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Test of alternator with Ohm Meter

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  • Test of alternator with Ohm Meter

    I can't find the reply someone gave earlier. Question:: Someone said to check from the field to ground on the alternator to see if it is a good alternator,What is the reading I should get? 2.5 ohms sticks in my mind, is this absolute or can it be above or below. This is a mid 50's mopar alternator.

  • #2
    2.5 ohms sounds like it's in the ballpark.

    The ouput terminal should read infinity (or at least many thousands of ohms) with the ohmmeter connected one way, and a very low value connected the other, because of the diodes in there.

    Having said that, I'd suggest you take it in to an auto parts store or auto electric shop that has a test setup. They can actually run the alternator under load, and see what it puts out. If the bearing is getting old, you'll hear it. Normally they don't charge for a test, because out of every "X" number of tests they do, they get to sell an alternator.

    Gord Richmond, within Weasel range of the Alberta Badlands
    Gord Richmond, within Weasel range of the Alberta Badlands


    • #3
      Mine reads 6 ohms. What does that say? Anything? It puts out 14.24 volts at idle.


      • #4
        If it brings the battery voltage up to 14.24 volts there is certainly nothing wrong with the alternator Curt.
        Electrically that is.
        Frank van Doorn
        Omaha, Ne.
        1962 GT Hawk 289 4 speed
        1941 Champion streetrod, R-2 Powered, GM 200-4R trans.
        1952 V-8 232 Commander State "Starliner" hardtop OD


        • #5
          I just went through troubleshooting and rebuild of a 80s vintage ford alternator. The shop manual gave a range of R for the field winding and suggested that the meter be left hooked up while turning the pulley by hand slowly. The value can be expected to fluctuate but not go to either 0 or open. I think the nominal value given for the ford was something like 4 ohms. I don't have the manual handy here to confirm though.

          I would be a little worried about that 14.25v at idle. The fully charged battery voltage is ~13.8v. In product design of things powered from the battery, we usually use 14V as the nominal voltage. Make sure that the voltage does not get higher as the engine is revved up. Your regulator could be bad and overcharge the battery. I had a bad regulator (reason for rooting around the alternator) on this ford I was working on and it would intermittantly put out full field current and cause the alternator to charge at 44A into the battery (I had a clamp-on ammeter on it so I know!). Boiled acid out and made a mess [xx(]. I measured >15V at the battery posts when it was doing that. I had rebuilt the alternator for the heck of it (20yrs old with 162k on it, figured it was due) and then had the overcharging problem 2 days later so I was rechecking my work!

          Jeff in ND

          '53 Champion Hardtop

          Jeff in ND


          • #6
            I put another new regulator on the car. The amp needle is steady. At 500 RPM idle the voltage reads 13.6 at the battery posts. Thanks all for the help. The old style contact-point regulator's cover snaps over the new regulator with zero alteration. I'm glad I saved the old style regulator when I put the other soid state regulator on this spring.