AXP340/322 ADS-B Transponder - AXP340 uncommanded switching from ALT to ON

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AXP340 uncommanded switching from ALT to ON

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=1797
Printed Date: 26 Mar 2026 at 1:01pm
Software Version: Web Wiz Forums 12.07 - https://www.webwizforums.com

Topic: AXP340 uncommanded switching from ALT to ON

Posted By: paulr
Subject: AXP340 uncommanded switching from ALT to ON
Date Posted: 08 Jul 2019 at 2:24am

Posting for my partner, who's had our airplane for the last couple of trips-- I haven't encountered this firsthand yet.

Apparently our AXP340 will sometimes silently switch from ALT mode to ON mode, which makes ATC get shirty. Manually switching it back to ALT causes it to start broadcasting Mode C again, as expected. I am not sure how often this happens or if there are any other indications, but I wanted to start this thread to open the discussion. I'm planning on having him contact support this week while he's got the plane and I'm OCONUS.

Replies:

Posted By: tony
Date Posted: 08 Jul 2019 at 4:30am

You are supposed to wire an air ground line to the transponder and leave it on all the time. This is to ensure that aircraft are visible to ATC surface surveillance systems at the larger airports. The days of camera, lights action are over.

Posted By: paulr
Date Posted: 08 Jul 2019 at 5:22am

Yes, I know. This AXP has been working flawlessly since it was installed in 2014, so it's not an installation or user error-- it's a new behavior that just started.

Posted By: Ibraham
Date Posted: 09 Jul 2019 at 4:16am

It could be an issue with the Altitude Encoder, or accidentally pushing Mode instead of Function button on the transponder (guilty of that, while trying to set the timer on the transponder)

Posted By: dwcason
Date Posted: 10 Jul 2019 at 2:23pm

I updated to the latest SW version a couple weeks ago. Now my 322 won’t stay in ALT. It automatically switches back to STBY mode (on the ground and in the air). It worked in the way to the shop for the SW upgrade, but hasn’t worked properly since. The shop said the SWAT setting may have been reset during the upgrade. I’ll update when I find out how to check it and if that fixes my problem.

Posted By: Ibraham
Date Posted: 10 Jul 2019 at 5:41pm

AXP340 Installation Manual

7.5.6 Squat Switch Source