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Transponder short

Dave S

Active Member
I am using a VP-X and Garmin GTX-345R.

The electronic circuit breaker keeps tripping.

I have checked the pins on both the Transponder and the VP-X and they doesn't seem to be any shorts between the pier and ground.

P3251 Pins 21 and 42 connect to a 3A circuit on the VP-X and continuity is good between them.

P3251 Pins 20,41,38,59 connect to aircraft ground and continuity is good between them.

I don't see any continuity between any of the grounds and power.

What am I missing or does anyone have any thoughts as to why I'm tripping the electronic breaker?
 
There are many potential places for a short to occur inside the transponder. Could also be a failure inside the VPX.
 
You may be looking for something like this , a small black spot on a white wire. I found this one on mine after repeated intermittent circuit breaker operation. Usually happened in rough air. BUT before I found this I replaced the breaker, suspected the Narco 165 transponder, bought a new GTX327 that did the same thing, and finally found this power wire cut thru the insulation on a sharp panel edge. Took about a year before I finally fixed the problem. Best of luck . Resized_20210301_135033_1887.jpg
 
I am using a VP-X and Garmin GTX-345R.

The electronic circuit breaker keeps tripping.

I have checked the pins on both the Transponder and the VP-X and they doesn't seem to be any shorts between the pier and ground.

P3251 Pins 21 and 42 connect to a 3A circuit on the VP-X and continuity is good between them.

P3251 Pins 20,41,38,59 connect to aircraft ground and continuity is good between them.

I don't see any continuity between any of the grounds and power.

What am I missing or does anyone have any thoughts as to why I'm tripping the electronic breaker?

I’d suggest removing the transponder wire from the VPX connector & connect a 12 globe to it & other side to ground - then power up the avionics ? circuit (assuming the Txpdr is on it) - if the lamp goes out then it’s the VPX ( most unlikely) if the lamp stays on the ‘fault’ is elsewhere .
 
Thank you all.

After speaking with SteinAir and Vertical Power folks, a few other things were checked to include the antenna wire. Nothing found.

They solution they both suggested was to bump up the amperage to 5A. In doing so I ended up moving the pin on the VP-X. Not sure why I had to but certainly user error was part of it.

Thus far that seems to have fixed the issue, no more transponder fault. Of course the aircraft was stationary in the hangar, so hopefully phase I when I get there won't result in any more issues.

On a positive note the ID 10T user error resulted in my finding out that when my PFD fails everything automatically switched over to the MFD. The PFD is on the same plug as the transponder, and its apparently recommended to plug it back in prior to testing. :)
 
Looking at the Garmin manual it seems the transponder should NEVER pull more that 1 amp. Note that reply rate will raise power consumption, so flying it will make a difference.

So either Garmin is wrong and there are momentary higher glitches... or 3A circuit on the VPX won't support 3A. Or a little of each. I'd love to put a dummy load on the VPX and pull 2.9 amps... and put a scope on the Garmin. Someone isn't holding up their end.

Or put a 2A fuse on the Garmin and see if it blows.
 
ON P3251, pins 21&42, bear in mind note 13 on the 345 install manual. Depending on your mounting location this may have been a factor.

- SINGLE 22 AWG WIRE WITH 3A CIRCUIT BREAKER IS ALLOWED WITH WIRE RUNS LESS THAN 20 FEET.
- SINGLE 20 AWG WIRE WITH 5A CIRCUIT BREAKER IS ALLOWED WITH WIRE RUNS GREATER THAN
20 FEET.
 
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