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Merc 160 high voltage problem

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howellm70

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Hi all, I've got a '67 Sabre Craft with a Merc 160 and she's been a pretty bullet proof boat. It's kept up at a lake far from any sort of repair facility so I'm trying to diagnose an electrical problem on my own and I'd really apprecaite any guidance. I'm a decent DIY mechanic but a professional by no means.

I was clued into something not being right when the radio started shutting when we were motoring around. Checked the battery with a volmeter and all checked out. Check the alternator and the voltage read 15.8v and rising when the motor was at 1500 RPM. I figured that meant the alternator was bad so I I just replaced it. Rechecked the voltage under load and got the same reading...so the alternator doesn't seem to be the issue.

Any ideas for what to do or check next?


-Mike
 
if you voltmeter shows a big difference between the alternator output and the battery, I'd suspect some of the original wiring is corroded.....if I'm getting the jist of your post.

If its an original alternator, it should have an external regulator. Since most of that stuff is NLA, most people use a single wire delco replacement....some minor wiring changes make it work.

As far as the troubleshooting goes, on the installed alternator, you need to make sure the proper voltage is maintained and adequate current is produced...in other words, a voltmeter by itself wont give you a definitive answer....if you are seeing 15+ volts at the alternator and the battery, you're gonna cook the battery.
 
Hi @makomark, appreciate your response. I was able to get back up to the boat today and took a couple of other readings. Before first start, battery was reading 12.79 volts. After starting and idling up to 1,500 RPMs, battery reading was 17.4 volts and alternator was reading 19.2 volts.

Obviously that's high. Does that suggest I need a new alternator or would you check something else first? FWIW, the alternator I replaced it with was a used unit so I suppose it could be bad as well.

-Mike
 
Sounds like a pretty big drop in the charging lead....where was the black lead when you made the measurements?

Do you know which alternator you have and is the regulator internal or external - they have a few versions back then????
 
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