GTX-327 Issue

[Nub_Pilot]

I recently had some repairs done on my plane which required turning the plane upside down (composite repair to belly). All repairs are complete and during first flight after repairs my transponder would not turn on. The shop conducting the repairs stated that they did not disrupt any dash items during the composite repair.

It is on its own lighted switch, when turning the switch on the light on the switch would be normal and bright, but when I attempted to turn on the transponder the switch light would dim (barely visible) and transponder would not turn on. To me it seems that the transponder is drawing too much current (based on dimming of the switch light) which could be a grounding issue when the plane was upside down.

Is it possible that during the repairs some of the dust and debris could have bade its way into the connectors and causing this issue? Has anyone had any similar issue with the GTX-327 transponder? I am just trying to figure out where my starting point is to troubleshoot the issue. The plane is an S-LSA and I will only be conducting visual inspections.

Thanks for any help you could offer.
Jeremy

 

[Daleandee]

I wouldn't think the transponder is drawing too much power. It's much more likely that it cannot get enough power either from a bad ground connection, or no ground connection and the ground is finding a different path to follow.

If possible put a volt meter on the wiring to the transponder, at the transponder end, to see how much voltage is actually there. Even then you might get a good voltage reading but not have enough amperage for the transponder to function properly.

Use a meter to check the ground connection at the transponder against a known good grounding point (grounding tab block).

Dunno if this helps ...

 

[jsstevens]

Because of the light on the switch dimming, I wonder if the upstream power connection is now loose? It sounds like the voltage at the switch is dropping when the load is turned on. That could be a short in the transponder but it seems to me that if that were the case, the fuse/breaker would let go. However, if the connection for power above the switch (at the fuse or breaker, or at the switch or somewhere in between if there are other junctions) were loose, (or corroded-but I don't know why that would show up now) it could account for the voltage to drop when under the normal load. On a 14 or 28 volt system it doesn't take much resistance to make that happen.

 

[Nub_Pilot]

thanks for the replies and I'll see what I can find this weekend during a visual inspection and I'll discuss taking a voltage reading when the shop does the transponder inspections.

 

[Aloui]

What did you find?

 

[Der Fliegermeister]

I'm confused. Is your 327 being powered through a just a switch? It it connected to a breaker on the avionics bus? Main bus? The unit only requires a 5 amp breaker typically.

Reference ground is a more complex issue for a composite airframe than a conventional aluminum bird. Is it possible the repair affected the continuity of the bus ground?

Also, (point of curiosity) what the hell kind of repair was this?