KAP 140 trouble

"Dan Luke" wrote:


Anybody have any ideas?


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.

--
Dan
T-182T at BFM

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.

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....

"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.

Matt
--
Matt Barrow
Performance Homes, LLC.
Cheyenne, WY

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...

"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.


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.

Thanks a bunch for your input.

--
Dan
T-182T at BFM

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?

On Tue, 26 Jun 2007 22:40:27 +0100, Peter
wrote:


Peter Clark wrote

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.

My KFC225 does the same, and I've had several of the baro transducers
fail.

However, reading later posts, I agree the problem must be in the gray
code connection... this could be within the KAP140 unit though, unless
changing that doesn't make the problem go away.

Aparantly a spare computer was swapped in and there's been no change.
Unless I've missed something here all that's left is the physical
link.