1. coalman

    coalman Well-Known Member

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    How much fluctuation should an alternator have at idle? I am going to run it in my car disconnected from both the field and battery post, and check it with my multimeter. Thanks
     
  2. Ricks70Duster340

    Ricks70Duster340 Child of the King, widower FABO Gold Member

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    The only fluctuation I've seen is when using the old style mechanical voltage regulators. They would often cause the charge to pulse, especially if the contacts were burned. Newer VR's don't have that issue. The charge shown on the ammeter should be smooth, but lower at low speeds and increase as the engine RPM's increase (if the battery needs more charging).
     
  3. coalman

    coalman Well-Known Member

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    Thanks for response, I am just trying to see if the alternator should have steady voltage not connected to anything by taking a reading off the battery terminal to ground through the multimeter.
     
  4. 80fbody

    80fbody Well-Known Member

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    Early alternators struggle with voltage at idle. Try one of the newer Nippondenso units.
     
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    • 67Dart273

      67Dart273 FABO Gold Member FABO Gold Member

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      Your "test" is not going to test anything.

      First, disconnecting the field will result in no output from the alternator. Second, even if the field/ regulator is connected, "that is not the way these work."

      The regulator circuit essentially forms a loop with the output circuit of the alternator. The regulator "reads" the output (via the "run" dark blue IGN wire hooked to regulator IGN terminal, which is supposed to be "same as battery") ---and the regulator ramps the field current up or down until the reading at the IGN terminal of the regulator matches the regulator set point. If you disconnect the output circuit, the regulator "will think" the battery is way low, because the voltage will never come up--as it is disconnected. THE REGULATOR WILL ramp up field current AND CAUSE the alternator to go to "full output" which will result in a VERY HIGH overvoltage reading at the alternator output
       
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      • 67Dart273

        67Dart273 FABO Gold Member FABO Gold Member

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        There are only so many causes of "fluctuation" in alternator voltage Some of them are

        A bad or intermittent diode or it's connection
        A bad stator. The windings can come loose and rub against each other causing internal shorting. You can see this at night with revving the engine while watching the alternator. Other than sparking at the brushes, you will see a "micro lightning show" in the stator windings

        Any bad terminal connection in the charge path from the alternator output, through the bulkhead connectior, the ammeter, and back out to the battery.

        it is rare BUT THE WELDED SPLICE under the dash can fail and has in a few cars.

        Poor brush connections, bad condition of the rotor slip rings, grease on / in the brush holders causing sticking, etc

        Especially in the old mechanical regulators, a worn out regulator

        Poor grounding and or "the usual" VOLTAGE DROP in the harness. "The usual" suspects are the bulkhead connector terminals, the ignition switch terminals, and the contacts in the switch itself
         
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        • Dana67Dart

          Dana67Dart The parts you don't add don't cause you no trouble FABO Gold Member

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          What problem are you trying to solve?
           
        • BillGrissom

          BillGrissom Well-Known Member

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          I have a small 10A Black & Decker smart charger which has an "alternator test" function. I think it works as what you are getting at. It looks for excessive ripple on the BATT+ wire, which could indicate a blown diode (of 6) in the alternator output. I think that is why the round-back alternator in my 1965 Newport wasn't charging sufficiently years ago. An easy test is if you measure ~14.3 V at the cigarette lighter with the engine running, your alternator is doing its job. The squareback alternator in my 1982 Dodge Aries (same as 1970's A's) kept melting the 3 positive output diodes every year almost like clockwork. I learned to peer in the rectangular slots and if I saw them missing I knew it was time to replace them again. I suspect they didn't get enough cooling on their floating bar in that transverse engine application. By cutting cooling slots in that bar, I could get them to last 2 years.
           
        • coalman

          coalman Well-Known Member

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          Thanks to all, it puts out a good 14 volts it just fluctuates a lot. I have done everything that has been on this forum for this problem. Every time I start it is new day, smooths out around 1500 to 2000 rpm. I just can't get it fixed...
           
        • Dana67Dart

          Dana67Dart The parts you don't add don't cause you no trouble FABO Gold Member

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        • coalman

          coalman Well-Known Member

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        • Dana67Dart

          Dana67Dart The parts you don't add don't cause you no trouble FABO Gold Member

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          Is your crank pulley stock?
           
        • Dana67Dart

          Dana67Dart The parts you don't add don't cause you no trouble FABO Gold Member

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          Post a video ov what you are seeing.

          Also a photo of your VR
           
        • coalman

          coalman Well-Known Member

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          Don't know it is a built high performance engine, I purchased it that way.
           
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