7.4L Mercruiser battery drain - Alternator Loom??? any help gratefully appreciated!

Discussion in 'Electronics Q&A' started by Imuck, Nov 2, 2013.

  1. Imuck

    Imuck New Member

    Nov 2, 2013
    454gm carb merc
    Hi guys!
    Hoping you can help diagnose the drain on house battery (0.8A) when everything was switched off.

    Everything functions normally (which is hot wired to the battery - such as bilge/mercathode etc).

    I came across a similar thread which suspected the Alternator, so I tested the wire loom (with everything switched off).

    I found a 0.4VDC voltage from the battery output and sense wire, with a closed circuit between the two. (Five wire Mando alternator).

    When the Alternator is running, it sits at 15.8VDC without fail and slips the belt a bit - indicating it's under high load.

    On a random day, the activation of the trim motor killed all the voltage to the ignition and the engine died while the trim switch was pressed.

    Any ideas would be greatly appreciated... the Owners supplement wiring diagram doesn't have the alternator on it, just the starter motor.

    the only other thing that could be wrong could be the current is getting past a (faulty) battery switch.....

    1997 Searay 250DA, 7.4L Mercruiser Carb TB5 ignition serial 0f809867

    Thanks in advance!
  2. Saverio

    Saverio Active Member

    Aug 31, 2010
    Long island new york
    1995 330 DA
    Twin 454 Webber Carbs
    V drives
    Fresh water cooled
    Re: 7.4L Mercruiser battery drain - Alternator Loom??? any help gratefully appreciate

    Hi did you check all your connection for corrosion, even behind the battery selector switch? It seems like you have a bad connection somewhere. 15.8 volts from you alt all seems pretty high? I had an issue a few years back where I was only getting 7 volts to the coil and engine would stall. It wound up being the harness plug on the engine was lose and corroded. The best way to check for a drain is put a test light in series with battery cable and battery if it lights up you have a drain. Start pulling fuses until light goes off. like you stated you will have a draw on the mercothode I'm not sure how much?

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