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Thread: 78 mercury 800 tach problem

  1. #1
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    Default 78 mercury 800 tach problem

    Hi guys. 1978 mercury 800 with jet. Having an issue with my tach that just started. Last week I was out on knik and the tach was reading 1000-1500 rpm high. The last time I went out it was stuck bouncing between 1800-2500 regardless of engine speed. Quick look over the wiring harness and found nothing obvious. I am kinda new to outboards and hoping someone can help point me in the right direction. Thanks

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    The tack gets it's information from the alternator you may have a problem with the alternator not the tack.

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    Thanks. Wouldn't happen to know what voltages I should see on the leads to the tach would ya? Or what the alternator should be charging the battery at? ...thinking its time to invest in a manual...

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    I have no idea what the voltage would be going to the tack if I had to guess I would say over 13vpdc which is hard to measure with a DVM. The charging voltage 13.8 to over 14v dc.
    I would also measure the ac ripple voltage to see what it is I would think it should be <1vac.

    Lusack library has manual, I would try them first. If they don't have the one you need get thing on mercury.

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    Thanks for the help. Good tip on lousac I would have never thought. I'm going to order up a manual, with the price of new outboards I'll probably have this thing a while... In the mean time I made a little run tank with a plastic 55 and my sled lift. I know I can check the charging at the battery with it running. The ac volts I'm guessing at the alternator? If I pull the flywheel to get a look at things down there do I need to worry about timing? Going to check the charge and look over wiring to the gauge first.

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    [QUOTE=lostinak;1422338] I know I can check the charging at the battery with it running. The ac volts I'm guessing at the alternator? If I pull the flywheel to get a look at things down there do I need to worry about timing? QUOTE]

    If your talking about ac ripple voltage it measure across the battery.

    I have no idea if pulling the flywheel will effect the timing. When I pull anything I always put a mark it so there no question as to it location. I forgot to mention you may have to rev it over 1500 rpm to get it to start charging.

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    Well I got a manual and think I have it narrowed down to a cooked rectifier. Fingers crossed for not having to pull the flywheel to get at the stator. I will update when I get the new part installed.

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