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  • Alternator Wiring Question

    Maybe this goes in Tech, but here goes........

    On the race truck - The Bosch alternator has just 2 wires - B+ and D+ (switched 12VDC). The folks at the rebuild shop said to be sure to put a light bulb or 10-15 ohm resistor in the D+ line to drop the D+ voltage to below 12 V - I think the stock value is about 9V. Here are my questions:

    1) Would an Ammeter in that line provide more or less the right voltage drop? It will be easy to add that to the dash right now if it'll do the job. I'm thinking a bulb would be ON all the time and that would be distracting.

    2) If I'll need the resistor, how many watts?

    Dan

  • #2
    I don't know how much resistance the amp meter has but off hand I think it would be less then 10-15 ohms. You could measure it. I think what is going on with the lamp and resistor is this. It the alternator is not producing the needed voltage, the alt is basically becoming the ground and the light goes on. As the alternator produces there required voltage to charge, it is more then the D+ with the bulb and reisistor and trying to pump the voltage back to the D+ extinguishing the lamp. You'd need at least a 10 watt resistor. 12v/15ohms=.8amps. Wattage is .8x.8x15=9.6 watts

    To me a voltmeter is the best way but the idiot light does get your attention! The alternator on my 66 Mustang just took a dump. I never noticed it went bad untill it wouldn't star. I never look at it'svoltmeter.
    Tom
    Overdrive is overrated


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    • #3
      Thanks, Tom. I'll look and see if I have an ammeter to measure (duhh - why didn't I think of that?).

      Anyone need a mechanical oil pressure or water temp gage? Yours for the shipping.

      Dan

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      • #4
        it's the same resistor that I used on my H3 to fool the computer to thinking the rear locker is still attached.... 10v 5%
        Doing it all wrong since 1966

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        • #5
          Dan, the ammeter will not function if you hook it to that wire. Nor will it provide sufficient resistance for the D wire.

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          • #6
            Originally posted by DanStokes View Post

            2) If I'll need the resistor, how many watts?

            Dan
            Dan you'll need a 5 watt, 50-100 ohm resistor.
            The Green Machine.
            http://s1.postimg.org/40t9i583j/mytruck.jpg

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            • #7
              Amp meters are apparently bad......AMP gauges at the dash are troublesome.

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