AXP340/322 ADS-B Transponder - AXP340 long delay for Press Alt at startup

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AXP340 long delay for Press Alt at startup

Printed From: Avidyne
Category: Avidyne General
Forum Name: AXP340/322 ADS-B Transponder
Forum Description: Topics on the Avidyne AXP340/322 ADS-B Transponder
URL: https://avidynelive.com/forum_posts.asp?TID=1174
Printed Date: 26 Mar 2026 at 2:12pm
Software Version: Web Wiz Forums 12.07 - https://www.webwizforums.com

Topic: AXP340 long delay for Press Alt at startup

Posted By: CardinalRG
Subject: AXP340 long delay for Press Alt at startup
Date Posted: 12 Nov 2016 at 11:44am

Recent problem with my AXP340. Was fine for past year or so....

Most days, on first flight of the day, I don't get Pressure altitude output for 10 minutes or so. It starts in ALT mode and transponder squawk works fine, but The AXP screen shows Pressure Altitude shows FL---. After 10 minutes or so, the AXP340 starts reporting the correct Press Alt.

This started a few weeks ago. No recent panel work done.

Any troubleshooting suggestions?

Joe Brunski

1977 C-177RG with two IFD540s + AXP340 + MLB100

Replies:

Posted By: DavidBunin
Date Posted: 18 Nov 2016 at 3:08am

Sounds like the heater is going out in your altitude encoder. Altitude encoders normally contain a heater to bring the pressure sensor to an above-ambient operating temperature (usually around 140°F) which means they can eliminate temperature as a variable in the pressure sensor logic.

Most encoders have a warm-up time of no more than 30 seconds before they go valid.

Have your altitude encoder removed and bench tested at an avionics shop. My guess is you'll be buying a new one. They are not terribly expensive.

David Bunin