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1979 mercury 7.5 HP stator voltage

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  • 1979 mercury 7.5 HP stator voltage

    I have a 1979 Mercury 7.5 HP and wanted to know if it's possible to check the voltage of the stator output at the rectifier. what should the voltage be? all the reference in the manual just gives you oms readings of it. I also wanted to know if you can check the switch box voltage output to the coils. what should the voltage be for switch box output? I have a DVA multimeter and have to check newer outboards just don't know the voltages of a 7.5 hp or if it can be done on older outboards electronics systems. thanks for the input! JJB

  • #2
    The manual refers to current testing of the stator with 0 shown for 1k rpms and for the 9 amp stator, 6 amps at 2k rpm and 10 amps at 5k rpm. Since the resistance of the stator is constant with a slight increase due to a temperature rise as current is drawn from such, Ohms law should give you your answer.

    Sounds easy, but the manual shows the resistance for the 9 amp stator at "0.1 to 0.5 ohms measured on the 1 ohm scale" with no temp specific. Cheap DMMS have 200 ohms for the lowest scale and read to the 0.1 Ohm so you would show a short. More expensive meters may have lower scales but I buy the cheap ones which suffice for my needs. Special note in the manual is that it will be less than 1.5 ohms and might read 0 as would be the case with a 200 Ohm min. scale. Course you have to zero your meter leads and remove that from your reading. On my cheapie digital multimeter the leads zero value is 0.4 ohms.

    Today's digital meters measure "True RMS voltage" on the AC scale and DC voltages on the respective scales. DC voltages and True RMS readings are considered equal in calculating power dissipation. Therefore if you are to get 14.5 volts at max rpms out of a FW rectifier with regulator (assuming the battery is up to charge at the time), and 16 or so volts out of a rectifier with no regulator, you have 2 diode drops in the FW bridge rectifier so you have to have another couple of volts for their drop at max current so the AC voltage would theoretically be around 18 volts.

    With that said, if I were to put my digital multimeter on the 20V AC scale and measure across the yellow leads going to the rectifier at full rated rpms I'd expect to see something on the order of 18V. At lower speeds the voltage would be less.
    If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

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    • #3
      Hey Texasmark . great info and thank you .. what about the switch box voltage to coil , what should the voltage be ? JJB

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      • #4
        This requires a more complicated answer. You need to get a manual for your engine to determine your trigger resistances and need a DVA adapter for your DMM to measure your trigger voltages for your engine. Triggers are very narrow, voltages in the 100s of volts, spikes occurring at the rate of the number of cylinders of the engine X the engine rpms. My manuals are for 75 thru 125 HP 2 stroke engines . Since the numbers in my manuals are different for the different HP engines, I assume that for your engine your numbers will be different from what I could tell you.

        Since the triggers are thin and the rest period is 1000X or so, you need something to flatten out the trigger so that the DMM can read it. That is what the DVA does. It captures the trigger through a diode, stores it in a capacitor (condenser type thing) considering 100's required to get the capacitor charged to the peak voltage, taking several seconds of running to get charged up, and a large resistor included to allow the capacitor to bleed off the charge when unused, or input signal amplitude changes.

        Expect voltages in the 150-350 volt range, a function of where measured and engine rpms. Then there is the switchbox bias voltage measured at less than 50 volts.
        If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

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        • #5
          Thanks, texas mark for the great info .. I have a Clymer manual not a shop manual for this and it only lists oms readings for it. I have a DVA meter and have check my 175 johnson before and it had around 220v at the switch boxes coil outputs at 2000 rpm. I guess since I don't have actual voltage values for switch box I can look for the same voltages for both coils outputs would give me a good idea if it's putting out equal to both coils . Thanks for the input .. JJB

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          • #6
            Sign up today
            Sounds like a plan.
            If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

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