Ignition? Wires heating up during engine operation

M

Matts5150

Member
Oct 5, 2020
30
Boat Info
2007 240 Sundeck
Engines
5.0
2007 305/350 MerCruiser engine with Bravo 3. The engine port side sits a bank of 4 20amp mini fuses 2 sets of the wires leading to that fuse bank are heating up and one of the 2 melted the fuse holder for that bank. I believe the two sets of wires that are heating up have something to do with the ignition circuit and possibly other one something to do with the alternator. The other two 20 amp fuses that are also sharing that bank of 4 at least one of them is for the fuel pump I believe which I'm not having any issues with any of those wires heating up. I cut those four wires out and spliced them into 2 new independent marine fuse holders and still noticed both of those sets of wires are still heating up one getting much hotter than the other. The wire that gets hotter than the other has to do with something like I said with the ignition if I pulled the fuse while engine running it still runs but I won't be able to crank it it'll just click until I put that fuse back in. I cleaned and replaced both eyelet wire connectors on the alternator (ground and positive) to rule that out. I already replacemend the 50amp circuit breaker that sits behind the horn in the center of the engine (between the bank of three relays and the fuse holder that I'm having issues with. I'm not sure what else to check I do know they are definitely pink wires one has pink and purple stripe that leads to a thicker red with purple stripe wire on other side of fuse. And I believe the second wire that's also heating up is a pink with green stripe that leads to a thicker red with purple wire on the other side of the fuse. Hoping someone could share some input to wear these wires go and what their suggestion might be for the reason why these wires are heating up and sometimes melting the fuse.

Could it be something with the key ignition switch at the helm.. perhaps dirty connections there? Or possibly one of those relays going bad? Or maybe somewhere attached to the starter possible corrosion with the wires down there? Keep in mind those pink wires are thin possibly 18 gauge. Also please keep in mind I have a Mercruiser Smartcraft system. I keep my engine very clean if that helps.
 

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Last edited:
scoflaw said:
Your going to need a wiring diagram first. Merc customer support will email you one. 920-929-5040.
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I believe I found one for this model just very difficult to read these things lol was hoping maybe someone has exact same wires with that bank of 4 fuse holder on top
 
You said the starter won't crank with 1 removed, so the starter is drawing to many amps. Have you cleaned all the connections at the 90 amp fuse and the ground cables that go to the back of the block? I'd start there.
 
scoflaw said:
You said the starter won't crank with 1 removed, so the starter is drawing to many amps. Have you cleaned all the connections at the 90 amp fuse and the ground cables that go to the back of the block? I'd start there.
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Thanks for the response yes when I pull one of the fuses it won't stall the engine but it'll just click until I put that fuse back in so assuming that one fuse has to do with the starter circuit. I thought about cleaning those connections on the starter but man it's extremely tight in my 240 Sundeck I have to climb in there with my head upside down I did however think about doing that I just know it's going to be a pain you getting down in there cuz I'm going to need more than just a few minutes LOL. I literally have to lay on top of the engine and squeeze my head over the back side and point my head upside down in order to see the starter wires unless I remove the starboard side exhaust manifold which I don't really feel like doing in the middle of the season LOL. However I do very much agree with you to start there and I'll probably end up having to do so. Thanks for the advice greatly appreciated
 
scoflaw said:
You said the starter won't crank with 1 removed, so the starter is drawing to many amps. Have you cleaned all the connections at the 90 amp fuse and the ground cables that go to the back of the block? I'd start there.
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One question I just thought of if it has something to do with the starter wires wouldn't the wires only heat up when I'm cranking the engine and then eventually cool down while engine running or is there continuous power going through that 90 fuse area at the starter while engine is running?
 
scoflaw said:
Those relays are cheap, I'd replace them all at 10 bucks ea.
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That's great to know they're that cheap I'll replace them and keep those at spares in case I'm stuck out in the water. what's the best place to pick those up are they genuine oem? Or doesn't matter with these?
 
If the fuses didn't BLOW, then there's a good chance it's not actually drawing too much current, but there was some corrosion on the connections (maybe at the base of the fuse) which caused the high resistance and heating until things melted.
 
km1125 said:
If the fuses didn't BLOW, then there's a good chance it's not actually drawing too much current, but there was some corrosion on the connections (maybe at the base of the fuse) which caused the high resistance and heating until things melted.[/QUOTE
I already ruled out the both fuse locations because I cut them out and installed two new ones still same issue. But I will check my grounds on the engine block and replace relays and then last but not least the starter location which is a real pain to get to I may have to take the manifold off because my 07 240 Sundeck there's no room only to squeeze my head in and stick it upside down which is difficult when you're 50 plus LOL. I'm wondering if they're could be a broken few strands of wire inside one of the jackets perhaps during manufacturer engine installation they bent one of the wires. But I really want to get down to the starter location and examine those wires and of course on both them and clean everything I just don't know how I'm going to get in there easily.
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Matts5150 said:
I already ruled out the both fuse locations because I cut them out and installed two new ones still same issue. But I will check my grounds on the engine block and replace relays and then last but not least the starter location which is a real pain to get to I may have to take the manifold off because my 07 240 Sundeck there's no room only to squeeze my head in and stick it upside down which is difficult when you're 50 plus LOL. I'm wondering if they're could be a broken few strands of wire inside one of the jackets perhaps during manufacturer engine installation they bent one of the wires. But I really want to get down to the starter location and examine those wires and of course on both them and clean everything I just don't know how I'm going to get in there easily.
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The fuse is there to stop any overcurrent condition from damaging the wires (or the fuseholder). If the current draw exceeds the fuse rating, it still should be less than what would cause damage to the wire or fuseholder - assuming that the wire and fuseholder is in good condition. If there is corrosion, then any continuous current will cause a heat rise and at some point that becomes too much to be dissipated and the connection will overheat.

Is the fuse the correct size for that application? Is it possible someone previously put in too large a fuse, to "repair" some "nuisance" blowing of the fuse? If THAT is the case, then you might indeed have some issue downline that's causing a higher current draw - less than the fuse rating but higher than the circuit is designed for. It would not surprise me if someone added some load to the ignition circuit and then just popped in a larger fuse to "accommodate" that larger load.
 
km1125 said:
The fuse is there to stop any overcurrent condition from damaging the wires (or the fuseholder). If the current draw exceeds the fuse rating, it still should be less than what would cause damage to the wire or fuseholder - assuming that the wire and fuseholder is in good condition. If there is corrosion, then any continuous current will cause a heat rise and at some point that becomes too much to be dissipated and the connection will overheat.

Is the fuse the correct size for that application? Is it possible someone previously put in too large a fuse, to "repair" some "nuisance" blowing of the fuse? If THAT is the case, then you might indeed have some issue downline that's causing a higher current draw - less than the fuse rating but higher than the circuit is designed for. It would not surprise me if someone added some load to the ignition circuit and then just popped in a larger fuse to "accommodate" that larger load.
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All the fuses that were in that bank of four fuse holder were 20 amp fuses I put the same number back in I am the second owner of the boat the original owner only had 110 hours on it. I never added anything to the ignition circuit. I'm guessing it's either a bad relay or bad wire somewhere such as a possible dirty connection or broken or frayed somewhere.
 
I also want to mention those 20 amp fuses to blow once in awhile and I've seen that 50 amp breaker pop out once in awhile where I had to reset it so something is going on just need to figure out what it is
 
Change the 90 amp fuse on the starter and clean those connections. Pay someone to do it, you need to start somewhere and this would be the place to start.
 
scoflaw said:
Change the 90 amp fuse on the starter and clean those connections. Pay someone to do it, you need to start somewhere and this would be the place to start.
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Thanks everyone for your help I'm going to start with that starter area and order that 90 amp fuse as soon as I get some time. I will definitely report back once I'm able to do those things
 
Thornton69 said:
Not sure what exact model your engine is, but most of the 5/5.7 diagrams show three fuses (2x15A and 1x10A). May want to try using the correct fuses to help narrow down the search for your problem? Pages 4E-15, 16?
service manual 5&57.pdf (atlanticmobilemarine.com)
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Thanks mine is different it's newer I think than the older ones I found a similar wiring diagram and it doesn't match mine mine definitely has a bank of four fuses and when you look up the parts using my engine serial number it shows you 20 amp fuses so I'm correct on the fuses I just wish I could find the actual wiring diagram for my engine I'm sure if something's are similar but I really need the exact one to trace out these wires and see if there's a broken connection somewhere on the runs and clean each terminal they terminate at
 
My engine serial number in case someone happens to come across the exact wiring diagram for mine
Engine s/n 0W657692

Thanks all
 
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