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Alternator upgrade, 1-wire or 3-wire?

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  • Alternator upgrade, 1-wire or 3-wire?

    As posted in a previous thread I fried my externally regulated alternator on my '68 Mercruiser 120. I had some wires with worn insulation in the alternator circuit that shorted when I hooked up the battery. I'm taking this unfortunate opportunity to upgrade to an internally regulated alternator. I have studied the wiring diagrams of the current setup as well as the diagrams for the later one wire and three wire setups.

    In the future I would like to use the boat for trolling more at low speeds. My concern is that the one-wire alternator, from what I've read, doesn't charge below 1000 or 1500 rpm depending on the source. Would I be better to install the three wire alternator?

    The wiring changes appear to be simple enough. The white lead (excitation) goes to the positive terminal on the coil, the red lead (sensing) goes to the positive lead on the starter slave solenoid, and the output lead goes to the positive side on the ammeter. I have an ammeter on the dash that I'll need to upgrade because it's a 50 amp scale. I believe I'll need to upgrade the guage of the wiring to and from the ammeter if I go from the current alternator which I believe is a 42 amp to the 63 amp unit.

    I also will need to add a circuit breaker between the starter solenoid and slave solenoid according to the wiring diagram in the 1974 Mercruiser manual. Since I doubt the alternator will ever be outputting the full 63 amps would a 60 amp breaker be appropriate or should it be lower that that?

    Thoughts? Thanks in advance for your input.

    I did a bad crop of the image for the external regulated alternator. It says at the top it is relay regulated. The diagram is the transistor regulator circuit which is what I have. On the page the description for the diagram was to the left below a lot of text not needed for this discussion.



    Last edited by adamjr; April 24th, 2013, 08:31 AM. Reason: content and spelling

  • #2
    Re: Alternator upgrade, 1-wire or 3-wire?

    The white lead (excitation) goes to the positive terminal on the coil,
    Just make sure you don't have a resistor wire or ballast resistor before the coil, or it will over charge your batteries big time.
    Don S.


    Please, no PM's (Private Messages) regarding boat/engine problems.
    That is what the forums are for.
    Only forum/moderator issues will be answered in PM's.

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    • #3
      Re: Alternator upgrade, 1-wire or 3-wire?

      Originally posted by Don S View Post
      Just make sure you don't have a resistor wire or ballast resistor before the coil, or it will over charge your batteries big time.
      Thanks Don S for your input. There is a ballast resistor before the coil positive. I was going to hook it up on a separate wire directly to the positive lead of the coil as opposed to on the resistor side so there won't be a resistor in series with the excitation lead and the coil positive. That is the correct way, isn't it? I just want to make sure I'm not missing something that will cause me another expensive or, more importantly, dangerous failure.

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      • #4
        Re: Alternator upgrade, 1-wire or 3-wire?

        I looked at the diagram again and I think I see what you mean. The circuit shows a lead going to the ammeter, then the ignition switch then to the coil via a resistance wire. So if I tapped the coil at the positive terminal then there would be resistance and over-charging. Looking at the complete wiring diagram from the Manual 1 (figure 6) it does not show a ballast resistor or a resistance wire although I know there are two ballast resistors on the mount for the harness connection terminal block. If this is the case and there is a ballast resistor to the positive of the oil (my recollection of point systems tells me there should be) where would be a better place to connect the excitation lead?

        Thanks in advance for clarification. I apppreciate your advice greatly.

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        • #5
          Re: Alternator upgrade, 1-wire or 3-wire?

          once a 1 wire is excited and charging ,it continues to provide output untill the motor is stopped. They need about 1000-1500 rpm to excite and provide output.So ,if you did a restart your voltmeter or ammeter would show no charge untill the rpm came up.
          NO PERSONAL QUESTIONS, THIS IS WHAT THE FORUM IS FOR.

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          • #6
            Re: Alternator upgrade, 1-wire or 3-wire?

            So Doc, what you're saying and what I missed is that in simple terms that the exciter is essentially like an on switch. Once it's on, it's on. So all I have to do to make it charge is to get over the excitation threshold once and then I'm golden until I turn off the key regardless if I drop the revs to idle (about 600) to troll? That might be the better and simpler way to go then. Is there an advantage to one over the other making it a better choice in most situations when upgrading?

            Thanks again to you both. I'm going to order up a marine unit hopefully by the end of the day so I can do the install next weekend and hopefully be on the water the following weekend.

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            • #7
              Re: Alternator upgrade, 1-wire or 3-wire?

              If you have an actual ballast resistor and not a resistor wire, just hook your excite wire on the white wire going TO the ballast resistor. Not on the side of the resistor going to the distributor.

              Your new wire in purple.

              Click image for larger version

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              Don S.


              Please, no PM's (Private Messages) regarding boat/engine problems.
              That is what the forums are for.
              Only forum/moderator issues will be answered in PM's.

              Comment


              • #8
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                Re: Alternator upgrade, 1-wire or 3-wire?

                Thanks Don, that was exactly what I didn't understand and makes plenty of sense. It also makes the wiring a bit more tidy which is something I'm a little of a stickler about. Nice, neat, and clean. I've had to clean up rats nests in every boat I've ever bought. The previous owner grounded a lot of things to the aluminum hull. That took some time to trace and correct. Might that explain the amount of pitting on my outdrive that has only seen fresh water? That in conjunction with the fact that the P/O painted over the anode.

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