On Nov 24, 7:15*pm, "Viperdoc" wrote:
I have flown with a couple of Garmin 327's and now have two 330's. The
pressure altitude readout on the transponder comes (as I recall) from the
blind encoder. It is obviously not connected to the altimeter setting.

So, the encoder could be culprit for the mis reported altitude. I suppose
there could be a difference in altitudes of 700 feet if the day was very
non-standard (such as very cold, high pressure, etc).

That is for sure. The altitude used by the transponder for all us
flying non-digial, non-flat screen bug smashers is not the same
necessarily as what you see on the panel. However, in my experience
when ATC is getting the wrong altitude its almost always either a
calibration issue or a wiring (i.e. dirty connectors, etc) issue. Its
usually something a good avionics shop can fix in minutes.

-robert