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  1. #31

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    One engine is the problem, the other engine is following ecm commands.... The engines are possibly linked together so when one engine goes down RPM are reduced on both... Which MPI do you have? L29 or the Lingenfelter set up?
    How would I determine which MPI I have?

  2. #32
    Join Date
    Jun 2006
    Location
    Spotswood, NJ, USA
    Posts
    5,142

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    The fact that the ECM`s may be communicating with each other is a sign there joined together and needs to be addressed first

  3. #33

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    First video shows the tachs. Second video shows the voltage gauge at the end. Mind you, I am not touching the throttles.


    https://youtu.be/3f1FmHtMqPc

    https://youtu.be/V2ME9a4iPak

  4. #34
    Join Date
    Jun 2006
    Location
    Spotswood, NJ, USA
    Posts
    5,142

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    says "unavailible, private"

  5. #35

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    Okay try it now. I had it set to "Social Distancing"

  6. #36
    Join Date
    Jun 2006
    Location
    Spotswood, NJ, USA
    Posts
    5,142

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    If the voltage drops when the engines die off it may be from combined electrical system , 1 dead alt, and the other alt getting ready to die. If they were truly independent only 1 motor would be affected not both.
    Like I said earlier test the system to find out beacuse it looks like the same boat I worked on a few years ago and it had the wires combined and drove me nuts
    If you still got my cell, give me a ring
    Last edited by Bt Doctur; 04-30-2020 at 03:50 PM.

  7. #37

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    Quote Originally Posted by Bt Doctur View Post
    If the voltage drops when the engines die off it may be from combined electrical system , 1 dead alt, and the other alt getting ready to die. If they were truly independent only 1 motor would be affected not both.
    Like I said earlier test the system to find out beacuse it looks like the same boat I worked on a few years ago and it had the wires combined and drove me nuts
    If you still got my cell, give me a ring
    Was the boat that drove you nuts in Forked River?

  8. #38
    Join Date
    Jun 2006
    Location
    Spotswood, NJ, USA
    Posts
    5,142

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    No PP, any chance you have the Silent Thunder exaust box on the transom?
    Aside from the motors blowing the 90A fuse on the starter to losing ign power but still able to crank,finding jumpers on the main buss bar under the dash. Lots of hours to get it back to true dual systems. Next was an exaust reversion issue rusting the valves in the heads on both motors .
    Found a fix for that problem , but developed a fix for it. The bad part is once we got it perfect he sold it to go fast person.Moved the battery switches to under the rear seat for hatch motor power access,
    and ran a dedicated hatch motor power lead there also .
    Last edited by Bt Doctur; 04-30-2020 at 05:11 PM.

  9. #39

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    I don't thing it's the same boat. I bought this from a retired cop (Bob) in Forked River. He had it one year and the wife hated it. They were used to more of a cruiser type boat. He said it came from the Lake of the Ozarks.

  10. #40
    Join Date
    Jun 2006
    Location
    Spotswood, NJ, USA
    Posts
    5,142

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    The one I worked on had the 6.2 mpi motors

  11. #41
    Join Date
    Sep 2010
    Location
    Jersey Shore
    Posts
    5,951

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    Drive it over to my Dock by Ocean Gate Yacht basin.. We will figure this issue out in short order.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  12. #42

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    Anyone know what this unmarked box with the black and red wires coming out of it that was wired to the alternator is?Click image for larger version. 

Name:	Black Box.jpg 
Views:	10 
Size:	93.0 KB 
ID:	23362

  13. #43
    Join Date
    Jun 2006
    Location
    Spotswood, NJ, USA
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    5,142

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    need a better pic but most likely a filter , should not affect the alt output.
    Now if it was added because of a alt whine thru the radios or stereo then the alt has a bad diode

  14. #44

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    Here is the update. I had the port side alternator benched tested and it was down 10amps from what it should have been. Replaced that alternator and finally got to sea trial the boat today and it ran like a champ. Thanks all for the help.

  15. #45
    Join Date
    Jun 2006
    Location
    Spotswood, NJ, USA
    Posts
    5,142

    Default Re: Twin Merc 7.4 MPI Electrical Issue?

    To be continued with combined electrical systems!

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