Nav lights only work battery 2 or both

[mr300z87]

Yesterday while showing our new to us 310 Sundancer to a friend, I told him I had replaced the masthead light on the windshield and the stern light on the transom, when I flipped the switch I had no nav lights, I was like wtf they were just working. The boat has 3 batteries, one for the port engine that has a on/off switch, this battery has to be connected but can be off for the stereo to work and the auto bilge pump circuit. Then there is a 2 battery bank connected to starboard engine with a 1/2/both/off switch, this is also the house bank. I had read that it's best to run on one or the other to help keep them charged properl. So yesterday's date being odd I ran on battery 1, everything was working except the nav lights. Switched to both batteries nav lights worked. Having a full understanding of how the switch works and plenty experience with 12v DC circuits I can not figure out how this can be. I belive the only wires connected to batteries are the 00 gauge primary wires going to the switch. So how would it be possible to isolate nav to bat 2? If connected direct to battery either at bat or switch nav lights would work regardless of switch position. The heat and a couple beers prevented any further investigation most of the wiring on the boat seems to be the factory rat's nest. I have worked in the network cable business for over 30 years, studied electronics in college (forgot most), even did both electrical/network wiring in my home renovation and have been playing with DC circuits since I was a kid so I will figure it out wires are my thing lol. Any insight as to where I should start is appreciated.

 

[dingbat]

Having a full understanding of how the switch works and plenty experience with 12v DC circuits I can not figure out how this can be.

If I had a dollar every time I heard this from customers…If you have a full understanding, why are you calling me?…lol

What battery switches are involved?
Nav lights normally wired direct to house. Follow nav circuit back to source

 

[mr300z87]

Dingbat what you stated is why this is so perplexing. I will try and follow the source in the rat's nest under the dash.
"Nav lights normally wired direct to house. Follow nav circuit back to source"
Port engine single battery with simple on/off switch. I have not really tried to see what is powered with only this battery on, although do know it needs to be connected, switch can on or off, for the stereo and auto bilge pumps to work.
Starboard/house 2 battery bank with 1/2/both/off switch. These batteries seem to run everything, when we took the boat out last week on battery 1 everything seemed to be working until I tried nav lights.
Honestly between work, crappy weather and a broken lawn tractor I haven't been back to the boat all week. Long holiday weekend and I'm off the rest of the week to look deeper into this. I can say it is super simple stuff as wiring goes. Neg terminals tied together then to negative buss. Each positive lead to its respective terminal on the switch. When I get to it I'll post my findings.
BTW I was told there is need for marine electrical techs in my area and was considering this as income source after retirement from my network cabling career.
Have a great holiday weekend.
Mike