I have a Narco transponder which tells atc that I'm about 1800' higher than I actually am. On 4 occasions I have been verified higher by 2 different atc's. Two times I was at 4500' and they were showing 5800'. I pulled the transponder and the encoder and sent them to Ellison in Texas (one of the few shops that still work on Narco). They can't find anything wrong and I decided to have them do an alignment. I still have the exact same issue.
The plane is a '66 Cherokee 180c and the encoder connects to the common static pressure line which goes to the altimeter, air speed, and vertical speed indicator. All of those other instruments read correctly as verified by the electronic panel on my Garmin GPSMAP696 so I assume the encoder is getting the correct pressure.
There is one thing I forgot to mention to Ellison. My altimeter sticks a little. I can tap the face of it and the indicator will jump to where it is supposed to be. Could that somehow effect the pressure the encoder is seeing? It's not bad, the dial might lag behind a hundred feet or so but will correct itself with the normal plane vibrations even if I don't tap the face.
The transponder antenna had grime on it but I have kept it clean since discovering this problem. If the encoder is getting the correct pressure yet the transponder transmits the wrong altitude, what else is left? The antenna? Has anyone ever heard of a transponder antenna going bad? Ellison never mentioned antenna to me and I didn't ask. I have verified continuity on all the wires going from the encoder plug to the transponder tray/plug.
Any guesses?
The plane is a '66 Cherokee 180c and the encoder connects to the common static pressure line which goes to the altimeter, air speed, and vertical speed indicator. All of those other instruments read correctly as verified by the electronic panel on my Garmin GPSMAP696 so I assume the encoder is getting the correct pressure.
There is one thing I forgot to mention to Ellison. My altimeter sticks a little. I can tap the face of it and the indicator will jump to where it is supposed to be. Could that somehow effect the pressure the encoder is seeing? It's not bad, the dial might lag behind a hundred feet or so but will correct itself with the normal plane vibrations even if I don't tap the face.
The transponder antenna had grime on it but I have kept it clean since discovering this problem. If the encoder is getting the correct pressure yet the transponder transmits the wrong altitude, what else is left? The antenna? Has anyone ever heard of a transponder antenna going bad? Ellison never mentioned antenna to me and I didn't ask. I have verified continuity on all the wires going from the encoder plug to the transponder tray/plug.
Any guesses?
