Garmin 327 transponder altitude errors

Having trouble with widely varying altitude errors reported by the Garmin 327 transponder, installed in my Model IV-1200.

Last time up, ATC asked us if we were REALLY at the 2600' they saw on their radar - when we were actually at 1100 in the TPA. So the avionics shop took out the existing ACK Model 2 blind encoder, put it on the bench, and it wouldn't hold pressure values. Okay, easy enough, so a new Model 5 ACK encoder was bench-calibrated and installed in the KF.

Parked on the ramp, the panel altimeter and the 327 agreed with each other. (We made the correction for 29.92, etc.) Took her up for a spin and first verified that the steam gauge and the GPS altitude were in sync within 100'. They were, and they remained in sync for the remainder of the flight.

But the 327 was all over the map. For the first 10 minutes it was +1000 over the panel and GPS, then +300, then +500, etc. Then, for five minutes, the 327 was in perfect agreement with the panel and the GPS. Then it was all over the place again and we had to notify ATC (again!) of the discrepancy and head home.

I would have figured that if the plumbing had a problem, both the panel altimeter and the 327 would have been off, but the panel gauge was very steady and verified by the GPS.

Anyone had an experience like this?


Rich Cunningham
Lafayette, CA
Model IV-1200 80 hp UL

Something simple.......Is it properly grounded. Sometimes home built aircraft radios and transponders are not grounded per mfg specs......Just a thought.

Also check your signal wires, you might have an intermittent connection on encoder to transponder grey code (digital) interface that might cause this erratic behavior.

Ariel
Los Angeles, starting a KF7 in January 2011

First off, thanks to Russ and Ariel for their input on possible solutions.
Ariel was pretty much spot on with his guess.

Here's what happened:

In the avionics shop, the problem appeared only once (randomly) after we'd hooked up the pitot testing system and cycled up and down to different altitudes for several hours.

So we started doing wiring checks, pin by pin, and the problem finally showed up at a damaged (but not entirely broken!) soldered connection on a data wire. The power ground wire was also damaged and we re-soldered that one too.

BTW: Although we attempted to induce vibration by tapping on mounts and connections while we were doing the altitude test cycling in the shop, we (obviously) weren't able to reproduce the vibration that was present when in flight, which had been sufficient to disturb the damaged solder connections.

Thanks again,

Rich Cunningham