'68 Mercruiser 120 smoking alternator. Details inside.

adamjr

Petty Officer 1st Class
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Jul 23, 2009
Messages
272
I was not sure if this should have gone in the electrical forum. I looked at that forum first and all of the threads I saw were accessory related. If that is where this belongs I'd be happy if it was moved by the mods. Thanks in advance.

This morning I got up with the intention of starting the boat for the first time this season. I did all the preliminary cleanup stuff like getting the winter cover off and cleaning out any leaves I missed in the fall before the cover went on. I got the battery out of the basement and went to hook it up. I hooked up the positive cables first and tightened the hex nut down. Then when I went to attach the ground cables I got sparks. I thought that was mighty strange since the keys were in my pocket and I verified at the panel that everything was off. I snugged the hex nut down anyway and went to look over a few things. Possibly a minute or two at the most had elapsed and I saw smoke coming from the port side of the motor. I looked closer and saw smoke coming from the back side of the alternator. I grabbed the wrench and immediately removed all of the cables.

I have the original charging system still in the boat with the remote mounted regulator. I am guessing one or the other is likely toast now. I'm broke right now being that I am back to work for a little over two months now following three and a half months off following shoulder surgery. This was not when I expected to do the 1-wire alternator upgrade. Even if I do I am not sure exactly where to start to troubleshoot where the short is that likely caused said toasting to avoid toasting an even more expensive set up.

I do have the Mercruiser manual that covers the year of my setup but I'm curious if someone has had the same experience that can lead me in the direction of what they found. I'm about to get on the road for a 220 mile ride for a deposition tomorrow morning and then I'm heading straight home so I appreciate any help. I apologize in advance that I won't likely be able to respond until tomorrow late afternoon at the earliest.

Thanks in advance.

Adam
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,344
Re: '68 Mercruiser 120 smoking alternator. Details inside.

you installed the battery backwards and destroyed the alt,electronic ignition components (if equipped), and any other electronic devices.
 

adamjr

Petty Officer 1st Class
Joined
Jul 23, 2009
Messages
272
Re: '68 Mercruiser 120 smoking alternator. Details inside.

Doc, Thanks for your reply but no. I'll be 45 in August. I've been wrenching since I was 8 when my dad owned an auto parts store. The first thing anyone learns is the difference between the positive and negative terminals. Red is positive and black negative. Now that that is clear at the end of last season I had the boat starting, running, charging, and shutting off perfectly. When I winterized the boat I removed the cables first from the negative side and ran a zip tie through the eyelets. Then I removed the cables from the positive side and also ran a zip tie through all of the wires. I did not reconnect them in reverse. First black to negative and then all red to positive. They went back on exactly as they came off when I had the motor running and charging perfectly. The harness is original 1968 and possibly has a short but no, I absolutely did not reconnect it in reverse. I double checked that (which would have been obvious to me even as an eight year old) when I disconnected the negative side first.
 

adamjr

Petty Officer 1st Class
Joined
Jul 23, 2009
Messages
272
Re: '68 Mercruiser 120 smoking alternator. Details inside.

But being that it's late the only thing I did reverse is the order in my last comment. Connected positive first and then negative, not the other way around. Been up way too long after barely sleeping Saturday night. If I am coming off snippy I don't mean to. Very tired and going to bed.
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,344
Re: '68 Mercruiser 120 smoking alternator. Details inside.

2 ways, either the alt took a dump over the winter and the diodes were back fed enough power to start it smoking or the batt cables were reversed. no other way that I know to burn it up like that.
just tell us witch way the motor turns hooked up the way you did earlier. If CCW ,batt cable are reversed
 
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