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Thread: Alternator charging issue

  1. #1
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    Alternator charging issue

    Greetings gents, Fairly recently I replaced the alternator on my 1955 Willyís utility truck with the I6. The old alternator was whining when I got the truck. Iíve noticed that this new one doesnít seem to be charging the battery at all. When I try to test it with a DMM it something interferes with my multimeter when I get close to the alternator. Even with the probes unplugged I get a reading that jumps all over the place so Iíve been unable to test it. Could this be another bad alternator or a bad ground wire somewhere? This is my first project vehicle so Iím pretty new but Iíve always been interested in restoring an old jeep. Any suggestions are appreciated!

  2. #2
    Super Moderator bmorgil's Avatar
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    Calling LarrBeard..... Hopefully Larry will see this and give an explanation. I know he dealt with a similar "random voltage" issue a while back. I'll search for it.

    You may want to make sure the engine and alternator are well grounded.

  3. #3
    Super Moderator LarrBeard's Avatar
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    " When I try to test it with a DMM it something interferes with my multimeter"

    Old Jeep trucks are electrically very noisy - think about the noise you hear in an AM radio. Modern digital multimeters are very high impedance devices - that means they are very good at picking up all of the noise around an old truck. Just wave the leads around the coil and watch the meter go nuts. My first suggestion is to go find an analog meter that will not go crazy over noise. On the '48 I can't even get close to it with a modern meter - it takes the old Simpson 260 I learned with back in '62 or so.

    https://www.amazon.com/Gardner-Bende...Q%3D%3D&sr=8-5

    What was the old alternator and what is the new one? If there was already an alternator on the truck, the replacement should (??!!!) have been pretty simple - but life doesn't work that way.

    Once we know something about the alternators, we can start guessing -- I mean analyzing.
    Last edited by LarrBeard; 10-04-2022 at 07:56 AM.

  4. #4
    Super Moderator bmorgil's Avatar
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    Man that was fast!

  5. #5
    Super Moderator LarrBeard's Avatar
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    We're not from the guvmint - we're really here to hep' you.

    Just pure blind luck overcoming skill, cunning and forethought once again.

    I'm hoping he has a "one wire" alternator like BigFishDave had earlier this year. He might not be spinning it fast enough to get it 'ginning the first time.

  6. #6
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    I’m trying to figure out how to post a picture of the alternator

  7. #7
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    E794A0E9-1B8B-4337-8F33-040136049BAE.jpgE794A0E9-1B8B-4337-8F33-040136049BAE.jpg

    Here’s a picture of the back of it. It’s a pretty old one. As far as getting an analogue multimeter I’ll have to look around online. Thanks for the advice!

  8. #8
    Super Moderator LarrBeard's Avatar
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    Alternator Won't Alternate!

    The alternator is a "one-wire" alternator. It requires only one wire to make it work, the BAT terminal you can see in the picture. Originally that wire was usually a Blue/white tracer wire. At the old voltage regulator, it should connect to the red wire that was on the ARM terminal of the old regulator. I recommend putting a fuse in the line if there isn't one already.

    The old Red wire connected to the BAT terminal of the circuit breaker, which is part of the light switch assembly. That breaker is old and it is slow and after all of these years, it might not keep the magic smoke in things if something goes wrong. The BAT terminal of the breaker goes to the + terminal of the ammeter.

    The alternator is self-excited. It uses the residual magnetism in the pole pieces to start generating current as the engine comes up to speed. Once the alternator reaches a preset voltage, the internal regulator switches in and connects the alternator to the battery. It is a simple circuit that works very well 99+% of the time.

    Some folks have an issue getting the alternator to start up the first time because of low residual magnetism. This usually happens when the generator to alternator conversion is first made and the alternator is running at too low a speed for given engine RPM's. Changing to a smaller pulley usually fixes this, but since an older alternator was charging OK, that probably isn't your issue.

    To get it charging the first time, you may have to run engine speed up to 2000 RPM or so - no need to red-line it though. Make sure you have a good, solid ground. I don't trust mounting bolts. Somewhere on the alternator case there should be a threaded stud or a place to put a good size screw. Run a #10 or so wire from a terminal on that stud to a good, solid clean ground on the frame - the battery negative connection to the starter housing is a good place if you can get to it.

    This is a first cut at a solution – let us know what you find out! And show us pictures of the whole truck – We like Old Jeep Trucks!
    Attached Images Attached Images
    Last edited by LarrBeard; 10-06-2022 at 06:23 AM. Reason: "one wire" ... not "wire wire"

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