Transponder malfunction

Good afternoon,

This weekend, when working with a towered airport, I found that my transponder was reporting altitude approximately 35% higher than the altimeter. The local antenna farm provides a solid reference for confirming the altimeter - and that was OK. The controller was kind enough to do some readings at different altitudes and the error seems to be a percent of scale error. All previous biennial calibrations have come out right on the money.

I have the plane grounded till I can schedule a trip to an avionics shop (home airport sits under a Class B); however, has anyone else had this kind of problem and how costly is it to resolve? Is this likely to be the transponder; or the encoder, or both?

Transponder is a Garmin 320A new at installation and the encoder is a Ameri-king , also new at installation.

Thanks for your input - transponders are pretty much voodoo in my little world.

Dave S
KF7 Trigear
912ULS Warp

I doubt it's the transponder. Most likely the encoder or a bad wire connection between the two. It should be pretty easy for the tech to track down once a calibrator is hooked up.