KAP 140 trouble

"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?

"Peter Clark" wrote:


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.

Yes. On the ground I have set the xponder to squawk altitude and turned the
altitude preset on the KAP 140 up until it gives the "1,000 ft. before"
warning. That happens at 13,000 ft.

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?

The GIA2. The GIAs have been swapped and the problem stays.

--
Dan
T-182T at BFM

On Tue, 26 Jun 2007 13:57:53 -0500, "Dan Luke"
wrote:


"Peter Clark" wrote:


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.

Yes. On the ground I have set the xponder to squawk altitude and turned the
altitude preset on the KAP 140 up until it gives the "1,000 ft. before"
warning. That happens at 13,000 ft.

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?

The GIA2. The GIAs have been swapped and the problem stays.

Sounds like at this point they're going to have to build a harness and
ring out the graycode lines end-to-end from the GIA2 output through to
the AP computer connector individually. If both GIAs are producing
the issue and two AP computers are doing it, there's got to be
something going wrong in the harness. It's all that's left..

Dan Luke wrote:
The GIA2. The GIAs have been swapped and the problem stays.

Is the previous owner paying for all this repair time?

On Tue, 26 Jun 2007 16:42:41 -0400, john smith wrote:

Dan Luke wrote:
The GIA2. The GIAs have been swapped and the problem stays.

Is the previous owner paying for all this repair time?

The OP says it's warranty.

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.

"Peter Clark" wrote:


The GIA2. The GIAs have been swapped and the problem stays.

Sounds like at this point they're going to have to build a harness and
ring out the graycode lines end-to-end from the GIA2 output through to
the AP computer connector individually. If both GIAs are producing
the issue and two AP computers are doing it, there's got to be
something going wrong in the harness. It's all that's left..

Seems logical to me.

The tech says he is talking to Garmin about the possibility that it is a
software problem, although I cannot imagine how such a thing could appear in
only one of many identical airplanes. Faulty memory chip, maybe?

--
Dan
T-182T at BFM

On Tue, 26 Jun 2007 17:45:51 -0500, "Dan Luke"
wrote:


"Peter Clark" wrote:


The GIA2. The GIAs have been swapped and the problem stays.

Sounds like at this point they're going to have to build a harness and
ring out the graycode lines end-to-end from the GIA2 output through to
the AP computer connector individually. If both GIAs are producing
the issue and two AP computers are doing it, there's got to be
something going wrong in the harness. It's all that's left..

Seems logical to me.

The tech says he is talking to Garmin about the possibility that it is a
software problem, although I cannot imagine how such a thing could appear in
only one of many identical airplanes. Faulty memory chip, maybe?

In two different GIAs and two different KAP140s? It seems rather
remote that all four componants would have a software problem that
only manifests itself on the one airframe. I think realistically it's
down to there being a short somewhere. I've not jacked into a KAP140
for quite a while so I don't remember if there's a diagnostic page
which shows the decoded input gray code, but one way for them to try
and isolate which wires on the harness are bad would be to pump the
aircraft up and see if the gray code as shown by the altimeter and
gray code as decoded by the KAP140 A) correlate and B) change with the
test set's altitude. Knowing which wires should be hot for a given
pressure will let you know which ones are bad. Have they tried
hooking up the pitot/staic testset and pumping it up?

"Peter Clark" wrote:

Have they tried
hooking up the pitot/staic testset and pumping it up?

Don't know. They're getting complete schematics from Garmin for further
trouble shooting.

--
Dan
T-182T at BFM