1996 40HP Merc engine fire?

Question

Had a small fire in my engine, pulled off the engine cover, then a cover on the starboard side of the engine, and found, what I think was a burned out rectifier (same as voltage regulator?). Lot's of magic smoke. Prior to this, I had lot's of trouble with the tach not working/working sporadically. Was this an indication of a developing problem with the rectifier/voltage regulator? I did install a dual battery system (Blue Sea Systems) back in May. Could that have slowly worn out the rectifier until it finally burned out? I have pictures here: http://boards.trailerboats.com/cgi-bin/trailerboats/ubb/ultimatebb.cgi?ubb=get_topic&f=4&t=003167#000007

Answer 1

jtexas is right on the money regarding batteries.  The only beneifit you get trying to charge two batteries at once with one healthy and one dead with the boat's positive amp output is two half dead or all dead batteries.

The reason is fundamental electrical and electronic theory.  If two batteries are connected in a parallel circuit and one is weaker than the other, the stronger one will give away its stored charge to the weak or dead battery.  

Your outboard also does not generate enough positive amps per hour to charge the batteries high enough to prevent them from both going weak or dead.  That would take a LONG time for you to charge them with your small outboard motor.

Answer 2

Couldn't get the smoke back in it, eh?  They just don't work without they smoke.

Rectifier converts AC generated by the flywheel & stator into DC for charging the battery & running accessories.

The tach gets its signal from the rectifier.  tach going out more often than not indicates bad rectifier.

Can't speak to that particular motor, but in general, outboards can have small (6- or 9-amp) unregulated charging systems all the way up to 30- 40-amps with watercooled voltage regulators.

Primary cause of rectifier failure:  disconnecting the battery while the engine is running, or corrosion/loose battery connections.

If your blue sea system does not "make before break" when switching, that's the most likely cause of your troubles.

If you're trying to charge a deeply discharged deepcycle battery (like, for example a trolling motor battery), plus the cranking battery with a 6-amp unregulated alternator, be advised that it just won't work - eventually you'll end up with two dead batteries.  I don't really think you can smoke a rectifier like that, but I guess it's possible.

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"Can you explain what you mean by 'If your blue sea system does not "make before break" when switching, that's the most likely cause of your troubles.'?"

"Make before break" means that when you (or an electronic switching device) switch between batteries, the next connection is made before the previous one is broken.  It closes the circuit you're switching to, before opening the one you switched from.  That would enable you to keep the house battery offline so devices don't drain your cranking battery, yet you could still switch to "both" after the cranking the engine, to charge both batteries.  Try that with a "break before make" switch, you'll smoke the rectifier.

the pictures tell the story.  One thing about boats, is that the shock & vibration and atmospheric conditions take their toll on electrics.   You gotta look out for areas where wires can get chafed, loose insulation.

The timing of this fire coming so close after your installation would make me nervous enough at least to inspect everything thorougly and monitor the battery voltage at least for awhile, to ensure the next rectifier is functioning normally.  If it's regulated, you should see a steady 14 to 14.5 volts, except at idle it'd probably be lower.  Unregulated, it'd be an almost linear relationship with RPMs.

Glad the fire was contained & nobody got hurt.