OG GarlicSalt

Working on a G450 with the original complaint being that the CMC cannot communicate with cockpit printer.

The CMC was showing 4 messages for LAN failure DMU, Printer, NICMRC2, NICMRC1. All Lan were showing inop as well.
 

Luckily when I looked these messages up in the FIM they are in the manual but zero instructions on what do or where to start for troubleshooting, so dead end on the FIM. Since we have a spare printer in stock the first thing I tried to do was slave in a new printer, which was no help. After that I wanted to do a good inspection of the DMU. Once opened I found that it was covered in coffee stains, but appeared to be working. I disconnected the DMU and the entire LAN was still down. Its my assumption that if one component had an internal failure that was crashing the entire system, removing it should bring the other items back to life. Did a good inspection of the wiring of the DMU all checks out good. 

At this point I decided to call Gulfstream Tech Ops for any pointers. They instructed me to check the shorting caps on the cockpit MX panel for a reading of 50ohms, there is BNC "T" connector under the printer that is also known for having issues, and if neither of those fix it a full wiring check.

 

I decided to take the low hanging fruit and checked the MX panel LAN shorting caps, removed them both and found they had the proper resistance. I wanted to take a short cut so I hooked up my generic "fox and hound" to the LAN system as it is run as COAX wire. I was hoping that I can get an easy find somewhere with this tool. I hooked up to the right side and just wanted to do a quick scan, and as soon as I passed over the LH Lan port I was getting a super faint tone, decided to do a better inspection of the actual port. Then I found that there is a male to female gender change fitting that is plugged into the port. I found that this adapter was maybe 1.5 - 2 turns loose and that it was basically causing the 50OHM shorting cap to not make proper connection crashing the entire LAN.
 

Tightened it up and everything was working perfectly again. Got pretty lucky with this one and was able to reconnect to the CMC.