Thread: Garmin GTX327 Displays Intermittent Pressure Altitude

My GTX327 transponder gives an intermittent pressure altitude display. Most times the pressure altitude is displayed but occasionally, I get a "_ _ _ _" where the pressure altitude is normally displayed. On two occasions, I have been told by ATC that I was not squawking altitude and these have coincided with the "_ _ _ _" display. I seem to get a consistent blinking "R" in the lower left corner.

I have inspected the ACK A-30 altitude digitizer/encoder and it appears to be intalled OK and the wires are not dislodged from it. I have inspected the transponder blade antenna on the belly of my plane and the coax running to this antenna. This antenna appears to have an aluminum plate at the antenna which acts as a ground plane and the wire shielding looks to be properly "captured" at the coax connector to provide a ground to the ground plane. Any thoughts would be appreciated. I do have a local shop that can look at this but I'm trying to learn more about this system before taking my plane in for inspection.

I would set the G327 up in one of the diagnostic modes where it shows you the gray code output. You could also check the pin outputs on the encoder. Just takes time.

If it's a wiring issue, I'd call SteinAir and get a new harness. And make sure you get it signed off by an A&P!

I had a problem where the G327 randomly displayed totally wrong altitudes ... like FL270! The avionics show said it wasn't the encoder, G327 needed to go to Garmin for refurb. The good part is that it's a fixed price no matter what's wrong - you know exactly what you're paying. The bad news is that's $600 plus shipping.

Garmin reflashed the software and called it good. Never got a good reason for what exactly happened, but it's working, at least as of yesterday. Today, I can't say, haven't flown .....

Doug

Doug, thanks for the reply. I went into the G327 manual and tried to understand how to use the G327's diagnostics setup and studied the pin-out diagrams. This is complicated to say the least and over my head. I'll monitor things and see if I continue to get "_ _ _ _" instead of the pressure alt. display or if ATC indicates that I'm not squawking altitude. If this continues, I will take the plane to my local avionics repair station with the hope that they can isolate whether the problem is the wiring, the encoder or the transponder. It appears that you were able to do this and decided that your G327 needed repair. I'm hoping that I can do the same. I sure don't want to send the G327 to Garmin and spend $600+ unless I'm fairly sure that this is causing the problem.

Dick

Having worked in the electronics industry for 32 yrs let me offer some "food for thought". Electronic instruments nowadays are very susceptable to power fluctuations. Any instrument with memory can loose or trash it's own memory simply by receiving a power fluctuation from it's power source. When someone relies solely on an avionics power switch to kill the power, several things can happen. One of these is "dumping" or corrupting any and possibly all of it's data. Calibartion constants stored within the unit are the most likely to get trashed. My recommendation is to turn off each unit by it's own switch. The collapsing power field generated by using the "master avionics" switch can and does cause all kinds of failures. Understand that each manufacturer considers THEIR equipment to be the only thing in the aircraft...they don't care about any other manufacturer. All these units consume power at different rates etc...so I feel the best solution is to turn each unit off with it's own switch. A pain in the butt for sure...but it could save you tons of money and headaches.

runtoeat, so what was the outcome? How is your airplane set up??

FD Avionics/Sales Types: Please let me know if the following is cogent thought: I too have a errant 327 mode C. I'm thinking in my case, no matter what wiring problem I find, the 327 and D100 bench test OK, that just having an extra piece of hardware like a dynon gray code converter, when it is not needed, is just more unnecessary wiring to go wrong overtime and maybe now. I'm guessing that FD installed the gray code coverters as a matter of policy before the type of avionics the first buyer wanted was known. Maybe it would be needed depending on how most transponders input data. The gray code converted is necessary for most transponders less the Garmin 327 and 330 and probably a few others, but the Garmin 320 needs it. The D100 itself can directly encode for the 327 without the converter if I read the Dynon Support web correctly. So, I'm thinking I might have a wiring problem from original MFG wiring policy and perhaps just taking the dynon gray code converter out of the diagram will resolve it by reducing its uncessary complex and potentially problematic wiring and setup. What do the smart folk think? Am I on to something here or just swinging wildly?

Bugbuster, my altimeter problem ended up being a mis-wired connector at the ACK altitude encoder. After purchasing my plane, I noticed that the "Pressure Alt" reading on the face of my Garmin 327 would show "_____" intermittently. Along with this blank readout, ATC would call and tell me to "reset my altimeter because I was not reporting to them". I originally thought loose wires and the A&P looked at all connections and they seemed OK. I called ACK and they said that it was very doubtful that the ACK-30 encoder was bad since they virtually had not seen any of the thousands they sold come back. Thinking the worst that I would have to send the 327 to Garmin for the $600 minimum and possibly more repair, I lucked out and found that there is an avionics shop close by and it is run by a smart and honest person. They checked the pin-out of my 327 into the encoder and found that there were two wires wired to the wrong pins. They said that they see this on the 327 because the sequential routing of wires does not go in a straight sequence in one spot and if one isn't paying attention when hooking up the 327, these pins can be cross connected. It was strange that this condition was not noted before I got my plane but I think this was due to the plane not being operated in Mode C airspace. You may have this same problem but it really takes someone who is a wire wizzard to figure this stuff out. If possible, try to find a shop on an airport that you can take your plane to for checkout. Side note - the avionics repair shop said that the Garmin line of transponders and radios we have in our CT's are just about bullet proof and they feel that this equipment is one of the best for long term, trouble free performance.

Every word makes sense to me runtoeat. Even the not used in in Mode C airspace. I think we're down to tracing wires, one by one, this weekend. Will let you know how our journey ends up. Thanks!

Bugbuster, looked at some of my notes and see that the #12 and #13 pins on ACK-30 were crossed. Hope this is your problem. I couldn't believe that an intermittent problem would be solved by a hard wired problem but no worries since wiring was corrected. I think the 12 and 13 wires take the communications between the 327 and ACK-30. When cross connected, the communication occurs intermittently.

Even Better! Yup, go figure, intermitent = crossed wiring. Sounds like semi-artifical intelligence-machine trying to work despite human error.

runtoeat,
I recant my last comment....because, at least in my setup, my log book is current with pitot static checks and no hint of a mode C problem then...So, even though previous operators may not have been concerned about mode C operations, the log says A OK! So, back to chasing wires tomorrow.
BugBuster