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Is my alternator working?

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  • Is my alternator working?

    I have a brand new battery. I am not running into any problems so far I just was testing to make sure everything was fine before taking the boat out on the water. When I measure volts across my battery with the engine running I get 12.21 volts. But if I go to the alternator and measure from grnd to the only positive terminal coming off the alternator I get like 10.5. that wire goes to some kind of amp looking device with about 5 bolt down wire fasteners on it. the lead to the battery comes off of that amp and so does a wire going to something that looks like a solenoid mounted to the engine block. if I disconnect the lead going to the battery from the amp the terminal on the amp reads the same as the alternator output I am getting. have let the boat run for up to an hour with no problems. However at that time I was not worried about this and did not measure voltage. I am not sure if this should work on a boat like I think it does on a car but i unhooked the negative cable from my battery and the engine died. I thought if the alternator was working it would keep running. Should I unhook the alternator lead and measure current? or should I take the alternator to autozone and have it tested? I looked up re[placement alternators in a mag I have and the cheapest was $500, is this true of all marine alternators>? Thank You in a Advance. Sorry for the long post


  • #2
    Re: Is my alternator working?

    It would help to know your eng and drive.

    But no your alt. is not working, and never disconnect your batt. from a turning alt.
    '76 Bayliner T1750
    Aq130D, 280
    Work in progress
    http://s301.photobucket.com/albums/n...anx1/bayliner/

    Flybridge restore thread
    http://forums.iboats.com/showthread.php?t=456072

    This one has reevits
    http://forums.iboats.com/showthread.php?t=550380

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    • #3
      Re: Is my alternator working?

      It is a 1972 mercruiser 250, 165 hp with a pre-alpha outdrive

      Comment



      • #4
        Re: Is my alternator working?

        I've heard that alternators can "partially fail" where it just puts out a lower than normal voltage, which I think just means you have to replace the diodes inside and that just costs about 6 bucks or so if you can do it yourself.
        Currently own a 1978 Sea Ray SRV 185, w/ 250 ci 165 hp inline 6.

        Comment



        • #5
          Re: Is my alternator working?

          Remove it from the boat and take it to a shop and have it diagnoised. Usually can get a new one at napa for + or - $150.00
          1984 - S2 Yachts - 23' Tiara Continential - 260 mercrusier - alpha 1 drive

          Comment



          • #6
            Re: Is my alternator working?

            Make sure a replacement is for Marine only. Can't use an car one in a boat. BOOM! ! !
            never mind crying about it, sigpic"Make it Happen"

            Comment



            • #7
              Re: Is my alternator working?

              battery voltage test with engine running should be over 13 volts. but sometimes the rpms have to be above 1000.

              Comment



              • #8
                Re: Is my alternator working?

                There is a simple test.

                Measure your voltage with the key on, but engine not running.

                Then, start the engine and run up to 1500rpm. Do not go higher...there is no need.

                If the voltage goes up, your altenator is charging. If it stays the same or goes down, it is not charging.

                If it's not, then there are two posibilities:

                1) Altenator

                2) Connections

                As you have measured from ground to the altenator and found a low voltage, you may have both.

                With a good connection, and good alt, the voltage at the alt should be the same as the battery

                With a poor battery to alt connection, and a good alt I'd expect to see the altenator voltage high, and the battery voltage low

                With a poor connection and a poor alt I'd expect to see what you are getting. No charge from the alt and a difference in voltage to the battery

                But wait! Some altenators need an external +12V feed and ground to run the regulator. The regulator controlls the field of the altenator, and therfore it's output voltage. This +12V source sometimes comes from the battery/altenator connection.

                So, fixing the connection between the battery and the alt may be all you need to do.

                IF the altenator is dead, the most likely culprit is the brushes. Now and again, the diodes will die, but it's less likely.

                Good luck

                Comment



                • #9
                  Re: Is my alternator working?

                  I reread your post...the solenoid looking thing is just a terminal block.

                  Pull the wires off and retest...I bet there is a bad connection there or at the alt or the battery.

                  Comment



                  • #10
                    Re: Is my alternator working?

                    Your alternator has an external regulator more than likely. By pulling the battery lead while running the engine, you proably killed it if it wasn't dead already. My suggestion is replace your alternator/regulator with a one wire alternator. The regulator is built into the alternator.

                    And NEVER disconnect the battery while the engine is running.
                    1998 Wellcraft Eclipse 24 Cuddy
                    Volvo Penta Duo-Prop 7.4L "LK"

                    2006 Sun Tracker Party Barge 21
                    Mercury 90 4-Stroke FI
                    "Common sense is not very common"
                    "Failing to prepare is preparing to fail." -- John Wooden

                    Comment



                    • #11
                      Re: Is my alternator working?

                      do not use a car alternator on a boat engine!!!!! unless you want to go boom
                      -BAproject
                      ----------------------------------------------------
                      "If it ain't broke then you'll probably have to fix it anyway"
                      ----------------------------------------------------
                      '91 Bayliner Capri
                      4.3L V6 Mercruiser
                      Alpha I Outdrive

                      Comment



                      • #12
                        Re: Is my alternator working?

                        Originally posted by BAproject View Post
                        do not use a car alternator on a boat engine!!!!! unless you want to go boom
                        yeah...that too!
                        1998 Wellcraft Eclipse 24 Cuddy
                        Volvo Penta Duo-Prop 7.4L "LK"

                        2006 Sun Tracker Party Barge 21
                        Mercury 90 4-Stroke FI
                        "Common sense is not very common"
                        "Failing to prepare is preparing to fail." -- John Wooden

                        Comment



                        • #13
                          Re: Is my alternator working?

                          Ok, thanks everyone! What is the amplifier looking device that my alternator lead goes into? And I think I already have a one wire alternator with a regulator built in. At least there is only one hot wire coming from the alternator and it goes to the amp thing. the only other wire on the alternator is the grnd. I also have a small rubber cap on the back of the alternator that says do not remove. Is this a 1 wire setup already? I will try to post a pic but I have never done this on this forum before. Thank you

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                          • #14
                            Re: Is my alternator working?

                            I definately cant figure out how to post pics, Sorry I know that would be a big help.

                            Comment



                            • #15
                              Re: Is my alternator working?

                              Originally posted by Grice7 View Post
                              Ok, thanks everyone! What is the amplifier looking device that my alternator lead goes into? And I think I already have a one wire alternator with a regulator built in. At least there is only one hot wire coming from the alternator and it goes to the amp thing. the only other wire on the alternator is the grnd. I also have a small rubber cap on the back of the alternator that says do not remove. Is this a 1 wire setup already? I will try to post a pic but I have never done this on this forum before. Thank you
                              It could be an isolator or a noise filter.

                              http://forums.iboats.com/showthread.php?t=285637
                              1998 Wellcraft Eclipse 24 Cuddy
                              Volvo Penta Duo-Prop 7.4L "LK"

                              2006 Sun Tracker Party Barge 21
                              Mercury 90 4-Stroke FI
                              "Common sense is not very common"
                              "Failing to prepare is preparing to fail." -- John Wooden

                              Comment


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