Possible alternator short question

Discussion in 'GM Electrical Tech' started by gexter, Sep 27, 2010.

  1. gexter

    gexter New Member

    I looked through the forums and so far all I came up with that the alternator can short the battery.
    I have a 1990 Diesel Burb that keeps killing batteries when it sits for a couple days and I spent way too much time chasing this down.
    I removed the charge wire from the battery and with the key off I get direct continuity from it to ground, I also use a test light from the positive on the Battery to the alternator charge wire and I get full brightness on the test light.
    Does this confirm a bad alternator?
    It also does not show a charge on the dash until I get some engine speed up and then it shows slightly under 13V on the instrument panel. I never tested the alternator directly or at the battery because I was chasing other things.
    I thought if I would just get the answer on the possible alternator short confirmed by what I tested I could end this nightmare.
    Its old and rusty but runs like a top with 420,000 K. If I would have known it would last so long I would have taken better care of it :)

  2. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    Sounds like the voltage regulator may have gone out. Depending on the year of the truck and the model of alternator, it either has an external or internal regulator, I'm guessing internal. Take the alternator and/or alternator on the truck to your local parts store and they should be able to test it for you for free.
  3. gexter

    gexter New Member

    Thanks for the quick response.
    I was thinking that there should not be continuity between the charge wire and the alternator body and if that is correct and then I do not need to take it off.
    I have had the truck parked for several years because my wife develop Chemical sensitivities and she can not a handle the smell. It still is one of the cleanest burning N/A 6.2 I have ever come across but its no blue diesel which my wife still cannot handle.
    She loves it and bought it years ago but can't use it or even be around it. She had to go to the dreaded minivan to get close the mileage capability. I was getting 27 MPG highway with the diesel. 25 highway with the van.
    I am wanting to fix what I can at little or no cost and just sell it and tell whoever buys it and tell them whats wrong with it. Mechancially it is in such good shape it would be a shame to drive it to the wreckers.
    I was using Battery kill switches at the posts for awhile as it seemed to charge if the RPM's were up.
    I have a banks exhaust, hellwig suspension, class three hitch and intake and injector pump upgrades, Somebody could get another hundred thousand miles or more out of it.

    Thanks again. I may have to take it off but I am really leaning to a bad diode I just want to be sure before I tell someone its a bad alternator.

  4. gexter

    gexter New Member

    Update on Alternator short

    The Charge wire that goes to the battery is a direct path to ground with the key off and grounds it which is killing the battery. When I disconnect the plug ( terminal 1 and 2 ) which one wire goes to the Alternator charge wire and the other wire to the ignition, I no longer have a short to ground through the charge wire. I assume a diode is bad in the alternator and allows the alternator to ground when the key is off and the plug is attached.
    With the ignition on 1 and 2 show 12+ volts which it should and the alternator is charging fluctuating between 15.11 volts and 15.55 volts. this variance shows up in interior lights as a flickering. I am not getting a over voltage but I am getting the fluctuation.

    I assume without really understanding anything because electrical stuff is way over my head that the ignition wire should be ground until energized by the ignition switch which in turn energizes the alternator to charge once the engine starts.

    So If the ignition wire is for sure a ground without the key on then that pretty much proves the alternator has a bad diode and there is continuity in both directions.

    Can any one verify that this is correct?

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