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  • 3 wire generators

    the field wire, specifically. 9v at run, 0v at off?

    if you want the explanation behind the question, go to my PF2 thread.... but there's the question - how much of a difference in voltage is optimal?
    Doing it all wrong since 1966

  • #2
    are you talking about a 10SI alternator?

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    • #3
      Originally posted by squirrel View Post
      are you talking about a 10SI alternator?
      yep
      Doing it all wrong since 1966

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      • #4
        do you have a post number or something, so I could find the reference? or just a quick explanation of why you're asking?

        I think you'll find that there is no "optimal voltage" on the wire on the alternator...it wants to connect to power through some type of resistor (an idiot light is one type), and the voltage at the terminal will depend on whether or not the alternator is making juice.
        My fabulous web page

        "If it don't go, chrome it!" --Stroker McGurk

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        • #5
          Originally posted by squirrel View Post
          do you have a post number or something, so I could find the reference? or just a quick explanation of why you're asking?

          I think you'll find that there is no "optimal voltage" on the wire on the alternator...it wants to connect to power through some type of resistor (an idiot light is one type), and the voltage at the terminal will depend on whether or not the alternator is making juice.
          I think the confirmation I'm looking for is that there isn't any resister in the brown wire if I'm getting 12v. Stupid me presumed that GM had a resistor in line.... apparently not. ah well. It does explain the failing alternators.....


          (it's a 76 Corvette)...
          Doing it all wrong since 1966

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          • #6
            I'd look for a brown with white stripe resistor wire in the wiring...somewhere....that's what Chevy generally used, with or without the idiot light. I have a 73 chevy service manual, it shows a resistor in the circuit. I don't have the 76 manual though.

            You will get full voltage at the terminal on the alternator, when the engine is running, because the wire is connected to the stator output (through it's own diode trio). The resistor lets the voltage the the alternator terminal change, depending on the situation--engine off, key on, or engine running, or alternator not working (the key on/engine off or alternator not working are the situations where there would be voltage across the resistor, and the idiot light would be on)

            But that's with the original type of voltage regulator. There's all kinds of weird crap with modern replacement alternators.

            My fabulous web page

            "If it don't go, chrome it!" --Stroker McGurk

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            • #7
              I pulled the plug out of the alternator to test voltages. The brown (field wire) should be less then the 12.18 the rest of the system is seeing if the resistor is in place (I can't imagine how it could fail to full voltage under what I said above (9v engine off, 12v engine on - not running).... On my FJ40, having the 3 wire system in place is good practice because it spends a great deal of time idling with a load (e.g. winching) which has the effect of collapsing the 1-wire field. You realize the problem when the motor finally dies because it doesn't have enough power to operate and now you can't start it because it's dead dead... but on the Corvette, it's different problems, so I'm going to simply cap and abandon the 3 wire set up for a one-wire alternator.... and have 3, new, warranty 10si on my shelf (which I'll use for sure).... heck, one will probably end up on the 50 Buick
              Doing it all wrong since 1966

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              • #8
                wait...you disconnected the plug, turned on the ignition, and measured the voltage at the connector, and you expect it to be less than the system voltage? It doesn't work like that...you have to have a load on the resistor, for it to be able to drop voltage. Your volt meter has a very high resistance, so it will not drop the voltage--you'll see system voltage there, even with the resistor.


                I dont know how much you understand about electric circuits...but if you don't understand why this is so, maybe read up on Kirchhoff's Law?

                My fabulous web page

                "If it don't go, chrome it!" --Stroker McGurk

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                • #9
                  Originally posted by squirrel View Post
                  wait...you disconnected the plug, turned on the ignition, and measured the voltage at the connector, and you expect it to be less than the system voltage? It doesn't work like that...you have to have a load on the resistor, for it to be able to drop voltage. Your volt meter has a very high resistance, so it will not drop the voltage--you'll see system voltage there, even with the resistor.


                  I dont know how much you understand about electric circuits...but if you don't understand why this is so, maybe read up on Kirchhoff's Law?
                  what I didn't expect was 9v at off.
                  but seriously, who told you I knew anything about this? seriously? who?

                  let's back up. I've now had 3 alternators, 2 from NAPA, one from DB Electronics that died on this car. it makes no sense to me why - sure, my luck with much isn't the best, but 3?

                  now I'll try a one-wire and hopefully that will be the end of the story except that I'm trying to get a grasp on something I never enjoy working with.... readings from your suggestion.... battery disconnected, 3.0 ohm with the ignition off and 13.0 with it on. I suppose I expect some level of resistance at off....
                  Last edited by SuperBuickGuy; July 12, 2021, 07:03 PM.
                  Doing it all wrong since 1966

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                  • #10
                    well, I know what killed this one, a short. whether it was a current draw that was so large it melted the insulation, or the insulation failed... it shorted....
                    Doing it all wrong since 1966

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                    • #11
                      I don't have much trouble with alternators. But I also don't put things in my cars that suck a lot of current. I also replace batteries before they die, when they get to be 5 years old.

                      I don't know what the answer is for your car....I doubt it's that it needs a 1 wire alternator instead of a 3 wire, though.


                      My fabulous web page

                      "If it don't go, chrome it!" --Stroker McGurk

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                      • #12
                        Originally posted by squirrel View Post
                        I don't have much trouble with alternators. But I also don't put things in my cars that suck a lot of current. I also replace batteries before they die, when they get to be 5 years old.

                        I don't know what the answer is for your car....I doubt it's that it needs a 1 wire alternator instead of a 3 wire, though.

                        it bugs me to 'give up' and go one-wire, but I can't keep throwing parts at it and hope it works. it works fine again, and now a new noise to diagnose (which I thought was the alternator)... it never ends.
                        Doing it all wrong since 1966

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