Alternator Amp test

Electrical and Ignition

  1. Darren

    Darren FABO Gold Member FABO Gold Member

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    Hey guys
    I'm doing some work on my 68 barracuda.
    It's a 383 S matching #. I have upgraded to MP electronic ignition and cracked back's headlight relay kit (that I still have to install) I want to leave the original amp meter (good idea ?) I bought a sunpro volt meter that I was going to convert and install in place of the amp meter in cluster but changed my mind.
    Any way my question is..
    Is there a way to test my current alternator to confirm the amp rating?
    Can a alternator repair shop spin it up?
    I want to leave the car as close to original as possible but want it to be safe..
    The car has original radio.
    Thanks for any advice.
     
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    • toolmanmike

      toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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      You need a tester with a inductive amp clamp. (pickup) They are available as a accessory for a quality DVOM (digital volt/ohm meter) or as a stand alone meter like this.

      ammeter.jpg
       
    • 65wgn440

      65wgn440 Well-Known Member

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      Also, yes any alternator repair shop can check amperage output. And if you do keep stock amp gauge, make sure your bulkhead connector is in A1 shape.
       
    • Darren

      Darren FABO Gold Member FABO Gold Member

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      So I assume you clamp around the alt output wire?
      Is there a specific RPM or time (first start up in morning) to do this test?
      Sorry for my electrical ignorance..
      Thanks.
       
    • Darren

      Darren FABO Gold Member FABO Gold Member

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      Yes I actually bought a new bulkhead connector and will be going through it ..
      Thanks
       
    • 67Dart273

      67Dart273 FABO Gold Member FABO Gold Member

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      Well there's more to it than clamping on an ammeter. You need a way to load the battery, the "go to" device back in the day was known as a "carbon pile" load tester. This is an instrument, often containing both a large value ammeter and voltmeter, and an adjustable carbon resistor, huge, 1 each, with a huge knob up front. You can simulate various loads at will as well as test starter draw, and determine battery condition.

      Test procedures are laid out in the service manuals, and you can download many years from MyMopar....free

      I don't recommend this brand--or not--just a photo for reference

      JET-H3655__87736.1619421378.jpg
       
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      • halifaxhops

        halifaxhops It's going to get stupid around here! FABO Gold Member

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        As above a carbon pie tester is the ony way to draw max amps SAFELY
         
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        • mderoy340

          mderoy340 Well-Known Member

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          If the car is running just turn on the high beams, fan, ac compressor, etc and see what the output is at 2k rpm.
           
        • halifaxhops

          halifaxhops It's going to get stupid around here! FABO Gold Member

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          That will just match the load, not full output.
           
        • mderoy340

          mderoy340 Well-Known Member

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          True but it will give the OP an idea of wether or not the alt is operating correctly without taking anything apart.
           
        • 67Dart273

          67Dart273 FABO Gold Member FABO Gold Member

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          Sorry it really doesn't. The headlights and such don't draw all that much in comparison to typical modern day alternators of 50-60 and more output. Many can do that much with a couple open diodes, even though this generates a lot of ripple. It WILL give you a "quick and dirty" idea that the thing is charging somewhat, and that is about all
           
        • toolmanmike

          toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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          That is a carbon pile battery load tester and voltage meter. It has a amp meter for testing battery load but no way to test alternator amperage output. It just measures alternator voltage output.
          upload_2021-6-10_12-10-46.png
           
        • 67Dart273

          67Dart273 FABO Gold Member FABO Gold Member

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          Of course you can use it to test an alternator. You simply use the device to create a battery load. You have to be able to load the system otherwise on a "big" alternator you may have system voltage "to the moon"

          This is the start of the alternator output test from the 1972 Plymouth service manual. Please notice the bottom line.........

          outputtest.jpg
           
          Last edited: Jun 10, 2021
        • halifaxhops

          halifaxhops It's going to get stupid around here! FABO Gold Member

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          stig eating popcorn.gif
           
        • Tooljunkie

          Tooljunkie King of cobble/master of the broken bolt FABO Gold Member

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          Loading an alternator unnecessarily without the proper equipment is a good way to release the magic smoke.
          Have at it.
          I have used the carbon pile test, but i dont go too far. Its not worth the risk.
          In order to load it, you have to pull those amps through the bulkhead connector and ammeter. And all that questionable wiring.
           
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          • Darren

            Darren FABO Gold Member FABO Gold Member

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            Hi guys thanks for all the info.. I don't mind pulling the alt and having it tested if that's still a option..
            Thanks.
             
          • halifaxhops

            halifaxhops It's going to get stupid around here! FABO Gold Member

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            Local parts store should be able to do it. Ask them what it is putting out they might just say "it's good" Napa is usually more informed on the older parts IMO.
             
            Last edited: Jun 11, 2021
          • Darren

            Darren FABO Gold Member FABO Gold Member

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            A old friend of mine works at a local alt/starter repair shop.. maybe he can help..
            Thanks again
             
          • halifaxhops

            halifaxhops It's going to get stupid around here! FABO Gold Member

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            Perfect!
             
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            • Mattax

              Mattax Just the facts, ma'am FABO Gold Member

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              Before you go to the parts store, identify the alternator.
              See if its the original, or possibly the original for the engine.
              Then copy the pages from the shop manual showing the service test.

              This is the only reference I know of relating the 'rating' to an actual performance test.
              If your friend's shop has more specific bench test reference, great. But I wouldn't count on it.
               
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              • my68barracuda

                my68barracuda Well-Known Member

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                The AutoZone store near me does free alternator testing. After the test they provide a print out that shows max alternator output among other items.
                 
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                • Tooljunkie

                  Tooljunkie King of cobble/master of the broken bolt FABO Gold Member

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                  Best place to take it.
                   
                • 67Dart273

                  67Dart273 FABO Gold Member FABO Gold Member

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                  Really? Then what is the risk of driving it that way (as we actually used to do) and burning up the stuff you just mentioned in "normal operation." That is the exact purpose of testing.............to find the shit and get it out of there If the bulkhead connector/ ammeter won't put up with a very short term load test, then maybe they are the problem........which they are..........
                   
                • 67Dart273

                  67Dart273 FABO Gold Member FABO Gold Member

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                  Some store / bench testers are "ok" and some aren't. The BEST way to test a charging system is "in the car" in which it is being used........as I mentioned earlier.......the "sections", the components, the output circuit, and the field circuit. Testing in-car can test the entire system as a whole.

                  Most bench testers don't have the horsepower to fully load a high output alternator. It takes a LOT of power to spin a say, 100a alternator up to full shaft output RPM
                   
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                  • my68barracuda

                    my68barracuda Well-Known Member

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                    Remember the qu
                    Remember the only question the OP had was ‘what is the amp out put of his alternator’. He was not asking about a complete check of the cars electrical system.
                     
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