Updated a customer's FM-750 today to version 7.79 for the RTX frequency change that happened on the first of the month. I also updated the NAV controller. Oringinally, I had put 9.70 CIH software on it by mistake. When he went to engage his AB line it says on the bottom "Autopilot Connection Issue Detected." Figured this was from the wrong software, so I went back out and put on Trimble NAV software version 13.51. This did not solve the issue. Check all cables and connections and everything looks as it should. The other guy that I work and I both think that the problem is in the NAV controller and we are going to swap them out the next time it rains. Is swapping the NAV controller out the next appropriate step?