Volvo 2002 oil pressure alarm problem

[Hachiko]

Hi,
I think I have a problem somewhere in the circuit of a Volvo Penta 2002. A week ago, when I switch on the starting panel, the oil pressure alarm was triggered before the engine start.
Despite the noise the engine started with no problem and the alarm instantly stopped at the same time.
But when I stopped the engine the alarm and the oil pressure light started again.
This light (oil pressure) is the only one that turns on
I also noticed that my rev counter is dead.
I tried to check the wires below the fuse box were I think is the problem, but my knowledge is very poor in electronic
Any help would be very appreciated
Thanks

 

[Senior B]

That is how the system works. The oil pressure alarm warns you of a loss of oil pressure. When you turn the key on, the system is energized and since the motor isn't running you have no pressure and you get the alarm. Start the engine, get pressure, alarm goes off. It's working as it should.

 

[Hachiko]

Thanks for the help...

The alarm and the oil pressure light are the only ones working (no water temp or alternator lamp).
I think I have a problem with the alternator (not charging or randomly charging) I still don't know if it is a wire, or something more important is broken.
Batteries are good, the voltmeter on the main panel shows 12.4V before the engine starts but it drops to 12.1 or less with the engine running (even in full throttle).

Maybe the box beneath the fuse has something to do with the issue, or is the problem itself. I tried to follow the wires after they leave the main cord but they are all green painted and is impossible to distinguish one from another.

I don't know how to isolate the problem