On Tue, 26 Jun 2007 10:11:22 -0700, "Matt Barrow"
wrote:


"Peter Clark" wrote in message
.. .
On Tue, 26 Jun 2007 06:06:23 -0500, "Dan Luke"
wrote:


John Jones, a fellow Cessna Pilots Ass'n member, has put me on the right
track, I believe. The autopilot receives its altitude information via
Gray
code from the Garmin transponder. If a particular line is grounded, it
would hold the autopilot at 13,000 ft.

Tomorrow I'll take this information to the avionics shop and see what
they
say.

That wasn't it. The incorrect altitude Gray code is definitely coming
from
the Garmin side.

If the gray code coming in from the transponder was wrong it would
hold the wrong altitude, not be unable to hold altitude....

As a really new (four months) G1000 owner, I must thank you for your
excellent running analysis here.

Perhaps to complete my reasoning - the original post says it drifts
off altitude when in level flight clicking ALT. AFAIK the only time
the KAP140 cares about the altimeter setting and the preselected
altitude is when it's in a capture mode, and it uses it's internal
pressure transducer to maintain altitude. Course, wouldn't be the
first time I've been wrong about the internal workings of the box, but
it's really not that big a deal to replace it and see - it's not like
there's no known history of the KAP140 computers being problematic...