What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Garmin GNX375 ADS-B Fault

CajanPittsDriver

I'm New Here
Hi, I could use some help from all Garmin GNX375 experts. I have one installed in an RV-8 that has been working fine until a month ago the ADSB out stopped working. I get the code “ADS-B out fault. Pressure attitude source inoperable or connection lost.”

The plane is new to me and the wiring diagrams are sketchy. It has two GRT HX Horizon EFIS’s in the panel which may be the source for the pressure altitude. Don’t know. Very hard to trace out. Thoughts?

Thanks in advance
 
Hi, I could use some help from all Garmin GNX375 experts. I have one installed in an RV-8 that has been working fine until a month ago the ADSB out stopped working. I get the code “ADS-B out fault. Pressure attitude source inoperable or connection lost.”

The plane is new to me and the wiring diagrams are sketchy. It has two GRT HX Horizon EFIS’s in the panel which may be the source for the pressure altitude. Don’t know. Very hard to trace out. Thoughts?

Thanks in advance
David,

The GNX 375 can receive altitude data over RS-232, ARINC 429, a Gillham altitude encoder, or a small Garmin GAE altitude encoder/config module mounted to the rear of the unit.

In the absence of having information on how your plane is wired and configured, it would probably help to provide screen shots of the GNX 375 RS-232 and ARINC 429 configuration pages.

To enter configuration mode on the GNX 375, just hold in on the main knob during power up.

With this information, we could perhaps at least determine how the installer expected to provide altitude data to the GNX 375.

GNX 375 A429 Config Page.png

GNX 375 RS232 Page.png

GNX 375 Sensor Config Page.png

You could also go to the Main Data Inputs section of the GNX 375 Diagnostics (with all of your avionics running on the ground) and see if Altitude data is being received. Probably not.

GNX 375 Diagnostics Choices.png

GNX 375 Main Inputs Page.png

As explained in the GNX 375 pilot's guide, you can conveniently capture great quality screenshots (like those shown above) to the SD card.

How to Screen Captures.png

Steve
 
Last edited:
I installed the 375 in David's airplane some years ago and it does not have the GAE module. It takes its encoder signal from the GRT WS serial #6 out, feeding into Serial #3 on the 375. I dont have the ARINC map in my immediate records, but I can say that the ADSb was functioning before this.

That said, we did have a GRT display die and the replacement required some substantial reconfiguration to get them back to normal. With all the button pushing, it seems reasonable to assume we changed the serial format on that chanel. That, or we have broken wire.

Assuming the wire is not broken, what format is the 375 looking for from the GRT? With the two units being generations apart, the selections in each configuration window for each unit do not align word for word.
 
I installed the 375 in David's airplane some years ago and it does not have the GAE module. It takes its encoder signal from the GRT WS serial #6 out, feeding into Serial #3 on the 375. I dont have the ARINC map in my immediate records, but I can say that the ADSb was functioning before this.

That said, we did have a GRT display die and the replacement required some substantial reconfiguration to get them back to normal. With all the button pushing, it seems reasonable to assume we changed the serial format on that chanel. That, or we have broken wire.

Assuming the wire is not broken, what format is the 375 looking for from the GRT? With the two units being generations apart, the selections in each configuration window for each unit do not align word for word.

Thanks, Michael,

The GNX 375 supports several serial (RS-232) altitude data formats. Not sure which one the GRT supports, but perhaps you do. Hopefully the solution is as simple as properly configuring the input port 3 on the GNX 375 to match the GRT output.

Transponder Frmt 1 - Allows remote control of GNX 375 from G3X Touch.

Alt Frmt 1 100FT - Sandia/Icarus/ACK altitude format with parallel Gray sourceor 100 ft encoding.

Alt Frmt 1 25FT - Supports Sandia/Icarus/ACK altitude format with 25 ft or lower encoding.

Alt Frmt 3 100FT - Supports Shading altitude format with a parallel Gray codesource or 100 ft encoding.

Alt Frmt 3 25FT - Supports Shadin altitude format with 25 ft or lower encoding.

ADC Frmt 1 - Supports Shadin G/S/Z ADC Formats.

Fortunately the GNX 375 has such great diagnostics that David can go to the Main Data Inputs Page (as shown above) to verify whether or not the selected format is working without ever leaving configuration mode. This, of course, would depend on having good wiring.

Steve
 
Last edited:
Serial #3 on the 375 is "Not Receiving" (in bold red letters) according to the main data inputs page. And yes, the GRT has several of the formats you listed available.

I guess that narrows it down to a broken wire?
 
Serial #3 on the 375 is "Not Receiving" (in bold red letters) according to the main data inputs page. And yes, the GRT has several of the formats you listed available.

I guess that narrows it down to a broken wire?

...or the GRT setup/configuration needs to be looked at again...
 
Thank you for your suggestions and Mike for his clarifications, this is really a learning process for me. When I can get to the plane I will try them and keep the group informed.

Thanks again!
 
Update?

Any update? I am considering adding a GNX 375 to my stack and have the Horizon HX EFIS as well. (Have an SL30 and SL40 for VOR/LOC Nav, but have had trouble with my transponder and Echo UAT ADSB cooperating with each other. Looking to add IFR GPS and solve that problem in one package.)

Have you been able to communicate weather data from the GNX 375 to the HX as well?

Hope it's going well!
 
Yes, we got it fixed - sort of. Pin to pin wiring was good, but apparently the GRT EFIS unit feeding the alt encoder signal stopped doing that. Swapping the encoder feed to the other GRT EFIS brought the 375 back to full capability.

Also understand that these EFIS units are old, and no longer in production.
 
Back
Top