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Chevy Alternator Not Charging

Discussion in 'Shop - Electrical' started by ChromolyKid, May 12, 2010.

  1. ChromolyKid

    ChromolyKid Well-Known Member

    Joined:
    Mar 15, 2009
    Location:
    Phelan, CA
    Just installed a new Chevy 350 in my truck, went to break it in and my voltage gauge was reading 11 volts. Voltmeter on the battery registered 11, and 14.7 at the back of the alternator. Figured it was a bad connection on the frame rail, since it was dirty, so I made a new one and went to drive the truck to an exhaust shop. Volt gauge read 11.5 and dropped to 8 within ten miles, engine died of course.
    Put cables on it, jump started it and it was reading 12.2 at the battery, and at the post on the back of the alternator, so the connection is good between the battery and alternator. I suspect one of 2 things:
    Field wire, we weren't picking up voltage on the field wire. Anybody know where that taps into the junction or fuse block?
    Ground, I believe these alternators are internally grounded, but is there a chance that the powdercoated accessory brackets keep it from grounding to the engine block?

    We took the alternator off and tested it by the way, it passed all 3 tests at Autozone. Any suggestions?

    I'm trying to find info on where the field wire is routed, so I can check for fusible links or loose connections anywhere. Maybe it's as simple as a fuse blown in the fuse block. I'll take a look tomorrow when I go to the exhaust shop, but if anybody's had this problem before I'd appreciate some help. Thanks.
     
  2. bigblock72

    bigblock72 New Member

    Joined:
    Nov 19, 2009
    Location:
    American Fork, Utah
    What style of alternator? old style with 2 spade connectors, or newer style with weather pac connector?

    On older style one wire is connected to keyed power and the other to the battery. Newer style just has one connected to keyed power.
     
  3. Co-Dog

    Co-Dog Active Member

    Joined:
    Mar 13, 2007
    Location:
    La Mesa, CA
    The powder coating is a likely culprit. Internal grounding means that the case must be grounded. You can do a quick check with a voltmeter. Put the red lead on the battery negative and the black lead on the case of the alternator. If you read any voltage with the engine running, that's your problem.

    I would install a ground strap anyways. It should be at least the same size as the alternator output wire. There should be either a stud or a tapped hole for grounding on the case of the alternator.
     
  4. ChromolyKid

    ChromolyKid Well-Known Member

    Joined:
    Mar 15, 2009
    Location:
    Phelan, CA
    Bigblock: It's the "older" (I guess?) style with a lug and harness for the field wire.

    Co-Dog: I'll try tapping a ground into the casing tomorrow. We picked up a brand new alternator today and tried it, still nothing. Funny thing I noticed, the field wire is only putting out 6 volts. Should be 12, no? I think the field wire isn't exciting the coil enough to charge the battery, but I'm not sure why. Could be a loose connection or frayed wire, I'll run a temporary wire to a 12v source tomorrow and see if it makes a difference.

    Maybe I'm looking into this too deep. Maybe the battery is dead and just won't take a charge? It's a couple years old... Hmm... I'll have to test that tomorrow. If it's not a ground, field wire, or battery issue, I'm rewiring the entire thing.
     
  5. 63chevyll

    63chevyll Member

    Joined:
    Oct 28, 2009
    Location:
    colorado Springs, CO
    i would say a ground also, and make sure the bolts are clean with no powder coat
     
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  6. Sparky

    Sparky Active Member

    Joined:
    Apr 4, 2006
    Location:
    Feeniks, Aridzona
    Sounds like the issue is in the wiring. The alternator passed the tests, so you have eliminated that.

    If you have an older 10-SI model (2 flat connectors in the plug) then there are 3 wires you need to hook up. Fat wire from heavy lug on the back of the alternator is the output wire. My guess is that this is the wire that is broken, and it is most likely the fuse link that is at the far end of that wire. Now the two wires on the connector - there is a thin wire that is your excite wire. It is a resistor wire. It is OK if there are only 6 volts at the alternator end of that because feeding 12 volts into it will kill the alternator quickly. It should be running to a source that is hot when you turn on the key. The other wire will be heavy and that is your voltage sensing wire. You said you had 14v at the output, so I'll assume for now that the sensing wire is OK. And having the 14v output but your volt meter is showing less is another indication that the charge wire is your problem.

    So, before you fry anything by running a straight 12v source into that alternator, go follow the charge wire. It should be going from the alternator to the big lug on the starter. You will probably find that it is broken or the fuse link section is fried.
     
  7. ChromolyKid

    ChromolyKid Well-Known Member

    Joined:
    Mar 15, 2009
    Location:
    Phelan, CA
    Co-Dog and 63Chevy are the winners! The shop couldn't fix it so we brought it home, and ran a 4-gauge heavy duty wire from the back of the alternator to a clean spot on the frame, putting out 13.5 volts now. Thanks for all your help.
     
  8. 63chevyll

    63chevyll Member

    Joined:
    Oct 28, 2009
    Location:
    colorado Springs, CO
    nice, glad you got it dialed.

    MOST people never check grounds, and thats always 1/2 the problem.

    I over ground everything
     

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