GTX 335 installation - mode C inop

[Igor_U]

Last couple of weekends I worked full time in my hangar on GTX 335 install to my M20F, with the help of my IA.

It took longer than initially thought but included the redesign of the RHS panel, see picture before and after, with adding Fuel gage, USB and CB for future avionics upgrade (IFD540 and 2x G5). This is a Transponder kit sold by Aircraft spruce with GAD-12 encoder and prebuilt harness by them.

 

I completed the setup as described in manual myself as IA was not available and plan was to take it to local shop for biennial altimeter/transponder check.

It turned out we have no Mode C while transponder and ADS-B GPS built in is working good.

Message received on  screen was: “Pressure altitude failed”. What could be the reason? Encoder is connected to the harness. I found online forum that states that altitude correction settings needs to be set to zero for each of set points to operate. Would anyone know Is that correct?

In my case we set only one point (-1000ft), to learn the process as it was expected that avionic shop would do the rest at the altimeter/transponder certification.

I would really appreciate your answer. We had to stop the troubleshoot as I had to go to work and IA will not be available next week.

 

On the other note, this RHS panel upgrade resulted in increasing useful load by more than 10lb. Removing ADF was majority of it but just the old Engine instrument 6-pack with connector is 2.5lb! I kept it only for fuel lever gages after the engine monitor install so now it’s gone.

 

And finally, I dumped the old (green) carpet that I had! It was really in a sad condition, with cardboard backing falling apart and after removing I knew I didn’t want to install it anymore.

I’ve ordered Aerotex wall and floor carpet kits and they turned really nice. Not only that, but are significantly lighter! New one is 22.6lb while old carpets were 35lb (both measured with box).  

I don’t remember actual new UL from W&B but  it’s just shy of 1070lb.

 

It’s one of the better Fs, IMO.

 

 

Before:

 

 

 

 

After:

 

Regards,

 

 

 

[Cruiser]

what are you using for altitude source?

did you configure the 335 to match?

[Lee Moore]

Did you configure the GTX to receive altitude from the GAE 12?


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[Igor_U]

1 hour ago, Lee Moore said:

Did you configure the GTX to receive altitude from the GAE 12?


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Yes, I have done that, as you can see:

 

It turns out I just received reply from Garmin support (on Sunday!) :

"Once installed in the aircraft, a transponder with a GAE 12 encoder installed must be calibrated by a shop with a pitot static test kit. If this has not been done, you will not receive the Pressure altitude required for Mode C transmissions. Service Soon is a message displayed when the GAE 12 has not been calibrated."

So, it seems RV forum post might be correct and all it needs is calibration by avionics shop. My IA only did a leak check to verify added static line is all good.

Other then this, I found transponder installation not that difficult as harness was 90% done by aircraft spruce and I needed only couple additional wires...

 

[Lee Moore]

 
Yes, I have done that, as you can see:

 
It turns out I just received reply from Garmin support (on Sunday!) :
"Once installed in the aircraft, a transponder with a GAE 12 encoder installed must be calibrated by a shop with a pitot static test kit. If this has not been done, you will not receive the Pressure altitude required for Mode C transmissions. Service Soon is a message displayed when the GAE 12 has not been calibrated."
So, it seems RV forum post might be correct and all it needs is calibration by avionics shop. My IA only did a leak check to verify added static line is all good.
Other then this, I found transponder installation not that difficult as harness was 90% done by aircraft spruce and I needed only couple additional wires...
 

That was going to be my next advice.

Glad you got it sorted out. Just curious...how long did it take Garmin to answer?


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[Igor_U]

5 minutes ago, Lee Moore said:

 

 

That was going to be my next advice.

Glad you got it sorted out. Just curious...how long did it take Garmin to answer?


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Less then 2 hours, on a Sunday! Pretty good if you ask me!

[MBDiagMan]

Igor,

Not related to your topic, but I have N9512M and it has almost the same paint job as yours.

[Igor_U]

 

On ‎3‎/‎24‎/‎2019 at 6:10 PM, Igor_U said:

Less then 2 hours, on a Sunday! Pretty good if you ask me!

Yes, but it is already old paint job for me (2007).

I saw your tail number. You got it from D. Maxwell recently, right? Funny thing, fellow Mooney owner sent me a picture of it when picking up his plane from his shop years ago. It didn't look as nice as now, though.

My buddy had 16M at the time (recently sold and bought 231) and also we had same cars for a while: Black Saab 9-3.

 

Regards,

[bradp]

The answer is what Garmin gave you. It needs to be calibrated. You can do that along with tour transponder cert at pretty much any avionics shop  

[Stanton R]

If you put all the test points to 0 it will start working. It still needs to be corresponded to you altimeter.

[MBDiagMan]

On 3/26/2019 at 8:30 AM, Igor_U said:

 

Yes, but it is already old paint job for me (2007).

I saw your tail number. You got it from D. Maxwell recently, right? Funny thing, fellow Mooney owner sent me a picture of it when picking up his plane from his shop years ago. It didn't look as nice as now, though.

My buddy had 16M at the time (recently sold and bought 231) and also we had same cars for a while: Black Saab 9-3.

 

Regards,

Yes, Don has had it around for a long time.  Went through it with lots of good mods, fresh paint, panel from scratch, topped a mid time engine.  Have had some issues while shaking it down, but it has now stabilized quite nicely.

Good job on the transponder by the way!

[Igor_U]

Yeah, I figured setting all the points to 0 would do it... Anyway, plane is at the shop's tie down and they'll do the calibration and transponder check this Thursday.

Then I'm off to go flying and collect $500 from the FAA.... and to calibrate fuel new gages.

Not to mention new carpet needs installation. Busy weekend or two in my hangar.