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(39)98 Mercruiser Mag MPI Bravo 3 Alternator

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  • (39)98 Mercruiser Mag MPI Bravo 3 Alternator

    This is a follow up from my previous thread...this boat has had multiple issues at the same time and I am sorting them out. Once again, I need some experience and expertise I have found here in the past.


    Boat is new to me. Had bad batteries and bad alternator, but hooked a new battery to it when launched and it ran great.

    Changed the alternator and had an intermittent stalling issue, and it was low on power. Threw codes 42 ( bad ignition Control Module) and 43 (knock sensor reading constant knock).

    Disconnected the alternator and the stalling problem went away but still low on power. I changed the knock sensor . I narrowed down problems with the ignition switch and changed it out. Reconnected the alternator and Took it out for a run and it ran smooth and fast. I was pretty happy, until it suddenly died with no warning.

    This time it would not start, and tripped the helm circuit breaker (15amps?).

    If I turn on the key, the guages engage, but and way down...volt meter reads 10 volts, (new batteries fully charged) gas guage is barely above empty (has half tank of fuel) and the engine will crank but not start. If I leave the key on for more than 10 seconds or so the helm circuit breaker will trip, and the ignition wire at the ignition switch gets VERY hot.

    So I am thinking a dead short. The problem is that intermittently I turn on the key and all is normal. Hit the starter and it fires and starts for a few seconds than all goes bad again. I went ahead and changed the ignition module, no change. I am checking every connection I can find, no change. I disconnected the new alternator, and poof! All is perfect again. Runs perfect. Seems the alternator is the root of many problems, but it's new (reman) from a reputable marine parts house.

    So I am thinking short in the ignition (purple) circuit, but only from the key to the alternator. Not the ignition, fuel pump, etc...all work perfect with out the alternator, and all other accessories on the boat work fine, windlass, cabin lights, etc...

    I am not an electrician, but as I read it, the alternator is the last stop for the purple "exciting" wire circuit. If there was a short anywhere else, it would show up even with the alternator disconnected, no? So I pulled the alternator to have it tested and because of the unique plugs used in marine alternators, no one knows how to test it. I know the current output is good, but there must be a short in the excitation circuit inside the alternator.

    Is there anything I am missing? I am hoping to replace the alternator under warranty, but is there any other possibilities?


  • #2
    I should add that during testing, ,I unplugged the purple Exciting wire from the alternator, leaving all others connections, and the problem went away when I turned the key on Reconnected it and the problem returned. The purple exciting wire at the alternator is the only connection to make a difference.



    • #3
      no ideas? I was on this site ALOT about 5 years ago, now back, and always got some real expert advice and feedback.

      Any suggestions?



      • #4
        Only use good batteries as bad batteries ca toast an alternator in minutes with exactly the 'hot' lead issue you mentioned. You may have damaged the alternator. Remove the alternator. Is the end of the purple wire smoked? Your boat has an internal regulator in the alternator. The regulator usually goes in this situation trying to push to much voltage to the bad batteries. Hopefully the windings are ok and can be saved. Low voltage 10V also will cause fuel system to fail.
        cglazier - "Fiftybucks"
        1995 2855 7.4 bravo II


        • #5
          Turns out the brand new OEM Mercruiser (and expensive) alternator was bad out of the box. Btw...I had replaced all the batteries and a new charger had them at tip top. I replaced the new alternator with a new one and all is good in the world!


          • #6
            How did you figure this out? Did you have the new (bad) alternator tested or disassembled to find a short internally? Or did you just install another one and the problem went away?
            Edmonds, WA
            "THE FIX" '93 2556
            Carbureted 383 Vortec-Bravo II
            The Rebuild Of My 2556
            My Misc. Projects


            • #7
              I had it tested by a local shop that specialises in rebuilding alternators and starters, usually commercial truck stuff. These guys know their business and I explained ed what easy going on. He immediately said it had to be the alternator, tested it and said the likely culprit was a short in the voltage regulator that was intermittent until it finally shorted out the we unit, causing a dead short to the helm and ignition, and everything else.

              No, we didn't do a full forensic autopsy, but since there really is no other explanation, and an expert analysing and testing the unit, I am happy with the explanation, and a new alternator from another supplier fixed everything.

              Actually a reply to my previous post on this topic gave me the clue...I had said the only thing I changed on the engine was the alternator, and the reply was that if that's the only change, that must be the culprit. I disconnected the alternator and everything ran perfect, again pointing to the alternator as the problem. That kept the alternator as a suspect through it all.



              • #8
                Do y’all happen to know where I can find out what alternator I need for a 2001 MerCruiser 3.0L v6 135 HP? (Capri)