Intermittent ignition issues

[MV Mystery]

On my 1989 Cheoy Lee 58 motor yacht powered by twin Detroit Diesel 8v92ti engines.
The electrical circuits per engine are as pictured. It's been trouble free for 18+ years.
(And it goes without saying, also not really touched in this time either)
Lately ,I've been getting an erratic-jumping rpm and battery gauge on my bridge- and occasionally I am not getting any ignition at the bridge .
I am wondering if the Ancor breaker or other issue at the panel is causing this?
And what are the two solenoids or relays on this panel for?

 

[rwaldrop13]

I have the service manual in PDF format for the 8V92's if you want - my email rwaldrop13@gmail.com. My first thought would be to take apart, clean/lube the connectors and put it back together. I had an issue with one engine acting odd but that cured it. The trouble is my start issue traced back to corroded/rusty connectors at the starter and starter solenoids. I replaced the terminal lugs and it never happened again.

 

[dsharp]

The jumping tach is probably a worn drive tang in the sender. Are they electric or mechanical tachs? The solenoid are probably used to carry more amperage to the starter solenoid. The cables don’t look large enough to be a battery parallel solenoid. You are just going to have to start testing with a volt meter to see where you’re losing power.

 

[JD5652]

Those connections in the photo are extremely corroded. IMO, that's the first place to start. As someone else already said, break every connection, clean and reconnect.

 

[rwaldrop13]

Just an add-on comment. When I replaced my starter solenoids and had starters rebuilt, I removed the metal tab that was the link from the solenoid to the starter and replaced it with a 12-inch battery cable. Worked great.