Mercruiser 120 battery/short problem

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floridaboater1

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I have a 1975 mercruiser 120 i put in a new starter yesterday everything was good today i came out and my negitive battery post had melted down to nothing. I put in a new battery today and everything seems ok but i dont want to burn up this battery too. How can i tell what caused the battery post to melt down?
 
Grandad

Grandad

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Re: Mercruiser 120 battery/short problem

Do you know if the post was at all damaged prior to changing the starter? I would assume that there was a poor connection, most likely at the terminal fitting due to corrosion or looseness to cause that much heat. The thing is, there's a lot of mass to the post so a lot of heat would be required for an extended period. Repeated attempts to use the old failed starter, particularly if it had an internal short could produce enough heat at a poor connection. If the post was OK after replacing the starter, I don't know what could draw enough current after the replacement, assuming you weren't repeatedly using the new starter. - Grandad
 
Grandad

Grandad

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Re: Mercruiser 120 battery/short problem

Just had a second thought. If your battery was totally flat, the charging circuit could produce enough heat on a poor connection, considering it might be generating high current for a long period of time. This theory still requires that there was initially a loose or corroded terminal connection. Did you perhaps forget to tighten it after your repair? - Grandad
 
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kd7isf

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Re: Mercruiser 120 battery/short problem

Make sure your battery cables are in good shape -- especially the + (red) one.

Make sure it's not sitting on a sharp edge somewhere and getting slowly worn through....

Did you have a big spark when you hooked up the new battery?
 
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Me1stBoat

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Re: Mercruiser 120 battery/short problem

I had this problem with my 170 mercruiser. Didn't melt the post but would get hot enough to burn you. Mine was a broken brush in the starter. It would intermittently ground out against the housing and cause a dead short sending all that amperage straight to ground. Replaced the starter and seemed to fix the problem. Every mechanic, marine or otherwise i have talked to told me its a grounding problem if it's building heat like that.
 
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floridaboater1

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Re: Mercruiser 120 battery/short problem

Grandad said:
Do you know if the post was at all damaged prior to changing the starter? I would assume that there was a poor connection, most likely at the terminal fitting due to corrosion or looseness to cause that much heat. The thing is, there's a lot of mass to the post so a lot of heat would be required for an extended period. Repeated attempts to use the old failed starter, particularly if it had an internal short could produce enough heat at a poor connection. If the post was OK after replacing the starter, I don't know what could draw enough current after the replacement, assuming you weren't repeatedly using the new starter. - Grandad
Click to expand...

All connections were good I had just cleaned them with a wire brush on a drill befor i put the new starter in. All connections are tight. I brought the battery with the melted negitive post in and had it tested. And it passed. Just said it needs a charge... The wierd thing is the battery cables are fine not getting hot or any sings of being hot. The new battery has been in the boat all day now and it is fine and not hot???? The battery post was ok after replaceing the starter and the only outher thing i have connected to the battery is the electronic fuel pump. My main thing is i dont understand how the battery post could melt without messing up the cables or wires at all and i really dont want to take it out and have my battery melt...
 
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floridaboater1

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Re: Mercruiser 120 battery/short problem

also no spark at all when i hook up the battery
 
NHGuy

NHGuy

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Re: Mercruiser 120 battery/short problem

So can we assume you ran the motor with the new starter after it was installed & before the terminal melted? I think you were smart to bench test the starter. You've gone this far, check the charging system too.
 
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floridaboater1

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Re: Mercruiser 120 battery/short problem

At this point the only thing i can think of is somehow the neg of the battery got a crack in it and got hot. But i would think if everything is off that would not of done anything.??? I would also think if any post would get hot it would of been the pos not the neg??? I have now had the new battery in the boat hooked up for 12 hours with no problems? I have just never seen or heard of anything like this.
 
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floridaboater1

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Re: Mercruiser 120 battery/short problem

and yes i ran the motor with the new starter after it was installed befor the terminal melted for about an hour then turned if off and cleaned out the boat for about a hour and a half and everything was ok then i went to bed came out this morning tried to start the boat. got nothing went and looked at the battery and the neg post of the battery was hanging on the neg batt wire off the batt. and the wire is ok with no signs of damage to the wire
 
Grandad

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Re: Mercruiser 120 battery/short problem

floridaboater1 said:
and yes i ran the motor with the new starter after it was installed befor the terminal melted for about an hour then turned if off and cleaned out the boat for about a hour and a half and everything was ok then i went to bed came out this morning tried to start the boat. got nothing went and looked at the battery and the neg post of the battery was hanging on the neg batt wire off the batt. and the wire is ok with no signs of damage to the wire
Click to expand...
Did you or can you take some pictures of the remains of the post? I'm wondering if the post had been deteriorating (corrosion?) below the top of the case and something during your starter replacement process stressed it to the point of failure. It's a characteristic of electrical failures that a poor/deteriorated connection causes heat which then increases the deterioration in a runaway acceleration of the process ending only when the connection finally breaks. The lack of heat damage to the cable makes me think that the hottest point may have been below the post at the top of the plates. - Grandad
 
Grandad

Grandad

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Re: Mercruiser 120 battery/short problem

Hi floridaboater1, the attachments you posted last night aren't opening. Others have had this same issue. I believe previously the poster was able to see the pictures, but no one else could, so they thought they were being ignored. If you find the posting pictures problem, let us all know what the problem and the fix was. - Grandad
 
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floridaboater1

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Re: Mercruiser 120 battery/short problem

To grandad. If you could give me a email address i could send you the pictures that way
 
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