Discussion in 'Catalina 309 Group Forum' started by Pompadi, Oct 16, 2016. Add this thread to a FAQ

  1. Pompadi


    Joined Apr 3, 2012
    84 posts, 0 likes
    Catalina 309
    OZ Sydney
    Hi all...
    We Recently had the engine service done, it was mentioned that the tacho was a bit erratic and an electrician had to have a look..

    The electrician tightend some connections and said the alternator is faulty and needs replacing...

    Now the tacho is a bit erratic below 1200 revs but otherwise ok, charging voltage is about 14 volts.

    My question is, If the alternator is charging why would it need replacing ?

    Also does anyone know the output amperage of the factory fitted alternator and if it is faulty, What would be a good size to upgrade to.


  2. Stu Jackson

    Stu Jackson

    Joined Feb 26, 2004
    20,461 posts, 894 likes
    Catalina 34
    224 CA Maple Bay, BC, Canada

    Take the alternator to an automotive repair shop and have it checked out. It would be a good thing for you to learn how to do that anyway, then you won't have to pay an electrician to tighten a few nuts holding a few wires together.

    Replacement alternators are a very large and long topic, starting with how you use your boat and moving onto your entire electrical system. If you hop from marina to marina, then few things need change. We just don't know enough to answer the question.

  3. Hello Below

    Hello Below

    Joined May 7, 2012
    588 posts, 59 likes
    Hunter e33
    CA Maple Bay, BC

  4. Benny17441


    Joined May 24, 2004
    5,741 posts, 415 likes
    CC 30
    US South Florida
    Do not believe a cheap alternator clone replacement will provide the same service over time as the OEM unit. Like Stu suggests take it to an alternator shop for testing and repairs if needed. Don't know what engine you have so I cannot comment on the relationship between the alternator and the tachometer. There are many considerations when sizing an alternator; from the limitations of the battery bank's charge acceptance ratio to the available horsepower required to run the unit. It would largely depend on your particular needs and the way you use your boat. Larger is not always better and an upgrade in size usually accompanies an upgrade of the battery banks.

  5. Pompadi


    Joined Apr 3, 2012
    84 posts, 0 likes
    Catalina 309
    OZ Sydney
    Thanks for replying ,

    Did some testing today with volt meter . One week since the boat was used.

    Before starting the engine AGM Batteries reading 12.78v (100amp hr) and 12.88v (200anp hr)

    Engine on:
    @ idle 14.5vto 15.0v variable , rev Counter unstable
    @ 1000rpm 14.5v
    @ 2000rpm 14.35v
    @ 2500rpm 14.33v

    So the output looks pretty good to me , but 15.0 volts at idle seemed a bit strange , but I've never checked that before, so don't know what to expect . Any thoughts

    The alternator is our only source of charging as we are on a swing mooring, AGM batteries seem to hold charge very well for a few weeks .

    Feed back appreciated..

  6. tahomathundr


    Joined Jun 4, 2009
    85 posts, 2 likes
    Catalina 309
    US Swantown Marina Olympia, WA
    The erratic tach readings on the factory tachs are not uncommon. Mine does it several times a year. I notice it more if it has been raining so moisture in the air could be causing it. I have a pedestal cover with a plastic bag under the cover so rainwater is not getting into the tach. There is an adjustment screw in the back of my tach that seems to cure it for a while but it always returns after a month or two. I doubt your alternator needs replacement but as Stu said have it checked.

    Bob C309 #20

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