Update: I'm an idiot... The no-start problem was because I tripped the 50A breaker on top of the engine and didn't know that I had to reach under it to reset it! The batteries are all (correctly) measuring right around 13v when not charging and the engines off.

Now I'm back to the other codes the VV is showing (from the 2nd and 3rd pics). As an extra slap in the face, I spent $700 on the Rinda DIACOM and it doesn't even seem to pick up DTS/Axius errors! It says there are no codes (other than two bogus codes for non-existant sensors) while the VV is showing the errors plain as day. I tried refreshing the VV and the errors still show up about 15 seconds after the restart. I have the Axius service manual on order, hopefully it arrives Friday but at this point it looks like my 4th of July boating plans are a bust.

One other thing to note, that port alternator is only putting out around 12v (I'm wondering if that's the cause of the problems). I'm trying to replace it but you wouldn't believe how much of a pain it is to get to on this engine when it has both Axius and Seacore, I had to buy a power steering pulley puller tonight in hopes that I can get through everything to access it.

Assuming your getting the 12V from the ALT post (orange wire) check that your getting power to the other wires

All with motor OFF
Red/Purple should be constant battery voltage, same as orange
Purple should be power when key is turned to ON
Orange wire should be the same as battery voltage

all marine engines for engine side cranking batteries ONLY - I install 1000 MCA cranking batteries for running the engine. deep cycle batteries for all accessories.

Test voltage back side of alternator looking for voltage drops thru out the power side systems - you can start at the battery to starter and work your way along thru out - load test your batteries - clean all terminals - and possible weak batteries - poor connections inwhich is common in many apps - possible alternator issues.

12.75 volts is a fully charge battery. When you load test a battery the recovery back to 12.7 voltages is very important - if it does not make it back to the proper voltage - new batteries are needed.

I did NOT read most of the post here but just a few. just trying to cover a jist and NOT to use deep cycle batteries for the engine side. Many boaters do know this about deep cycle batteries

BUP: thanks for responding, at this point the problem isn't with the voltage, it's with the errors being shown on the vesselview. If you have any idea what "TVM:FourWay Pin Fault" or "TVM:Four Way Coil Failure faul(t)" mean then you would be my and my families hero! I'm assuming all the other errors are related to these two. I tried pulling the codes with my Rinda DIACOM but it shows nothing but sensor faults for sensors I don't have, I guess it can't get errors from the TVM/DTS...

Have to ask is a OEM Merc certified dealer out of the question for the fix ? Also many will tell you diacom is needed inwhich you have correct ? wrong many do not know this -- Diacom is a great tool to have but it does not cover the depth of Merc G3 scan tool that is needed here. OEM dealer for G3 inwhich is pass coded to operate.

TVM is all related to axuis / joystick bs. An axuis manual is needed here as well. TMV = TRUST VECTOR MODULE and there was some updates to the software to be had depending on serial numbers.

You will need a Merc oem dealer to scan with G3 with the latest versions and see if software updates are needed for your app and the possible fix for the faults showing. Sorry man - the BEST advice and answer here period.