Thread: 327 mode C home diagnostics

Any ideas from the field about easily trouble shooting/isolating transponder component (encoder, antenna, trans, grounding) problems just short of finding a shop?

Background: Watched my plane being ferried on FlightAware.com. Pretty cool for following flights, commercial and private.....But, the ferry pilot said ATC was contacting him during flight following indicating they couldn't see him. I think the tower said it showed up but no Altitude. I think she said all I get is a Primary something or another, but not good enough for class B...So, I think no mode C is the issue. Its a 327 and it had it pitot static check this past Dec. I don't know when the problem started. Hopefully I described it correctly. The ferry pilot thought pull the panel and check antenna connection and test antenna before sending in the 327 in to Garmin. Ferry guy said he's experienced bad antennas. Unfortuately there is nobody around me that can does avionics work. Any suggestions?

From the GTX327 manual, you can access the Configuration page. While off, push & hold FUNC as you turn the unit on.
Keep pushing Func until you see the RS232 input page. If not selected to 'off', you should see pressure alt from a serial source.
(I think this is typically with planes having D100 / D120)
Push func some more to see Gray code page if you have a parallel encoder.
Look in your planes maint manual to be sure which encoder you are supposed to have. It may be document in the
last static system test.
At first, only observe stuff on the Config. pages, and write down anything you might have the urge to change.
Good luck.

Thanks 248CT

The D100 was out before it was delivered to comply with fuel filter SB. How likely is that a wire could be affected by working behind the panel. Also, I'm didn't notice of Pressure Alt was showing when 327 was on. If it doesn't show is that where your suggestion comes in?

BB

You should see the Pressure Altitude on either the RS232 or Gray code page, depending upon wiring as serial or parallel.
Opening a single serial wire could cause the mode C missing problem, but opening a single Gray code parallel
wire would only cause 'strange' altitudes. If you are lucky, maybe it is wired serial, but just turned
off in the transponder. The Dynon d180 manual says its output should be set to Icarus format to drive a GTX327.
I had a D100 installed at the dealer (and no D120) and an external parallel encoder.

If you google for "gtx327 installation", you can find the manual; it doesn't seem to be available on the Garmin site.