Optimax 250 melting fuse

[One Ocean]

Hello everyone,


I need some help with our 250 Optimax (2B091968).


It is melting, what I believe to be the main power fuse. I unfortunately can't remember which fuse it was when looking down into the box, but it is a 20A fuse and the engine won't run its fuel pumps without it in place. I can certainly go and look which one it is if that helps.


What bugs me, is that the fuse will melt, not just pop.


What I have done so far:


Clean out the old contacts as I believed it was a contact issue. New fuse melted as well.
Cut the wire and soldered brand new clean connectors on. The fuse melted again.
The fuses that were used were OEM fuses, so I don't think they are the issue here.


There are no fault codes and I had the harness off and cannot find any broken wires or shorts or similar.


I have some experience with outboards and if a fuse melted in the past I could usually track that down to an arc somewhere in the engine, e.g. a broken spark plug boot. I cannot find any arcing at all, which obviously doesn't mean there is none.


Any input or suggestions would be highly appreciated


Thanks


Thomas

[Joe4d]

What component does that fuse protect. ? Issue is most likely that component itself. if it isnt wiring

[One Ocean]

Good questions,

I will go and check out exactly which ones it protects once I got some time on my hands.

I know for sure the fuel pumps and please don't hold me to this, but if memory serves right the trim won't work either. If I'm not totally off I think the direct and fuel injectors are also wired through this fuse.

Again, I'm not 100% sure and will have to confirm this on the boat. Unfortunately I didn't get as much time to sort things as I would like to.

What is so odd to me, is that the fuse is melting, which kind of defeats its purpose. I toyed around with the idea of sticking in a bigger fuse, but again that would defeat its purpose.

[Savage]

Post pictures.

[One Ocean]

I wont get to the boat today, so cant get pictures of the inside of the box.

If I'm not wrong the fuse that melted is the bottom left one, looking from the top onto the box, having the latch facing yourself.

[tomloans]

Visually inspect your coils for cracks and/or white puffy material coming out of a crack in a coil.

[Joe4d]

Have to check a fuse diagram,, but if its running that many items,,, sure dont narrow it down much.,,

[One Ocean]

Visually inspect your coils for cracks and/or white puffy material coming out of a crack in a coil.

Interesting that you mention that, because this did happen a few month prior to the melting fuse. One of the grounds was loose and the coil got fried. I have re-checked all grounds back then and did so again a few days ago. I can't find anything wrong and the coils, nor the grounds. They are all working perfectly with all cylinders firing as they should. Back then the fuse did not melt and it started a good 100 hours after this incident.

The engine runs awesome in general and has no hiccups or anything I would complain about - just the melted fuse.

Unfortunately I have been held up and couldn't get the picture, but hope to be able to do so in a few hours.

[EuropeanAM]

Interesting that you mention that, because this did happen a few month prior to the melting fuse. One of the grounds was loose and the coil got fried. I have re-checked all grounds back then and did so again a few days ago. I can't find anything wrong and the coils, nor the grounds. They are all working perfectly with all cylinders firing as they should. Back then the fuse did not melt and it started a good 100 hours after this incident.

The engine runs awesome in general and has no hiccups or anything I would complain about - just the melted fuse.

Unfortunately I have been held up and couldn't get the picture, but hope to be able to do so in a few hours.

Also UNPLUG each coil, and inspect the pins inside the connector for burnt, damaged, or melted components or material.

[One Ocean]

Hey,

finally managed to get some time on the boat this morning.

I have unplugged every coil and double checked the pins of the coils as well as the female part of the harness. There is no obvious damage whatsoever. Everything looks clean and shiny. As mentioned above the boat runs great, I have an old DDT and verified with a cylinder misfire test that all cylinders are online. I have done this in the past and repeated it this morning, everything works as it should.

See attached a picture of the fuse box.

[lpugh]

Poor connections create heat, heat will migrate from its source, heat will cause a fuse to fail at much lower current flow than it is rated for, The fuse will melt down in the area closet to the heat source. They will not just pop like a like short to ground will do, Your picture Cleary shows this. Also elevated current flow can cause a connector to degrade in this fashion. Replace it and check current flow on that circuit. Be sure there is no water intrusion as well, looks like there may have been

[EuropeanAM]

Agree. Going to need to replace the harness, or the fuse holder/connector in order to rectify the problem you are having.

[billnorman1]

Ugh. That burned area will suck up wattage due to it being a poor conductor. Watts is heat. Either replace the entire connector or remove the wires to the affected connection and splice in a new circuit by cutting back all the damaged wires. There will probably be some wiring damaged under the insulation which has to go also.
I bought some single blade style fuse holders from Amazon that would take a 20 amp fuse, complete with wiring already attached, but it was not marine rated wires which could work in this situation.
In hindsight, looking at the photograph, you may need to replace the connector and fuse panel due to corrosion. The reason the area burned was probably loss of tension on the female sockets, and that might spread.

[lpugh]

amps x volts = watts. 20 amps X 14volts = 280 watts. resistance reduces current flow. 10 amps x 14 volts = 140 watts. watts does not cause heat, it provides the power to do so, resistance does, watts is the amount of power provided or used
Another example is a battery that has 1000 ca at 12.5 volts would have 12500 watts of power. Once up on a time a long time ago, batteries actually used watts as rating for comparison purposes
think of it as how many gallons of fuel that is in your tank, both can be converted in to BTUs and compared as to energy available for work purposes

[One Ocean]

I have a new harness coming in, but I just wanted to make sure that not a accessory like the fuel pump or coil or whatever could cause this.


The picture is of the first fuse box and I agree that it doesn't look awesome. That is why I have cut back the broken connectors and soldered brand new ones back on and stuffed these into a fuse box I had lying around. This box was not new, but in the end it's just a plastic housing and the connectors I put in were brand new. The result was the same and the fuse melted. Of course I can not disregard the possibility that the new connectors gave a poor connection as well.


Hopefully the new harness will not get the same issue and I'm just chasing a ghost here.

Thank you for all the awesome input, I will update once I have fitted a new harness which will probably be a few weeks from now.

[billnorman1]

The burn could have come from a weak connection OR too much current in the circuit. Kind of like the chicken or the egg axiom. I'm no big fan of those connectors but if they work fine and last a long time I'm for that. Be sure to clean the male item that plug mates with if you don't replace it. A little dielectric grease won't hurt either to keep water/moisture out.

[One Ocean]

A little dielectric grease won't hurt either to keep water/moisture out.

Thanks for the suggestion. Are there any connections which should not be connected with dielectric grease? I know that the relays shouldn't be coated in the grease, but was wondering if there are any others I should be aware of.

Thanks in advance

[Savage]

If you didn't find the exact problem with the harness, then you have another part failing, the will require diagnosis.

[EuropeanAM]

Being one who usually likes to know for certain what is going on- I personally would be inclined to (for testing purposes ONLY), wire in a replacement fuse holder for ONE 20 amp fuse (bypassing the original holder).

THEN check to see if there is any high-amperage condition occurring in that circuit. If there is NOT, you could then replace the harness with a solid expectation of "success".

Just my .02 worth...

[lpugh]

Sometimes .02 cents can be worth 100.00 Plus and that's my .02 cents worth...….