I have a mercruiser I/O 4 cyl. 3.0 and was in the process of
winterizing it on the trailer. I always disconnect the battery
terminals while its on the slip between uses because it drains rather
quickly if I don't (it's my first season with this boat and I suspect
that the radio/equalizer that the prior owner installed may be
grounding out and draining the battery). Anyway, I had it on the
trailer and connected the battery, positive first, then negative, then
went forward and switched on the key to start it and warm the engine
for the oil change when I heard a "click" from the engine compartment.
I looked back and there was brownish/white smoke flooding the engine
compartment, which seemed to be eminating from the alternator, but I
didn't waste any time and immediately disconnected the battery. The
terminals were warm to the touch. It took several minutes for all the
smoke to clear, but when it did, I didn't see any signs of damage to
external wires. Can someone please help me with this?!? I don't want
to ruin the alternator or the battery by hooking it back up if I'm
missing something, and I certainly don't want a battery blowing up on
me! My only thought is that maybe it was caused by the boat being on
the trailer, and there was a grounding issue, but I've winterized it
at the end of last season with no problems and it worked all of this
season with no issues. The only thing different from last fall is that
I didn't have the muffs on the lower unit yet when I connected the
battery. Thanks!!!

-Smokey