GTX345 reporting wrong altitude

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Pre-takeoff checklist
Joined
Jul 4, 2012
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496
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Ames, IA
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david
Flew with someone yesterday who has a GTX345. The unit displays the altitude it is sending. For the first part of the flight it was mostly correct, although it occasionally jumped to an incorrect value (2k or 3k higher) and back. Later in the flight it just reported the wrong altitude the whole time.

Apparently he's had problems with the altitude reporting for a while and the shop hasn't been able to track down the source of the error.

Curiously, the altitude it reported went up as we descended and down as we climbed.

Anyone have any ideas what could cause this?
Is this just a bad encoder?
 
Had similar problem. Turned out to be the encoder. Cheapest solution, fortunately.
 
Encoder or wiring is bad. These systems are pretty simple so it should be pretty easy to get the problem nailed down.
 
When I had my 345 installed, the shop strongly recommended a new digital encoder. Their experience was that old encoders caused problems with alt reporting in exactly the way you describe. Either a good coincidence, or they are spot on. The encoder is cheap enough too.
 
Did you get the little Garmin module for GTX 3x5, or did you get a real encoder like SSD-120? If the latter, did you switch to RS-232? I heard from certain quarters that the Garmin add-on encoder has a flimsy connector and loses contact with the body of the transponder too easily.
 
Mine was reporting off by 300' but the discrepancy varied with altitude. Ended up being a simple static line leak.
 
zaitcev said:
Did you get the little Garmin module for GTX 3x5, or did you get a real encoder like SSD-120? If the latter, did you switch to RS-232? I heard from certain quarters that the Garmin add-on encoder has a flimsy connector and loses contact with the body of the transponder too easily.
Click to expand...

I think it uses the configuration module connector, since it takes place of the confit module. The module is hidden inside the connector shell but that encoder is external. The wires are somewhere around 28 gauge, very very tiny.
 
zaitcev said:
Did you get the little Garmin module for GTX 3x5, or did you get a real encoder like SSD-120? If the latter, did you switch to RS-232? I heard from certain quarters that the Garmin add-on encoder has a flimsy connector and loses contact with the body of the transponder too easily.
Click to expand...

My installer advised the SSD-120 via RS232, so that's the way I went.
 
Shop agrees that it is likely an encoder issue. They are replacing the encoder. Hopefully that will fix the issue.
 
That Trans-Cal SSD120--xx-RS232 must be pretty popular, I to installed a new SSD120-30N-RS232 encoder with a new blade antenna. I figured why hook a new $5kish transponder to old equipment?
My SSD120 runs channel 1 & 2 RS232s to a GTN and the GTX. The parallel data out runs an altitude clock.
 
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