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Altitude encoder

tkatc

tkatc

Well Known Member
I'm trying to prepare myself for my first condition inspection. I am a buyer, not a builder so the learning curve is high.

My mode C worked intermittently at best. It was usually accurate when it was transmitting but lately it has been erroneous if it even transmitted at all. (+-400') My transponder is almost due for its check so I will have that done first. I assume that check will determine if the GTX327 is operating properly. I do believe that it is.

We used to believe we had a connection problem in the harness, and while I have not ruled that out completely, I am sick of doubting the Ameri-King AK-350. I am about to purchase the Trans-Cal encoder which is a direct plug and play replacement. The confusing thing to me is wether or not to go with the gray scale only unit or the R232 unit. The more I read the more confusing it becomes. I don't understand either of those comm formats, so i need a lesson, along with buying advice as to go with a gray scale unit or one that is R232 compatible. Pros, Cons, etc :confused:
 
go with the rs-232 trans cal. All it needs is power, ground, and a couple of wires going to the 327. Then just configure your 327 to accept the 232 and you're up and running.

232 is a digital way of transmitting your altitude info from your encoder to xpdr. Grey code uses more wires to basically give a different combination of 1's and 0's that the xpdr recognises as an altitude.
 
Most shops, like myself, have a test box they can plug into the encoder plug to test all the data lines to the transponder, doing this quick check will confirm your wiring and xpdr is good.

Assuming your wiring is good there is really no advantage to switching over to the more expensive 232 unit and you would have to run an additional data line from the new encoder (seperate plug) to the xpdr connector. So my suggestion would be check with the shop that will be doing your xpdr cert and ask them to check the encoder lines, if thier good then just put in the transcal (I prefer the TC over the AK).

If you plan on doing a full panel upgrade at some point (glass panel, 430 etc) then having the 232 data is a nice feature for the future.
 
When the encoder sends a signal which fluctuates +-400ft, then its in 95% the encoder which is bad.
I would buy the trans cal SSD120-30NE-RS232, and connect the RS232 bus directly to the 327. That solves all the problems you can have with the grey code, and its a simple single conductor shielded wire. The trans cal encoders are very reliable.

kai.
 
Yep, I am hearing good things about Trans-Cal and bad things about Ameri-King. I think the best approach is to take Walt's advice (as usual) and have the shop do a quick check on the wiring before they do the txpr check. Pending a good check I will either make a new harness and/or buy the new encoder.
 
Yes, I think thats a good way. Maybe there?s a fault and they can repair it, so you can save some money.
Let us know what it was...

Kai.
 
ATC

Make sure it is not ATC causing the problem CLT always says mine is bad but no one else. ATL always has me dead on when I ask. I just now know to fudge on the low side when calling CLT....
 
My avionics guy says I have a bad encoder. He thinks the transponder and wiring are fine. So I ordered the trans-cal and it came today.

I installed the new encoder using the existing harness. On the ground it was rock solid. I was confident I had fixed the problem. That is until I was climbing through 4500... I noticed the altitude blanking out and ATC confirmed it. The 327 seems confused, it sometimes shows me a down arrow even though I am climbing. At times it doesnt show any arrow even though I am climbing or descending at a rate that the arrow should be on. That is a bit troubling.

I won't be able to get back to my avionics guy until next week so stay tuned...
 
Seems to be that one wire from the encoder to the transponder is bad.
I would change the wiring from gilham to RS232. Then all problems should be fixed.
 
Ok. So I FINALLY have a working transponder with Mode C. I decided to purchase the trans-cal encoder SSD120 RS232.
http://www.trans-cal.com/prodServ/Nano.html#SSDRS232

I wanted to use the serial out so I could discard the current harness, use less wires, and get an education all in one. I got a bit more than I bargained for.

I ripped out the old harness carefully noting where each wire went and checking the Garmin install manual to verify it's purpose. I got the pinouts for each unit and attempting to try my hand at micro soldering, under the panel no less.

the pinout is pretty simple. The Transponder gets power and ground from the aircraft bus, then it has a pin for power and ground that leads to the encoder, thus everything gets powered up once the transponder is turned on. Simple. Then there are 2 wires that send a signal from the encoder to the transponder for data. (one is ground) So I have it all wired up and I think I am done. I installed everything and power it up. So far so good. Just one thing, there is no altitude readout on the 327. Hmm. So I disconnect the d-subs on the encoder and make an attempt to see if it is getting power. Maybe my soldering connections sucked?? My voltmeter leads were too thick to fit into the small d-sub holes so of course, I improvised. I stuck a small piece of solder in the hole to make the connection and while holding 10 things upside down under the panel the solder bent under the pressure of my volmeter lead and touched something it shouldn't have. Out of the corner of my eye I see the transponder go dead. :eek::eek:

I figure it was a fuse so I go ahead and check them.....nope. I confirmed I had 12v to the transponder but it wouldn't wake up. Sick feeling.

The avionics shop tells me they jumpered the internal fuse but the display was still jacked up so off to Garmin it goes. For a complete overhaul at the tune of $600.

I don't want to wait that long for the unit to come back so I buy another unit (Thanks TurboSaaber) off of a fellow controller on this board. I figure I will sell the refurbished unit after I get it back from Garmin. I received it today and before installing it checked all my connections. Everything seems good. I installed the new 327 and it works but still no altitude readout. Now I am not about to make the same mistake twice. I called TransCal tech support and explained how I have everything wired. The tech said your perfect except you won't get any altitude yet. The TransCal unit transmits, by default, at 1200 baud. The Garmin is looking for 9600 baud. I need to jump the 7 and 8 pin on the serial d-sub and it should be fine.

I am happy to say the things finally works perfectly. At least in the hangar. I will flight test it shortly but I am pretty sure I finally have this problem licked. It was a costly experience/education. Had I known about the jumper, I might not have shorted the transponder and this story would be a little brighter. But maybe somebody will read this and learn from it.

The bad part is I never really figured out what was wrong with my original set-up. Oh well, I am happy its fixed.
 
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