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Author Topic:   Battery Polarity Reversal
gibsol52 posted 06-28-2009 03:09 PM ET (US)   Profile for gibsol52   Send Email to gibsol52  
I installed a new battery. After installing and trying to start [my outboard motor], I heard a low whine sound. Not realizing something was wrong, I tried it several more times. Then I checked the battery to see I had reversed the cables. After correcting the hookups, I tried to start the moter. The motor turns over quickly (new strong battery) but doesn't start. I ran the battery dead trying to start it. Are the diodes the problem? Could I have done any other damage? Any help would be much appreciated.
jimh posted 06-28-2009 03:19 PM ET (US)     Profile for jimh  Send Email to jimh     
Depending on the type of electronics and charging circuit, reversal of the battery polarity can often destroy the rectifier diodes in the battery charging circuit of the motor. If your motor no longer starts, this is good evidence of further damage from the battery polarity reversal. With solid-state rectifiers there is not much tolerance for reverse-bias current flow which exceeds the maximum rating of the device. They usually melt into a non-diode state.
jimh posted 06-28-2009 03:22 PM ET (US)     Profile for jimh  Send Email to jimh     
One aspect of mistakes like this is that they are only made once. For some prior discussion on this topic which is likely to be completely applicable in this situation, see

jimh posted 06-28-2009 03:32 PM ET (US)     Profile for jimh  Send Email to jimh     
One big problem I see here is the battery which connected with reverse polarity to the outboard motor was fully charged and raring to go. So when it was presented with a path for current to flow through many now-reverse-biased diodes it really took off with a vengeance.
gibsol52 posted 06-28-2009 09:18 PM ET (US)     Profile for gibsol52  Send Email to gibsol52     
Thanks for the information folks. Not knowing much about the 90hp mercury force motor the question I have is this: are the alternator diodes replacable? Does the entire alternator need replaced. Is is something I could do myself. I am fairly mechanically inclined. Worked on a lot of car sytems but never boat motors.
deepwater posted 06-28-2009 09:39 PM ET (US)     Profile for deepwater  Send Email to deepwater     
Try getting a shop manual and see if you have the tools for the job,,or rent them
jimh posted 06-28-2009 11:23 PM ET (US)     Profile for jimh  Send Email to jimh     
It is quite typical on an outboard motor that there is a permanent magnet alternator. The stator coil is under the flywheel and the diodes are generally separate and easily replaced. Depending on exactly what happened during the polarity reversal, it is possible that the stator windings might have been damaged from over current, too.

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