Mode C mystery continues
Thanks for your thoughts, Cecil. ATC is still reporting intermittent mode C, despite what we've tried so far (see interspersed below). I'm going to try to get troubleshooting help from the local avionics shop - I'd welcome theories of what is going on to share with them.
My thoughts;
Possible issues
1) Efis is providing the Alt out data intermittently?
2) A loose gray code wire from Efis to Transponder?
3) Transponder is acting up?
Tracking down the problem
1) Does the Efis have any intermittent or flashing data in its altitude display?
Check the Efis recorded internal data (Black Box viewer) to see if Altitude is intermittent.
These looked OK. The altitude display on the EFIS has been solid throughout, and nothing looked out of the ordinary on the MGL black box viewer.
2) If OK then check gray code wiring between Efis and Transponder? Make sure all are good.
We unplugged the wiring from the back of the EFIS, checked for anything obvious, and sprayed connector cleaner. Same from the inside of the tray, after pulling out the transponder. My helping mechanic reported that wires/pins looked solid.
3) Stop by an avionics shop and get there help or borrow a Transponder checker and monitor transponder altitude output.
Borrow a GTX-320 or 327 and swap it with your unit (327 will show altitude on its display which could tell you directly if the Alt output is intermittent), GTX 327 should be directly swap-able with the 320 without touching your tray or wiring.
If its wiring when you get a 327 you could change your Efis to Transponder Alt data wiring from gray code to RS232 or just fix the bad wire.
On the ground, the altitude is solid with a just-put-in, used GTX 327, and if I start it in setup mode, none of the gray code lines are flickering.
Our VFR transponder check passed OK on the 327. In the air, the 327 displays the correct pressure altitude the whole time...even when ATC reports our Mode C has blinked out (!?). This is the part that really confuses me. ATC has not reported our squawk dropping out, though.
Our Echo ADS-B iPad app in monitor mode is occasionally showing 0 pressure altitude in flight, too - it is set up to wirelessly sniff the squawk and pressure altitude. I noticed a problem with the relative altitudes reported on ForeFlight the same day ATC first alerted me to the intermittent mode C (that was when I had the 320a in).
Thanks