Okay, I’ll admit that I can be a little dense sometimes so for any of the rest of you that may be afflicted with the same problem I thought I’d share a simple learning. This is so obvious after I connected the dots that it’s a little humbling but nonetheless…..

Serial port 2…. The EFIS Main Cable wiring diagram shows Serial 2 connected to a transponder to send altitude information out to the transponder via the white wire in pin 13. The white/blue wire is connected to ground at pin 21 (shared with serial 3 ground). This is the way the wiring harness came… serial 2 & serial 3 ground attached to the same pin and pinned into pin 21.

Great…. So I have a GTX 330 transponder and need to have an input back to the EFIS for Traffic (TIS). The input should be pinned to pin 25 – serial 2 input. In my infinite wisdom, I ohmed all the connections out from the 330 (and everything else) to the EFIS before covering up the panel some months ago. I found serial 2 white/blue pinned to 21 even though elsewhere in the manual it tells me that I need white/blue pinned to pin 25 as input from the 330. It’s always difficult to keep documentation up to date with product development so I just made a note and kept going.

That’s the history… now current. I get the 4500’s in and start to install them. Get everything figured out and working (well, everything is a relative term ) except the 330 on serial 2. I find my note on serial 2 is pinned to pin 21 ground and realize that it should only be on ground if the serial port needs a ground. Oh, I get it . I pull the cable off , open it up and extract pin 21 and insert in pin 25.

Should work. Right. Well not quite. Pin 21 is serial 2 and serial 3 ground and they are pigtailed together and have a common, one wire output to the pin. I still get ground on the physical wire because serial 3 is connected to a SL30 and it requires a ground.

You would think that I would have caught the physical pigtail when I had the EFIS cable out the first time but I didn’t. It was buried in the bundle and all neatly covered by heat shrink.

The obvious answer (now… told you I was dense) is to clip the pigtail. Serial 3 white/blue still goes to pin 21 – serial 2&3 ground and add a pin to Serial 2 white/blue and pin it to 25 on the EFIS Main Cable.

Dots connected.

Doug