ACK A-30 Fails to FL 290?

[the_other_dougreeves]

I'm having some strange issues with my ACK A-30 (Mod 4, gray code, 30,750ft max) connected to my G327 XPDR. When first turned on, the altitude encoder reports the correct altitude. After 10-15 minutes, it jumps to FL 289-290, occasionally giving the correct altitude for 2-3 seconds. Imagine my surprise when Center asked me if I has suddenly climbed to FL290....

When I unplug the encoder, the G327 looses the Mode C and reports no altitude. The FL290 problem is reproducable on the ground and does not seem to be caused by vibration (i.e., works engine on or off). It always takes several minutes after the encoder turns on for the problem to show up. All connectors are secure and moving connectors and wire bundles has no effect.

Now that I've looked at the G327 install manual, I see that there is a page where I can see the gray code input. I'll go look at that today, which may help troubleshoot particularly for shorted wires.

My guess is that the encoder itself is the problem. Another possibility is a short in one of the grey code wires.

Any other thoughts?

TNX,
TODR

[kevinh]

Yes, I think the wiring is probably the fault (with a slim chance for the xpnd or encoder). From the following table:

http://www.airsport-corp.com/modecascii.txt

I'm guessing the A1 line is shorted to one of the other encoder signals (or ground). xpndr signals are active low, so "1" on that table means ground.

[the_other_dougreeves]

Update: Local avionics shop says that the encoder checks out Ok, transponder does not and that it needs to go to Garmin. Flat service rate for Garmin is supposedly $650. Yikes...

TODR

[fl-mike]

ya might give these guys a call:
http://www.transponderrepair.com/
They fixed up my KT quick and cheap, verified my encoder, and provided the cert.
Looks like they can work on Garmins, but call first.