Coil/Alternator Question - 1975 Merc 165

[fourfather]

I was given a boat in bad shape and while fixing it up discovered the alternator was making 17 volts (sometimes more), so I ordered an alternator. The coil on this boat says right on it that it needs a ballast resistor but there isn't one present, so knowing what excess voltage does to points, I filed and reset them (but I didn't buy a ballast resistor yet). Before the alternator arrived I got far enough on the project to give her a test on the water. If I run her wide open for 4-5 minutes the RPMs will fall off sharply and then she'll die. If I wait till she cools off, she'll start back up and run great for 4-5 minutes. Then the process repeats itself. When running wide open the volt meter is buried (I didn't check it with my diagnostic volt meter). BTW, this boat will idle for long periods without any issues.
I should add that this is a 1975 Mercruiser 165 (straight 6 Chevy).

Here's my question: do you think I was overheating the coil with the excess voltage causing the problem described or should I look elsewhere?

Thanks,
Jesse

 

[Don S]

Re: Coil/Alternator Question - 1975 Merc 165

The coil on this boat says right on it that it needs a ballast resistor but there isn't one present,

Mercruiser uses a resistor wire in place of an actual ballast resistor.

With 17+ volts, going to the coil, it may be on it's way out as well as the points and condenser.
Might want to check the battery to. Probably real low on water at this point, if not damaged.

 

[Eric 3.7LX Alpha 1]

Re: Coil/Alternator Question - 1975 Merc 165

Not sure if you have a repair manual for it or not, here is one for you, The original mercruiser service manual#2 for your engine and drive.

http://www.4shared.com/document/3AHpVLYw/Service_Manual_02.html

It's a big file so will take a little bit. Check that manual for wiring diagram for your engine. It's should have a resistance wire powering the coil. (purple) which acts as a resistor and lowers the voltage to the coil except for starting.

 

[eldplanko]

Re: Coil/Alternator Question - 1975 Merc 165

Without attempting to hi-jack the "hijacker's" thread I have the same problem on my....

...let me repharase this.

He's making 17+V out of his alternator. Should he bother figuring out why? Bad voltage regulator? 1-wire or 3-wire alternator?

 

[fourfather]

Re: Coil/Alternator Question - 1975 Merc 165

Thanks for the replies. I replaced the alternator yesterday so I'm back to a normal voltage range. It is a 1 wire. The boat ran several hours with no issues. I am going to check the points again. I only managed 26 mph which seemed a little slow. I was hoping for 35. I am a new boater but realize there are a great many factors that go into that. Do I need to run high test gas in this boat? It does pre detonate a little from time to time.
Now my biggest issue is shifter related (which I addressed in another thread).
Thanks again for your help and the link.
Jesse