Man D2848 LE403 coolant level alarm.

Roger wilco. I should be there next Sunday.

Maybe this will help. Diagram from the manual with a little explanation on how it works.

They are not "elecronic" in the modern sense, correct. But there are some sensors in there. I just learned for instance that they will sense, at higher engine RPM's , if the tranny pressures are way low. It then sets off an alarm. But they are not diagnostic ?.

Interpretation please. What does that tell us; disconnected from the probes/sensors, one alarm sounds and the other doesn't? It seems the sensors are not the issue? Dunno

Then the alarm monitoring circuit board is bad for the starboard engine or the display at the helm with the alarm lights. Do you have digital hour meters? If so are they reading correctly? Try swapping the connections at the 2 displays on the dash with the lights and see if the problem stays on the starboard side or moves to the port side.

I'm still thinking you have a short up the line. Have your voltage meter handy - whether something "burned" out, or other wised failed, you've got juice going to that alarm. Track it back to the source?

Can someone with more knowledge than I (everyone?) confirm how the sensor works the circuit. Do it close the circuit or open the circuit when its in a "non-alarm " state, and there is sufficient coolant in the expansion tank to cover it? That schematic and the OPs comments that the wires are disconnected from both sensors seem at odds?

Alarm monitoring boards are located in the large MAN black box in the engine room. On yours, the actual displays "may" be the computer/board/display everything all in one, if you have the displays with a row of like 6 lights.

Can you get to the lamp and buzzer or relay with a multi meter ?
Can you get to the sender with a multi meter.
How hard is it to remove the signal wires from the sender ?

You should read 24VDC across (+) & (-)
If (S) is disconnected from the sensor, you should read zero across (+) and (S)
Assuming (S) is not common to any other component.

You say the alarm stays on even if the sensor is disconnected.
If sensor is disconnected, & you read 24 VDC across (+) and (-) then the cable has failed.

I don't know the distance but you can get three conductor signal cable and connect it in place of your existing cables temporarily to see if the problem goes away.
This signal cable is pretty cheap - you could even use regular single strand wire from Home Cheapo just for a test but be sure you mark them on both ends or get different colors as you don't want to goof up the polarity.

If the problem persists then you know it's not the cable

The symbol with the diode may be a relay.
does the manual have symbol definitions ?
Possibly the relay might operate the buzzer and is stuck closed.
Swap relays with the other engine.

If the signal is as simple as that drawing then it seems easy to trouble shoot.
If, indeed the cornucopia of information that is "MAPISM" is correct and there are no real " Electronics" present.

I like mechanical engines for the same reason he does.

I agree, what the OP is reporting appears weird, and intuitively I also can't think of any other reason for the port alarm behaviour, other than a short somewhere along the wires.
Either from engine to the black box, of from there to the dashboard, but I'm just guessing.
Will check on my boat when I'll be back onboard...

No way.
The display you have in mind is the so-called MMDS, made by Boening for MAN electronically controlled engines.
I don't think it's even possible to retrofit that display to mechanical engines, because even the probes are different.
What I suppose KC must have in his dashboard is a set of analogue gauges, plus an array of red lights which go off together with a strong acoustical buzzer.
The only LCD bit probably is the small hour meter embedded in the lower part of the rev counter, nothing else.
Thanks God!

Far from wishing to teach you to suck eggs, but just in case you didn't consider this:
Don't assume that since the engine starts and runs fine, there's nothing wrong with the coolant (or anything else).

I mean, it's rather likely that what you've got is a sensor defect of some sort and there's nothing really wrong, in this case.
BUT generally speaking, even if you would have no coolant in the circuit, no oil, no raw water circulation, or any other serious trouble, your engines would still start and run till they physically can, because all warnings are just that, warnings.
There is no protection/limp mode or other electronic tricks to CYA, as with more modern blocks.

Good to hear that the problem is on its way to be fixed, but I'm a bit confused.
What do you mean exactly by "small black electrical sensor box"?

I suppose you aren't talking of the one in my post #9, because that's not so small.
But I can't recall to have any other black boxes in my boat...

Or do you possibly mean the smaller box attached to the side of the larger one which I posted, where the lights are located?
I never opened that one, so I have no idea of how it looks inside!

Have you figured out how it got water in it (in the Engine room) to begin with?