On Tue, 26 Jun 2007 13:07:02 -0500, "Dan Luke"
wrote:


"Peter Clark" wrote:


Perhaps to complete my reasoning - the original post says it drifts
off altitude when in level flight clicking ALT.

Sorry, that was incorrect. If ALT is selected in level flight, it will hold.
Altitude capture is the only thing not working.

Ah, yep that makes a big difference...

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...

The KAP 140 has been swapped for a known good one. No joy.

The wiring has been checked for a ground on the Gray code lines & found to be
ok.

The tech has plugged a laptop into the system and sent various coded
altitudes to the KAP 140 and it has decoded them correctly.

Have you tried changing the baro in flight to some number of inches
off the real pressure to see just how far the system is mis-reading
the baro? That will tell you something.

I'll have to go pull my wiring diagrams out since I don't recall off
the top of my head, but does the KAP140 actually get it's graycode
from the GTX, or does it get it from one of the outputs on GIA#2? Have
they tried swapping GIA#1 and GIA#2 to see if there is any difference?