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Thread: AHRS Problem with Garmin G3X Touch

  1. #1
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    Default AHRS Problem with Garmin G3X Touch

    2 weeks ago I encountered a AHRS Problem with the Garmin G3X Touch, for which I might need some support in order to solve it.
    Airplane is an CarbonCubEX with about 100h on the counter equipped with a Garmin G3X Touch with GDU460 using 'the full monty' of Garmin's LRU's (GTX23 ES / GNC 255A / GSU 25 / GMU22 / GEA 24 / GNC 225A...)
    The airplane has been built by a multiple builder with a lot of experience and love to detail and quality.

    After starting the engine the picture I was presented (see attachment) was not exactly what we would like to see.
    TAS, Altitude, Vertical Speed and OAT were 'redcrossed' - Message was: AHRS Hardware fault
    To me somehow contradicting...?

    Has anybody of you seen this or has anybodyIMG_1817.jpgIMG_1818.jpg experience with this situation?

    Many thanks in advance for good inputs! ;-))

  2. #2
    Senior Member Andy's Avatar
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    Default Re: AHRS Problem with Garmin G3X Touch

    Quote Originally Posted by aerofly View Post
    After starting the engine the picture I was presented (see attachment) was not exactly what we would like to see.
    TAS, Altitude, Vertical Speed and OAT were 'redcrossed' - Message was: AHRS Hardware fault
    To me somehow contradicting...?

    The display indications are that you have good attitude data but no air data. Since all air data parameters come from the GSU 25 ADAHRS the warning message seems quite reasonable. (Perhaps the confusion comes from Garmin contracting ADAHRS to AHRS but it's still the GSU 25 that is the source)

    Assuming you have the GSU 25 wired with the primary CANBus link and the secondary RS-232 link it seems likely that you really do have a bad GSU 25. The attitude data, which is good, is transmitted over the same data links so it's not likely to be a wiring problem.

    Was it ok before engine start?? It's normal to power the basic G3X system on IBBS before engine start so you have the engine display available.

    Edited to add -

    Your photo shows IBBS off. If you started the engine with IBBS off the G3X could have been trying to power up with low and fluctuating voltage. Do you see the same fault if you simply switch on IBBS only and don't start the engine?
    Last edited by Andy; 12-08-2020 at 03:35 PM.

  3. #3
    Administrator Pete D's Avatar
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    Default Re: AHRS Problem with Garmin G3X Touch

    Did re-booting the G3X, or engine shutdown and restart clear the problem?

    I have seen, very rarely, issues with an ADHRS when the engine is started with the IBBS in the OFF position. One of the reasons the IBBS is ON for start is it keeps steady power to the G3X and it's components if the bus voltage gets pulled down by the starter. If the engine is started with the IBBS off, that momentary blip of the voltage going below the threshold that the G3X units need can cause them to partially reboot and get "stuck". If that is the case, pulling the ADHRS breaker for 30 seconds, resetting it has cleared it up for me, and using IBBS ON for starting prevents this.
    Pete Dougherty
    R & D Shop Manager
    Cub Crafters Inc

  4. #4
    Senior Member Andy's Avatar
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    Default Re: AHRS Problem with Garmin G3X Touch

    Looks like my edit and Pete's reply overlapped. We are thinking the same re IBBS being off in your photo.

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    Default Re: AHRS Problem with Garmin G3X Touch

    Quote Originally Posted by Andy View Post
    The display indications are that you have good attitude data but no air data. Since all air data parameters come from the GSU 25 ADAHRS the warning message seems quite reasonable. (Perhaps the confusion comes from Garmin contracting ADAHRS to AHRS but it's still the GSU 25 that is the source)

    Assuming you have the GSU 25 wired with the primary CANBus link and the secondary RS-232 link it seems likely that you really do have a bad GSU 25. The attitude data, which is good, is transmitted over the same data links so it's not likely to be a wiring problem.

    Was it ok before engine start?? It's normal to power the basic G3X system on IBBS before engine start so you have the engine display available.

    Edited to add -

    Your photo shows IBBS off. If you started the engine with IBBS off the G3X could have been trying to power up with low and fluctuating voltage. Do you see the same fault if you simply switch on IBBS only and don't start the engine?
    Hello Andy
    your assumption regarding the wiring is correct and yes, was OK before engine start.
    After the problem occurred I have tried to reboot the system with engine ON/IBBS OFF and engine ON/IBBS ON and the same procedure with engine off. Result was the same.

    Your second assumption which was also taken up by Pete, that IBBS was off during engine start is correct too - that's the way it's instructed in the Checklist. But after getting familiar with the IBBS (did some reading...) I totally agree that IBBS should be ON during engine start (Pete is giving the reason...)
    and we might be on the right track here, that the problem has to do with low voltage during engine start. OAT was 3 centigrade and the battery was not delivering full power due to the fact that after the 100h service the screw attaching the power cable to the negative pole was not really tightened the way it should - so the low voltage blib could have caused the problem we see...
    Next will be that I will follow Pete's proposal and try to reset the 'boot mechanics' of the GSU 25 by depowering it via circuit breaker for 30 seconds.

    Let's see if that helps, or if the GSU 25 unit has to replaced....

    Thanks both of you for you valid input!! Will keep you posted...

  6. #6
    Senior Member Andy's Avatar
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    Default Re: AHRS Problem with Garmin G3X Touch

    "Next will be that I will follow Pete's proposal and try to reset the 'boot mechanics' of the GSU 25 by depowering it via circuit breaker for 30 seconds."

    I think Pete's suggestion to pull the GSU breaker was to attempt to reset the GSU 25 while leaving the rest of the G3X system powered. The GSU 25 cold boots every time you power the airplane.



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    Default Re: AHRS Problem with Garmin G3X Touch

    Quote Originally Posted by Andy View Post
    "Next will be that I will follow Pete's proposal and try to reset the 'boot mechanics' of the GSU 25 by depowering it via circuit breaker for 30 seconds."

    I think Pete's suggestion to pull the GSU breaker was to attempt to reset the GSU 25 while leaving the rest of the G3X system powered. The GSU 25 cold boots every time you power the airplane.


    Thanks for the additional note - that will make a difference!
    I am also asking myself if it is possible to 'repair' the GSU 25 Bootstrap by reloading the GSU 25 specific SW/Firmware...

  8. #8
    Senior Member Andy's Avatar
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    Default Re: AHRS Problem with Garmin G3X Touch

    You won't be the first one to experience this GSU 25 fault -

    https://vansairforce.net/community/s...d.php?t=146943

    Was it caused by engine start, low temperature, or simply random? I doubt you'll ever know but I suspect the fix will be to replace it.

  9. #9
    Administrator Pete D's Avatar
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    Default Re: AHRS Problem with Garmin G3X Touch

    Quote Originally Posted by aerofly View Post
    Hello Andy
    your assumption regarding the wiring is correct and yes, was OK before engine start.
    After the problem occurred I have tried to reboot the system with engine ON/IBBS OFF and engine ON/IBBS ON and the same procedure with engine off. Result was the same.

    Your second assumption which was also taken up by Pete, that IBBS was off during engine start is correct too - that's the way it's instructed in the Checklist. But after getting familiar with the IBBS (did some reading...) I totally agree that IBBS should be ON during engine start (Pete is giving the reason...)
    and we might be on the right track here, that the problem has to do with low voltage during engine start. OAT was 3 centigrade and the battery was not delivering full power due to the fact that after the 100h service the screw attaching the power cable to the negative pole was not really tightened the way it should - so the low voltage blib could have caused the problem we see...
    Next will be that I will follow Pete's proposal and try to reset the 'boot mechanics' of the GSU 25 by depowering it via circuit breaker for 30 seconds.

    Let's see if that helps, or if the GSU 25 unit has to replaced....

    Thanks both of you for you valid input!! Will keep you posted...

    If the master is on, cycling the IBBS won't remove power to any of the G3X LRUs. Breakers need to be pulled or master and IBBS cycled together.

    What checklist do you have that says IBBS off for starting? The Executive Glass Touch supplement in the CCK/CCX-1865 manual says IBBS then Master on for the starting procedure.

    CCK CCX POH Starting engine.png
    Attached Files Attached Files
    Pete Dougherty
    R & D Shop Manager
    Cub Crafters Inc

  10. #10
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    Default Re: AHRS Problem with Garmin G3X Touch

    Quote Originally Posted by Andy View Post
    You won't be the first one to experience this GSU 25 fault -

    https://vansairforce.net/community/s...d.php?t=146943

    Was it caused by engine start, low temperature, or simply random? I doubt you'll ever know but I suspect the fix will be to replace it.
    My personal conviction of what caused the problem is the following:
    Cranking the engine led to restart of system due to low voltage. Guess the second try cranking happened during AHRS initialization (engine started, but due to low voltage the system restarted again...) which busted the capability of the GSU 25 to start up properly...
    And of course the Checklist, which was prepared by a friend of the former owner of the CC, is wrong... IBBS has to be on.... (see comment of Pete...)

    I have read the post in the vansairforce.net a few days ago, searching for ideas how to explain my problem.
    Interesting....!

    Based on everything I have seen so far I guess my GSU 25 will have to be replaced and of course I would become a very happy Garmin customer, if I would get Garmin to support me by exchanging my GSU 25 -
    The installation is brand new (100h) and done very carefully, so I don't think that the problem was caused by some misstep during the built process...
    As I am new in this area I actually don't know who I have to address in order to give it a try...
    Any idea, I am based in Switzerland, not really the same EXPERIMANTAL scene as in US... ;-)))

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