so i've got this boat thats been unused in the backyard for about 10 years, slowing composting into a pile of $hit. 1979 140hp mercury inboard-outboard, uses a 181 ci. chevy straight 4 cylinder. carb, points, etc. it cranks and runs good and the kids have been using it for the past few days.

new battery in it. at the end of a day of use, i put the battery charger on it, and see that it charges hard at about 7-6-5 amps for a few hours. so i figure the alternator is not charging, and sure enough, its down to about 11 volts. i charge it back up and get about 12.2. start the engine, and the battery still measures 12.2. its not charging. alt belt ok, so i check the wiring.

alternator has 4 wires. 1 big output to battery. 1 ground, and 2 smaller control wires. merc service manual shows several different wiring diagrams, but labels those 2 control circuits on all of them as "sensor circuit- red wire" and "excitor circuit- white wire". it has an internal regulator.

i find the white excitor circuit to be unplugged at the alternator by the previous owner from about 15 years ago. so i plug it in and start the engine. measure volts at the battery, and find 14.2. ok. now its charging and i'm a genius. so i'm looking at it and notice an electrical smell... uh-oh. i let it run for a minute or less. alternator body is getting very hot, more smell, now smoke. shut it down. maybe i'm not a genius.

i remove alternator and go up to autozone. look up '79 chevy c10 pickup w/straight 6 cyl. that alternator is identical to mine, except for the flameproof screens and marine sticker. i buy it, take it home, and install it. i connect the big output, the ground, and the red sensor circuit. i don't connect the white wire (excitor). start it up, 14.2 volts output. fine. shut it off.

i connect the white excitor wire. start it up, 14.2 volts output. not quite fine. engine idling sluggish, as if the alternator is applying load to the engine, trying to stall it. something is not right.

turn off key - uh oh, engine keeps running. disconnect battery - still running. had to yank the coil wire to kill the engine.

disconnect white excitor wire. start it up. 14.2 output. no load on engine. ignition switch operates properly. so i leave it this way.

i study the merc wiring diagrams a little more, and it shows the white excitor wire being connected to the coil, and from there to the ignition sw. so when its carrying 12 volts, then that explains why the ignition sw wouldn't shut off the engine, as the excitor lead was providing power to coil. so i sort of understand that.

question - what the hell is the white excitor circuit, and when is it used/not used ?
and why would it be applying braking load to the engine ?

 

i was thinking the same ? before i read yours at the end of the post. maybe its there to excite you when you try to figure out what its for. everyone need a little excitement in their life.

i just had a thought, maybe its so you can disconnect your battery and use it for the trolling motor and have the engine running off the alternator or something. i dunno.

 

http://forums.iboats.com/

click around on this site, or become a member and post your question there. i was reading this site for too long (i have to get some sleep because i have to work tonight) and the one thing i saw that may apply is the exciter stud may be grounded on your motor.

 

Asked my best buddy (guy I boght my RAM from) who is a Mercruiser/Volvo-Penta authorized repair station. He doesn't like diagnosing stuff without seeing it, but he replied with this:
"Somebody "F'd" with the wiring at some point or there is a short somewhere from a previous issue.
I would put the proper Merc alt on - check all wiring and retest (the automotive alt is unsafe for a marine application."

 

on that boat forum i saw where the guys were saying bad things about automotive alternators on boats. something about spark arrest and explosions.

 

yeah. the marine dutys have a spark arresting screen on the front and rear to contain brush sparking. due to the enclosed engine compartments, there is risk of gasoline vapors and any sparking can make it go boom.

i appreciate the concern, and its wrong for me to do it, but i did it anyway. as a precaution, i always open the cover before and when starting. i'm sort of paranoid out on the water.

i've found out that napa sells a marine duty fits-all alternator. one for single-wire system and one for 3 wire. they also sell an automotive like the one i got, plus they offer a marine adapter kit that provides the flameproof screens.

the good news, is it ran all day sunday and came back with a fully charged battery (and didn't go boom). the bad news is they broke every ski rope we had.

 

good to hear that its working right. did you ever find out what the exciter thing is for? you have my curiosity peaked. when i searched for it i was seeing a bunch of references to generators and turbines. then when i narrowed my search i was seeing that its a dc to ac conversion deal. i'm so confused. its bugging me that i can't find exactly what it is for on a boat. i'm going to have to go to mercury's site and email them or something, this is going to drive me crazy not knowing.