So a few days ago, I turned on the avionics to find that nothing was communicating. How could this be? It was all working perfectly, and then something happened? At first i thought perhaps I hadn’t trickly charged the batteries enough and a low voltage was causing some LRUs to drop offline. Nope. Speaking with Garmin they told me to disconnect LRUs and reconnect individually to try to identify a bad unit. That didn’t work either. The only other possible culprit – the CAN Bus. So i pulled the connector on the GDU 460 and buzzed it out. I got the appropriate 60 ohms across CAN HI and CAN LO. Which told me that the CAN Bus was contiguous and terminated properly. Perhaps a short to ground? Bingo. CAN LO was buzzing as shorted to ground. Great. Finding that isn’t going to be easy. The only real way to do so, is to divide the CAN BUS in two, and figure which is the bad half, and keep doing that until the offending segment / connector is isolated. So i did that, and found that the bad area was the GMC 507 to GEA24 to GTR20 segment. The GEA 24 is really hard to reach, and is where i had just been cinching wires together. I buzzed it out, and thought i determined taht was the bad connector. Reinstallation of the connector didn’t solve the problem. But i did it from inside the airplane. Which, given the GEA24 is down by the fuel pump… is difficult. I decided to pull both segments out of the plane so i could diagnose and correct on the bench. Long story short, I isolated the leg that was bad. Then tried to determine which connector was bad. Ultimately, even after cutting both ends off the cable, the cable still buzzed as shorted to ground. I completely tore the cable apart and examined everything with a magnifying glass. I was unable to find the offending section.

I built two new segments and reinstalled them in the plane. With all the troubleshooting this was a major time sink, and quite frustrating. But everything is back to 100% functionality. While human error cannot be ruled out, i was extremely meticulous in testing the cable on the bench. I really think there was a flaw in the cable somewhere that was exposed when i began cinching wires with zip ties.

About Peter | Certified Flight Instructor (Single & Multi-Engine, Instrument, and Advanced Ground) | Commercial Helicopter

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