[Michael Burbidge]

I had my airworthiness inspection scheduled and then found out that my transponder was not sending pressure altitude. It is sending transponder code and everything else appears to be correct. Had to cancel the inspection. The FAA inspector did not want to come out until it was fixed.

I'm trying to figure out how to diagnose the problem. The wire harness for my transponder/encoder was made by SteinAir. I sent them my encoder and transponder so that they could verify everything was working. Of course that was a couple of years ago.

I had the data correspondence check done by the local FBO a few weeks ago. I took over my altimeter and encoder for that test. Everything checked out.

I've verify that the encoder is getting power and gnd on the right pins, from the harness.

My understanding is that the AK-350 is not an RS-232 device, but that it sends data over the grey code pins.

I've gone into the configuration section of the transponder. There is a section for configuring the two RS-232 channels. One is set OFF, the other is set ICARUS ALT. But I assume it doesn't matter since the AK-350 is a grey code device. Just in case I tried setting the other channel OFF also. It didn't help.

There is also a "Gray Code Input Page". You can't set any options, but it shows the code coming from the encoder for debugging purposes. The code is all zeros. (00000000000) This is supposed to be the binary representation of the altitude sent by the encoder. So obviously the transponder, for some reason is not getting the altitude.

So, I'm not sure what to do next. I'd appreciate any suggestions. Aircraft Spruce sells test devices that emulates both the encoder and the transponder so that you can figure out exactly what the problem is. It is $300 though.

Since it was working for Stein, I'm just wondering if there is something obvious that might be wrong.

Thanks for any help,
Michael-

 

[BobTurner]

First the obvious: power to the encoder, no fuses blown?
The gray code zeros indicate open circuit.
I would unplug the connector from the encoder; power up, watch the gray code page; short one of the gray code pins on that cable connector to ground.
If one of the zeros turns to a one, the problem is the encoder; if nothing happens the issue is in the cable or the 327.
At least you'll know where to start.

 

[pilot2512]

Did you check pin 6 of the ak-350 for ground? This enables the output of the encoder. Also are you allowing enough warm up time? The encoder needs a few minutes before it will put out altitude. Also concur to remove plug and ground the code lines 1 at a time and see if it shows.

Jay

 

[Michael Burbidge]

Thanks...

I've checked power, and GND (pin 15), but not pin 6 (Strobe). The encoder is powered of the switched power pin from the transponder.

I have waited for the encoder to warm up. I was aware they can take a while.

Thanks for the tip on the gray lines. I will try what you suggest.

Michael-

 

[Michael Burbidge]

Pin 6 empty!

My problem is that pin 6 on the dsub connector for the encoder is empty. I pulled all the gray pins to GND and one bit flipped to 1 on the transponder gray input screen, for each pin. So all those are correctly wired.

It clearly should have a pin in it and that pin should be connected to GND, for the Garmin 327.

Thanks,
Michael-

 

[pilot2512]

Great. No hurry up and get that aw inspection and first flight. I need some motivation!

Jay