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Garmin OAT inoperative…G3X with GSU 73

8MikeXray

8MikeXray

Well Known Member
I have a G3X with a GSU73 in my RV8. I’m close to the first flight, but unfortunately the OAT has been intermittent. It was working until two days ago (12.3) and now it is inoperative. The problem first appeared after system software updates were performed.

All wires/connections/ground as well as the GTP59 test good.

I found a post dated 2013 with the same issue I am experiencing.

https://vansairforce.net/community/showthread.php?t=100930

It stated the fix was to downgraded the software to 7.4 then back up to 8.0. I did the software downgrade/update today (7.4-8.0-12.4) and it did not fix the inoperative OAT.

Is there anyone with more information/ideas that can help me troubleshoot this problem and get my OAT working again?

Thank you for your help!
 
Last edited:
I'd check the ground at the GTP59

I had a similar issue in my RV-14A. The OAT would either be x-ed out or would be obviously wrong (160F for example).

The problem was intermittent. Sometimes the OAT was correct for an entire flight and then the next time I flew it would be inop again.

I checked the local ground wire at the GTP59 and found that the connection to the airplane was not ideal. A bit of roughing up the aluminum on the airplane and retightening the connection and the problem was gone.

bruce
 
8MikeXray said:
I have a G3X with a GSU73 in my RV8. I’m close to the first flight, but unfortunately the OAT has been intermittent. It was working until two days ago (12.3) and now it is inoperative. The problem first appeared after system software updates were performed.

All wires/connections as well as the GTP59 test good.

I found a post dated 2013 with the same issue I am experiencing.

https://vansairforce.net/community/showthread.php?t=100930

It stated the fix was to downgraded the software to 7.4 then back up to 8.0. I did the software downgrade/update today (7.4-8.0-12.4) and it did not fix the inoperative OAT.

Is there anyone with more information/ideas that can help me troubleshoot this problem and get my OAT working again?

Thank you for your help!
Click to expand...

Mike,

If you unplug the P732 connector from your GSU 73, what do you measure for resistance between pins 16 and 15, and also between pins 16 and 35?

GSU73_GTP59.png

The resistance between pins 16 and 15 should be close to zero.

The resistance between pins 16 and 35 should be between 500 ohms (at 0 deg C) and 549 ohms (at 25 degs C).

There is no requirement that the GTP 59 be grounded to the airframe to work, so that is not your problem.

Is your airspeed valid or invalid (shown with a red X)? The system cannot calculate OAT without a valid airspeed. Zero airspeed is ok, but it must not be shown as failed or OAT will also be shown as failed.

Steve
 
8MikeXray said:
Hi Steve,

I powered up the unit this morning and now the OAT is working again!

No physical changes to anything was made overnight.

When I powered it off yesterday, it was not displaying OAT.

Yes, airspeed is displayed.

https://share.icloud.com/photos/03dbgTtXStGe0WYizlL0klcow

The GSU73 is a new unit but I’m having doubts about its integrity.

Thoughts?
Click to expand...

Wiring. It is almost always a wiring problem.

You must have an intermittent open or short in one of the 3 wires to the GTP 59. If you have any inline connectors (like one at the wing root) - get rid of them!

Steve
 
We had a similar problem in our early-adopter G3X (with a GSU73) in our RV-3 when it was new. It turned out to be a bad connection in the ground somewhere along the line - I got tired of chasing it one day, and simply re-wired from the sensor to the GSU, paying close attention to the grounds along the way - and haven’t had a repeat of the problem in a decade….
 
I had a similar failure enroute from the paint shop. Replaced the mini molex plugs with dsubs in heat shrink. No problems since. BTW - I hate mini molex plugs. :mad:
 
Also, check that the grounding ring terminal is installed “as indicated “ with is going inbetween the sensor and the skuffed up aircraft skin.
 
Steve,

Guilty! I do have a CPC wing root connector. I will hardwire the GTP59 and press on. Have you seen many/any GSU73 failures that could cause the OAT to become inoperative?

Thanks
 
GSU 73 has multiple failures

Hi Steve,

I hardwired the GTP59 and upon completion the OAT was inop. Went home frustrated and returned today ready to completely change out the wiring on the temperature sensor and as soon as I powered up the G3X the OAT was working.

I decided to take the opportunity to complete the remainder of the set up tests.
I previously was able to perform the pitch/roll offset calibration for the GSU73.
I decided to see if the pitch/roll offset calibration test would function.
Today the pitch/roll offset calibration failed, and the engine run up test failed on AHRS 1 (GSU73)

I then switched over to AHRS 2 (GSU 25) and successfully performed the pitch/roll, offset calibration and engine run test.

I then attempted the magnetometer (GMU44) magnetic interference test and magnetometer calibration, both which also failed.

It seems that since the OAT has been intermittent through various cycles of wiring changes, and the GSU73 will not perform internal self tests, that the unit is defective.

Dual screen non touch, GSU73, GSU25, GMU44

Your thoughts please…….
 
OAT Probe Wiring

8MikeXray said:
Hi Steve,

I hardwired the GTP59 and upon completion the OAT was inop. Went home frustrated and returned today ready to completely change out the wiring on the temperature sensor and as soon as I powered up the G3X the OAT was working.

I decided to take the opportunity to complete the remainder of the set up tests.
I previously was able to perform the pitch/roll offset calibration for the GSU73.
I decided to see if the pitch/roll offset calibration test would function.
Today the pitch/roll offset calibration failed, and the engine run up test failed on AHRS 1 (GSU73)

I then switched over to AHRS 2 (GSU 25) and successfully performed the pitch/roll, offset calibration and engine run test.

I then attempted the magnetometer (GMU44) magnetic interference test and magnetometer calibration, both which also failed.

It seems that since the OAT has been intermittent through various cycles of wiring changes, and the GSU73 will not perform internal self tests, that the unit is defective.

Dual screen non touch, GSU73, GSU25, GMU44

Your thoughts please…….
Click to expand...

Have you checked the resistance of the OAT probe, as outlined by Steve in this post?

Thanks,

Justin
 
Justin,

Yes, I have checked the OAT probe forward and backwards and it checks out 100%. It seems that the BITE (built in test equipment) internal to the GSU73 is compromised. Thus pointing to my conclusion that the unit is defective.
 
Sent GSU73 in for repair and received an overhauled unit. Was never informed of the reason for failure from GARMIN even though that was our agreement when the unit was sent in. All features operational now. I just wish they told me what was defective in the GSU73.
 
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