I know this belongs in electronics but thought I'd put this here for more exposure. This will be a bit long winded, so excuse me.

A bit of background. 2013 MTI w/ 1100s. Boat had older Vesselview and Garmins that I had a shop change out to new Garmins and a Vesselview 502 this winter. From what I can remember, boat did ok the first couple times out with this setup.

I recently lost an AGM cranking battery, so replace both with new Dekas. Used the boat like that once or twice, no issues. I began to notice that the starboard motors voltage fluctuated quite a bit. Running or key off, the voltage would move about 1.5 volts... while the port motor bounced around .1 volts, no more. This weekend I boated to the marina, all was well. The next morning keyed the boat on to a starboard map sensor fault. Cleared it and it never came back. However, what did come back was a Helm 4- Inner Stbd Engine Fault (whatever that is). Throughout the day the starboard motor would fluctuate voltage. Every five minutes or so I'd lose all starboard engine data (motor ran fine) and then suddenly it would come back for five minutes. This continued for an entire day.

In my testing, I've discovered the following. I can hook a scan tool to the CAN network. The scan tool will show the same fluctuating starboard voltage that the vessel view screen reports. HOWEVER... as soon as I turn the vesselview screen to standby mode, the voltage immediately stabilizes and is rock solid. This is very repeatable... turn the screen on... goes to crap... turn it off... perfect.

So far I've checked the following:

- each and very NMEA and CAN connection I can
- confirmed clean power is good, fuse good, no breaks
- confirmed all terminals clean and tight
- confirmed voltage at motor and at batteries is stable

I'm lost. Clueless. In my head the voltage displayed on the vesselview is just what is "reported" to it... so how can the screen being on or off cause this? HELPPPPPP

Try disconnecting the Garmin from the NMEA network and see what the Vessel View does.