understand your set up. There is no GNS 430 or GTN 650 or similar GPS navigator, correct? Your G3X is not one either. I'm not sure about this. It is a moving map GPS with flight planning capabilities. It just does not have AHARS or ADAHRS. So it can't be used as a PFD but it is as much of a moving map as the internal GPS on the GRT EFIS
You will have to follow your wiring. I would check with Stein if they originally made the panel or Jesse's crew may have made a wiring diagram. The switch may indeed allow you to select either the GRT GPS or G3X GPS to send RS232 GPS information to your autopilot. This will provide horizontal steering only, no vertical steering and no GPSS steering. I could look for the Stein information but there wasn't any harness info. It was just a page of instructions for connecting the equipment the builder purchased. I don't think it even went as far as the individual pin and plug level of detail.
I'll take you word for on the steering because I can't refute it. BUT using the A/P control head before I injected the ARINC into the equation, the A/P would fly GPS NAV of my flight plan in the GRT EFIS, and I was also able to specify through the A/P a preset altitude to climb or descend to, and at what rate of climb or descent. I guess that was the A/P capability aside from the information the EFIS was sending to the A/P. And yes, I believe the toggle switch was simply allowing me to independently decide which flight plan I wanted to send to the A/P. That is, the GRT plan, or the G3X plan.
I have older Sports also and ARINC, but no autopilot. My ARINC is connected to serial port 4 of my primary Sport. However, in General Setup page, serial port 4 in and out are set to OFF since I am not using the serial port 4 provided on the ARINC module. I think GRT tech support may be mistaken here. The ARINC module connects to the Sport over the Sport serial port 4. Since that makes that port unavailable for other uses, GRT added a serial port to the ARINC module to replace the one the module took away from the EFIS. If you use the "extension port", you have to tell (define) it's use to the EFIS just like any other port. But you aren't using it, so your Serial Port 4 in and out should be OFF. This has been standard in the software for years. This is my understanding and exact setup as well.
Bob is the autopilot expert, certainly not me! But, does your autopilot work correctly by itself? Meaning just using it's front panel controls? Then if you connect just the RS232 signal, does the EFIS control it properly? This should be how you have been using it until now. When I got the A/P installed, I flew it using the A/P controls themselves. It would let me hold a course, climb and descend, and use control wheel steering (That is, turn deviate from the selected course by turning the knob on the A/P. I could also move past that initial control level (which you get when you turn the A/P ON) and turn on the GPS NAV mode which would fly the flight plan in the EFIS. I never tried to actually control the A/P directly from the EFIS menus. Can you use the switch and control the autopilot with the G3X? I believe I tested this successfully but I've tried to keep the G3X out of the discussion because I figured that just complicating things. If the EFIS and GPS and ARINC all work, I can worry about the G3X working later.
GRT Sport EFIS require an upgrade to support vertical autopilot commands. Do you have this? I don't believe I do. I never did the upgrade. So, like the G3X, I figure I can worry about that later (unless it might be part of the current problem!!)
You might want to put your thoughts together and the results of your investigation and post it on the GRT Community Forums. GRT's Jeff, who does all the software programming monitors that forum and provides answers. He is probably the most qualified person to assist. I may try this but it seems like I just have some type of novice issue that is a simple oversight. Maybe it is more sinister than that, but I just seems like it should be a simpler problem.
Keep us posted!
Jim Butcher