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ContinuousWave: Small Boat Electrical
Reversed Battery and Blown Fuses
|Author||Topic: Reversed Battery and Blown Fuses|
posted 10-02-2007 04:54 PM ET (US)
I did the stupid thing. I have a 2007 Mercury 40. I had some blown fuses and replaced them. When I turn the key, the warning buzzer sounds but as soon as I turn the ignition switch, I blow the 25 amp fuse that goes to the main [relay] and accessories. I replaced the remote control box because I had an extra but get the same problem. Any ideas where my problem(s) may be?
posted 10-02-2007 09:42 PM ET (US)
Where or when did the battery polarity reversal occur?
posted 10-02-2007 09:59 PM ET (US)
If you reversed the battery terminals for more than a VERY
short period of time (one second?), you HAVE blown the
alternator diodes. If they failed open, things will run,
but the battery will not charge. If they failed closed, they
will by and by, fail open. See above.
Blowing the 25A fuse is "the failure". Go get a stock of 25A
What boat? Is the 25A fuse under the cowling or somewhere on
posted 10-03-2007 09:32 AM ET (US)
I connected them backwards in my driveway, hooked up my flush kit and turned the key. I heard no warning whistle, was puzzled, turn the ignition and the posts melted off the battery! Once I changed the battery, corrected the reversal problem, changed the fuses under the cowling, and switched the remote control box, I am down to this recurring problem. I can turn the key on and get the warning whistle and right when I turn the switch to crank it, the fuse blows and no motor activity. I thought it may just be the voltage regulator but it sounds from other posts that there may be a little deeper problem.
posted 10-03-2007 05:14 PM ET (US)
Replace the fuse. Turn the ignition switch on and wait one
minute. Turn the ignition switch off. Is the fuse blown?
If not, the problem is in the starter circuitry. If yes, it
could be anywhere, and you are pretty much down to remove
everything you can. If it still blows, the bug is in something
you can't remove. If it doesn't, add things back one at a
time and see what blows the fuse.
But I'm at a loss to explain a current path that would melt
posted 10-03-2007 09:49 PM ET (US)
I concur with Chuck's analysis. Any connection of a battery with the polarity reversed will just about always cause damage to the diodes in the outboard motor charging system.
My inference from reading the description that the terminal posts of the battery melted is that the amount of current flow was very high and was allowed to continue for a prolonged period. Based on this I would guess that the diode failure eventually resulted in the diodes blowing into open circuit. However, before that happened there could have been high current flow through other portions of the charging circuit, including the windings of the alternator coils.
Your problems are most certainly in the alternator diodes. The stator coil may also be damaged from over current and over heating. After that it is harder to predict what else may have been damaged.
At the moment of connection the forward biased diodes in the alternator were conducting, and they more or less protected other elements of the circuitry. But when they blew to an open state, current could easily flow via other paths.
It is an unfortunate circumstance that connection of the battery with the polarity reversed can cause so much damage. This is one of the reasons I recommend careful color coding of the cables and the battery terminals to prevent such errors.
posted 10-04-2007 07:59 AM ET (US)
Definitely pull the battery cables from the rigging tunnel and inspect them for damage. The insulation could have melted and you could have a dangerous situation. This is probably not the cause of your problems but considering the battery terminals melted there was some serious heating along the current path.
posted 10-04-2007 04:35 PM ET (US)
Thank you very much gentlemen. I am going to start with these suggestions and go from there.
posted 10-04-2007 05:23 PM ET (US)
The fuse that is blowing is the 15A not the 25A if that helps.
posted 10-04-2007 08:04 PM ET (US)
Jimh, when the diodes failed open, current no longer flowed
through the alternator because it couldn't get through the
diodes any more.
The battery cables in the rigging tunnel aren't directly the
15A, 25A, is not terribly important to debugging. BUT REPLACE
posted 10-04-2007 08:15 PM ET (US)
Chuck--Here is what I said:
"before [the diodes blew to an open circuit] there could have been high current flow through other portions of the charging circuit, including the windings of the alternator coils.
Then you said: "when the diodes failed open, current no longer flowed
The observation that the current flow through the windings of the alternator coils was dependent on the diodes remaining forward biased and conductive was implied in what I said. As for damage to teh coils, it depends on how long the current flowed. The current flow created heat, which could have melted the wiring or its insulation if the temperature became high enough.
The short answer--when you reverse the polarity of the battery you've got a mess on your hands.
posted 10-04-2007 10:34 PM ET (US)
I agree that you have a mess on your hands with reverse
polarization, especially when there was enough current flow
to melt the battery terminals.
But a short in the alternator coil wouldn't explain why the fuse
posted 10-22-2007 04:56 PM ET (US)
It was the alternator diode adapter. A lucky $30 fix.
posted 10-28-2007 10:37 AM ET (US)
Just one more case for painting the postive post and cable RED and the negative post and cable BLACK.
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