Testing a charging system

Electrical & Ignition

  1. Kern Dog

    Kern Dog FBBO Gold Member FBBO Gold Member

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    My Brother in Law's 72 Duster has been at my place since 2007.
    He recently said he wants to use it as a fair weather Daily Driver at his place in Phoenix AZ.
    He likes these cars but has always had me around to guide everything and advise him. Once he takes the car, he will be on his own.
    I'm trying to periodically when time permits, work on this car to ensure that it will be safe and reliable. Today I decided to check the charging system. The gauges are not working, that is on the list of things to do. I suspected that the alternator might not be charging so with the engine running, I removed the negative battery cable. The engine shut down. I should have checked the voltage with my meter but I did not, I just grabbed another alternator from the shed, a nice clean one with a rebuilders tag on it. It showed a 13.9 volt reading at the alternator and the battery was at 12.8. I pulled the negative cable and it shut down again. There is a ground cable from the block to the firewall. Why would the engine still stall if the alternator was putting out the right voltage?
     
  2. 1972GY8SE

    1972GY8SE Well-Known Member

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    1.1v drop between the alternator and the battery.
    Maybe high resistance in the amp gauge?
    533.PNG
     
    Last edited: Jul 3, 2019
  3. Glenwood

    Glenwood Well-Known Member

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    The regulator is probably shot and there may be short or some other load on the system which may explain the variance.
     
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    • dvw

      dvw Well-Known Member

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      That is not how you check. A voltmeter reading of approx 14.0-14.5v is in the ball park. Actual output needs a load applied and a amp reading.
      Doug
       
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      • Derwud

        Derwud Well-Known Member

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        That is a good test for a Generator, not so good for an Alternator, since the take juice to make juice..
        I would throw a new Regulator at it, clean the mount, firewall and regulator for a good ground and move on to the gauges..
         
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        • Crackedback

          Crackedback Well-Known Member

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          The alternator is charging as it's putting out 13.9 volts.

          Sounds like a bulkhead/wiring to the battery issue unless there is a huge amp load in the car itself.
           
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          • Pops1967GTX

            Pops1967GTX FBBO Gold Member FBBO Gold Member

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            • Dennis H

              Dennis H FBBO Gold Member FBBO Gold Member

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              Put ac in it for him so they don’t find him somewhere cooked. Who is going to deliver it?
               
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              • Kern Dog

                Kern Dog FBBO Gold Member FBBO Gold Member

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                HA!
                I'll probably have to tow it out there!
                 
              • moobster

                moobster Well-Known Member

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                Do the same test you tried earlier but this time lift the positive cable off the battery, it should stay running.
                 
              • 1972GY8SE

                1972GY8SE Well-Known Member

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                Your transmission is shot and you need news tires.
                 
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                • 1972GY8SE

                  1972GY8SE Well-Known Member

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                  But I only brought it to you for a oil change.
                   
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                  • 1972GY8SE

                    1972GY8SE Well-Known Member

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                    You're welcome!
                     
                  • Glenwood

                    Glenwood Well-Known Member

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                    Nice catch! I missed that...of course it will shut down when you remove the negative or ground reference. Duh!
                    Even so, the unregulated voltage from the alternator should be higher depending on the rpms.
                     
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