Man D2848 LE403 coolant level alarm.

Aha, I see.
You are now making me curious, and I'll check out also that smaller box as soon as I'll be back on my boat.
I will also tell you about my findings on her ASAP.

Btw, interesting to hear that you are talking of a Manhattan 56, because she's a boat I carefully considered.
If you don't mind sharing some of her basic numbers (cruise & max speed, and possibly approximate fuel burn), I would be interested.
Besides, what's your experience with her unusual sliding hi/low swim platform, is it working smoothly?
I was warned about its reliability by a yard who struggled to fix one...

Interesting to hear that, because it fully matches my findings while I was looking for a used boat in that size/type bracket.
180 l/h at 24 knots means 7.5 l/nm, which is fine for those engines and boat size/type.
Max speed according to the builder's specs should be up to 33 kts, but just FYI there's a widespread consensus that she's genuinely a 28kts max boat - 30 at the very best, if superclean and running on fumes.
So, nothing wrong with yours, also in this respect.

All that aside, I'm now back home, i.e. living aboard.
So, if there's anything you would like to cross check with the behaviour of another pair of identical engines and their electrical bits, just ask!

Doh! You are right, I missed that bit of KC post, in my previous reply.

I guess I must have mentally removed that while reading, because 180L/engine doesn't make sense.
Those engines are rated for 162 L/h each at 2300 rpm and full load, so there's no way he can burn 180x2 even at WOT - let alone at cruise speed!

Not sure if in the meantime you sorted the problem, but today it popped to my mind that I told you I was going to cross-check the behaviour on my boat with the very same engines, so here goes.

On my dashboard, when I turn an engine key to the contact position (i.e. without actually starting the engine), three warning lights go on: alternator, oil pressure and coolant level.
The first two stay on till the engine is started, and go off as soon as the alternator kicks in and the oil pressure builds up.
The coolant level instead stays on just for a couple of seconds, and then it goes off regardless of whether the engine is started or not.

Besides, in order to make a full comparison with your situation, I also disconnected the coolant level sensor to see what happens.
And on the dashboard, with the plug disconnected, the coolant level warning light doesn't turn on at all.
I know this doesn't seem to make a lot of sense, but that's the way it is, and it happens on both engines, so I would think it works like that by design.

Last but not least, I also asked my wife to turn the key for me, while I checked what happens on the engine black box.
And the result is in the video below.
The first half of the video show in real time the "normal" behaviour of the warning lights on the black box.
When the two upper lights turn green, the coolant level light on the dashboard goes off.
The second half shows what happens with the coolant level sensor disconnected, when as I said the dashboard light doesn't turn on at all.

I hope this helps your troubleshooting, if not already sorted.

Yup, I'm onboard as I'm writing.
It's actually an exception when I'm NOT living aboard, rather than the other way round!

Not a problem to open one box and take a photo, but I'm a bit skeptic that you will be able to compare the wiring accurately.
In fact, I already attached to my post #9 a photo of the box interior which I took some time ago.
Below is the very same photo, just re-attached to this post for reading convenience.

But if that's not what you meant, and/or you are interested in some specific details, just ask.


PS: I only just noticed that this old photo was flipped vertically. I guess I did that just for easier reading of the Bosch label.
I don't think it really matters, but just for clarity, the actual box orientation is as shown in the previous video - i.e. reversed, compared to the photo.

PPS: I also have the original, much higher resolution photo.
If you think it could be useful for you, just pm me your email.

It appears that also the smaller box in your pics is the same as mine.
Ref. the wiring, check out the pics below.

Obviously I didn't detach the small box and the wires, as you seem to have done. I just opened the cover and took some pics.
The first shows a label which I found inside the cover, while the others show the whole box from different angles, and the detail of the wires.
It wasn't easy to take a good photo of the latter, particularly of the green block on the back, but I can confirm you that there are just two wires attached to it, both white, and connected to positions 4 and 10.

Lastly, don't ask me why the contacts in the label scheme don't match the actual wiring.
With anything boat-related, I'm a fan of the principle "if it ain't broke, don't fix it".
So, I'm not going to mess with something which is working just fine!

Happy to have been helpful!
Yep, the rubber seal seemed original, and was there in both boxes.
But as you surely have seen, the larger box has several holes where the cables pass through.
So, I think it would be a good idea to protect well the whole thing from any drops or spray of water.
In my boat, they are placed in a bulkhead where they have zero chances to get wet, unless someone deliberately throws water at them...
Looking fwd to reading that you'll have fixed everything!

PS: interesting to hear that the wiring was the same in your boxes.
Makes me wonder even more what is the logic of the label inside the cover, but... Hey-ho!

Agreed. Frankly, I was also a bit surprised to see the circuitry inside the small box (never opened it before).
After all, they are fully mechanical engines and those black boxes only seem to handle a couple of sensors with their alarms.
The silver lining is that your wallet could suffer MUCH more when anything goes wrong with electronically controlled engines...

My boat was built by Dalla Pietà, and she's a 56 footer flybridge like yours, but it's unlikely that you ever heard the name.
It was a yard based in Venice, founded in the 50s, which up to a dozen of years ago used to build semi-custom boats from 50 to 80 feet.
Historically, they always struggled to keep up with the domestic demand, and never had the wish nor the need to export.
That's the reason why I guess the name might not tell you a lot.
Sadly, when the financial meltdown stroke in 2008, the demand went down like a lead baloon, orders cancelled, etc.
And that happened right while they were enlarging the production facilities and investing in new moulds, with a 100' in development.
Long story short, sadly they were unable to stay afloat...