Cat 3412 1997 problems

Indeed don’t run with the parallel switch always on as it will mask a failing battery and leave you stranded.

same with running dual racors with both filters on line. Bad habit but very common.

you can’t upload a video here, only pictures. Put it on YouTube and post the link

If the parallel switch is always on, the solenoid windings would be fried by now.

Some boats use solenoids to parallel the batteries ( Hatteras for instance) but many others use battery switches which can be left on. Not a good idea indeed

I have never seen that on a boat and don’t see the point. It s just one more potential failure point. Redundancy is critical on a boat so if you have 24v system, you put two batteries in series for each engine and you re done. No need to complicate things. Alternator and starter will both be 24 v anyway.

Ocean Yacht and others boat mfgs do it.
The whole boat is 12volt.
Chargers are 12v.
Generator starters are 12v.
All alternators are 12v.
The big main engine starters are 24Volt.

Two G31 batteries in a parallel 12V bank are twisted by a fancy solenoid setup and makes a serial 24V bank for starting.
When the engine is running, batteries go back to a 12V bank for the house and I think, gen-set(S).

It saves weight and keeps lots of cost down.
There is no DCv back up or redundancy.

Delco made a tall bakelite solenoid with 4 battery connects just for this task.
https://www.texasindustrialelectric.com/relays_1119844.asp

I have also witnessed ugly rigs made with single solenoids. Not pretty.
Got a headache just trying to figure out this bird-nest, the first time.

For me; I would find room and ad more batteries.
Separate them all out for their task, (House, gen-sets, electronics, mains) and now have a couple of banks for any back-up need.

I do hope this is not installed this way for the OP.
You do need to disconnect the batteries for testing.

Did he say what mfg/brand boat he has?

Typical penny pinching by builders.

when I repowered my hatteras 53, I went the other way. Cummins was charging about $1500/2000 per engines for 24 volts factory reman so i got 12v engines with an 8D per engine but used 24 v for the house. I don’t like 12 v house on anythjng over 40’. Especially for the windlass.

only downside for now is that I can’t charge the house bank with the engines but we always run the gen anyway.

I could probably run a set of wires from the each engine 8 Ds to each of the two house 8Ds with a parallel switch if we loose the gen. But if we loose the gen, we we not staying out without air con anyway

You can imagine my Frankenstein designs; I'm still mostly 32V with 24v for the Inverter, windlass, 10Kw radar, Master head & 12v for the gen-sets and electronics.
14 Batteries on board.
So much for 32V weight savings.

At the very least change the oil and filter before trying to fire it up.

I can’t help wondering if firing it up is a good idea without further testing. Apparently you got away with some kind of water ingestion once but this may be pushing your luck.

pressure testing the cooling system is pretty easy at least on the coolant side. When I had our 16V2000s rebuild they pressurized the coolant overnight to make sure it was all good before starting.

I don’t think salt water can get in but could be wrong. Aftercoolers maybe ? Are they fresh or raw water cooled on the 3412s? I don’t remember.

and send oil and coolant samples to a lab to know what s in your oil and if any metal shows up