jjconstant

jjconstant

Well Known Member
I have an issue that I could use some troubleshooting help on. I have a Garmin 300xl as GPS1 and the internal GRT GPS as GPS2 on a GRT EFIS system. The EFIS drives the TruTrak digiflight II VSGV.

When using GPS1 the auto pilot is rock solid and displays track and a flashing plus. This is telling me it has a valid gps track that it's using. It smoothly flys a multi waypoint flightplan. When I switch GPS sources on the EFIS to GPS2 (internal GRT gps), even though the moving map works, ground speed works, and it doesn't annunciate "GPS2 lost", which it does in the hangar, when I try to couple it to the auto pilot, the auto pilot track indicator starts rapidly changing displayed track, with wild swings in what it thinks the gps track is, then switches to heading mode (it has lost a valid gps feed) and the heading keeps rapidly and wildly changing.

What I'm unclear on is if the auto pilot is being fed by the same pin out on the EFIS no matter which GPS is driving the EFIS, and both GPS sources seem to drive the EFIS fine, why is there such a difference in terms of what the auto pilot behaviour is? I have moved the GPS2 antennae to a known good location, but I don't know if the quality of the antennae is an issue. I know some Chelton guys who had weird problems until they went with a higher quality, powered antennae. I don't know if that's available with the GRT or if it would solve the problem.

I plan to test it again, turning things off to see if it's RF related, but it beats me why the same pin from the EFIS would supply two very different quality signals unless the source GPS signals were different...

Jeremy

RV7A
 
Are you sure that the GRT has not switched to the heading mode and following that. If the heading is a way off the GPS 1 track it will try to find the heading dialed up. Mine works well on both GPS 1 and GPS 2 .
 
When you switch GPS sources, does the flashing indicator change to a question mark? This indicates that the track being received is vastly different from the previous track being flown.
 
Lucas, you are correct, when I switch the EFIS source from external gps 1 to internal gps2 the a/p annunciation goes from TRK and flashing + to TRK ? to HDG and then the heading on the a/p starts fluctuating, even after I hit the a/p disconnect switch!

I've tried it in heading mode on the efis, with the heading agreeing with the displayed heading shown on the a/p as well as in enav mode. Same problem each time.

If the received tracks are vastly different I don't understand how the moving map still functions correctly, showing me moving across the map in the correct direction, at a believable ground speed. It doesn't change at all when I switch gps sources. I know that the GRT EFIS will do ded reconning when gps fails, but I also know that in the hangar I get annunciation of gps2 lost when it can't see the satellites.

It also doesn't explain why the a/p heading fluctuates when disconnected from gps2. I have not tried to see what the a/p heading does when I turn off both gps sources...I would have to pull the antennae out on gps 2 and simply leave gps 1 with the switch off.

So, we have the 2 gps's sending vastly different tracks. What's the next step?


Jeremy
 
How do you have all of this wired? Do you have the ARINC module as well (I'd highly recommend it if you have the VSGV autopilot)...

What are the serial port settings?

If the internal GPS wasn't working I would not expect the moving map to work either..

Is the 300XL connected directly to the autopilot in any way?
 
Yes, I do have the Arinc module in DU 1. I checked all the settings including serial settings with someone looking over my shoulder. I'm not at the hangar now, nor do I have my documentation (would be handy at home!:rolleyes:).

I have a switch to change the source driving the auto pilot from the EFIS (which means everything/anything hooked up to the EFIS can drive it) to just the 300xl only. That, too, is not working properly and I've gone in and checked the ARINC out settings on the 300xl and it doesn't appear that the info is getting to the GRT. I called Stein who did my panel, they called Garmin and garmin's response is "it's out of warranty". Before I send it off to spend an unkown amount of money on an unkown fix, I want to be sure I've exhausted all the possibilities on my end.

I am electronically challenged but am embracing this as yet another learning experience. A friend suggested that I make a flow chart of the flow of electrons to and from everything and I think it's an excellent suggestion.

None of this is stopping me from flying! Just trying to ensure that the functionality that I paid for is available!

Jeremy
 
jjconstant said:
Lucas, you are correct, when I switch the EFIS source from external gps 1 to internal gps2 the a/p annunciation goes from TRK and flashing + to TRK ? to HDG and then the heading on the a/p starts fluctuating, even after I hit the a/p disconnect switch!

I've tried it in heading mode on the efis, with the heading agreeing with the displayed heading shown on the a/p as well as in enav mode. Same problem each time.

If the received tracks are vastly different I don't understand how the moving map still functions correctly, showing me moving across the map in the correct direction, at a believable ground speed. It doesn't change at all when I switch gps sources. I know that the GRT EFIS will do ded reconning when gps fails, but I also know that in the hangar I get annunciation of gps2 lost when it can't see the satellites.

It also doesn't explain why the a/p heading fluctuates when disconnected from gps2. I have not tried to see what the a/p heading does when I turn off both gps sources...I would have to pull the antennae out on gps 2 and simply leave gps 1 with the switch off.

So, we have the 2 gps's sending vastly different tracks. What's the next step?


Jeremy
Click to expand...

Are the baud rates of the GPS's different? The AP baud rate has to be changed manually. I see in your post below you have verified everything as set up correctly, but just wanted to check. The other thing to check would be to verify the serial connection from GPS 2 is solid all the way to the AP. Perhaps if the wire is loose at the switch, we could see this problem.

If all else fails, we will be happy for you to send the AP to us for testing here at the factory. Thanks!
 
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