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3G alternator help - low voltage

Discussion in '6.9L IH & 7.3L IDI Diesels' started by renjaminfrankln, Jan 12, 2021.

  1. renjaminfrankln

    renjaminfrankln Full Access Member

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    Hey folks,

    Need some help with my 3G alternator. This was installed by the previous owner. I am just getting around to troubleshooting why it is only charging the system at 13.5 volts. At both the batteries and the alternator output post (to rule out charging cable)

    As far as I can tell, it’s hooked up correctly. The yellow wire goes to battery positive terminal. Green looks like its hooked into the wiring harness, I believe for an indicator light. The white connects to the connector on the alternator you can see in the pic. The charging wire is heavy gauge and goes straight to the battery.

    The only thing I don't see hooked up is any sort of ground wire, which I see listed as optional in some wiring diagrams, and looking with a mirror at the back of the alternator, I don't see where I would attach one.

    D63C4FAD-E33F-4962-910A-A3BE7111D3E7.jpeg
     
    Last edited: Jan 12, 2021
  2. Reggie f250

    Reggie f250 Registered User

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    If the alternator is bolted to the engine it is grounded, unless the bracket is corroded. There is a regulator and brush assembly screwed to the back of the alternator that can be replaced. That can bring it back to good as new. Only thing to watch for, there are slip rings inside that the brushes ride on that can wear out. The alternator has to be disassembled to replace them. I did this on mine because i just rebuilt mine with a high amp rebuild kit. Not hard but time consuming.
     
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  3. renjaminfrankln

    renjaminfrankln Full Access Member

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    Yeah it is grounded fine. I confirmed this by clamping a jumper cable to the alternator case and then battery negative terminal, same result.

    After some more searching around I just put an order in for a new regulator/brushes.
     
    Last edited: Jan 12, 2021
  4. Booyah45828

    Booyah45828 Full Access Member

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    Rev it up and see what your voltage is. Did you load test it at all? What was the voltage/amps at idle vs raised rpm?

    There's specific tests involved that you should perform before you replace a regulator. Yeah 13.5 is lower then what I would normally expect to see, but that 13.5v doesn't indicate a fault if you're in a high load low rpm state.
     
  5. renjaminfrankln

    renjaminfrankln Full Access Member

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    13.5 at all rpm’s
     
  6. MtnHaul

    MtnHaul Supporting Member Supporting Member

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    Since the P.O. did the work just thought I'd mention that there ought to be a fuse or fusible link in the battery to alternator charging cable. Fairly certain all the factory setups used a fusible link but when I did my conversion I used a MegaFuse & fuse holder.
     
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  7. renjaminfrankln

    renjaminfrankln Full Access Member

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    Appreciate that. There is no fuse. I may or may not add one.

    Also my aftermarket regulator and brushes arrived today. Will post outcome when I install it tomorrow.
     
  8. renjaminfrankln

    renjaminfrankln Full Access Member

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    Damn it... new regulator installed and still low. Rock steady at 13.45V even revving up to 1500 RPM or so. This is measured at the battery posts with a multimeter.

    The brushes on the old one looked fine and the surface that the brushes ride on inside the alternator did as well.

    From what I can tell the alternator is a new-ish 3G reman.
     
  9. typ4

    typ4 Supporting Member Supporting Member

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    It could be the meter.
    Or 1 diode
    IIRC the diesel trucks had a lower set point voltage, less computers to run.
     
    Last edited: Jan 18, 2021
  10. renjaminfrankln

    renjaminfrankln Full Access Member

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    Meter is out, cross referenced with another one.
     
  11. gnathv

    gnathv Full Access Member

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    Do you get the 13.45 v with the truck running and all accessories on?
     
  12. renjaminfrankln

    renjaminfrankln Full Access Member

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    With basically every conceivable combination.

    One interesting point is it always squeals the belt for a few seconds after a cold start. All belts are in good condition and tight. I thought this may be because at 13.5v the batteries aren’t charging fully, and after the glow plugs and a little extra cranking the alternator has to work hard
     
  13. Reggie f250

    Reggie f250 Registered User

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    Maybe one of the batteries is going bad? Or they need to be charged?
     
  14. renjaminfrankln

    renjaminfrankln Full Access Member

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    I just disconnected them, resting both showing an even 12.5 just after disconnection. I will let them sit for a minute and check again.

    a good suggestion, they crank the truck just fine but I haven’t really checked them out. They are optima red tops of an unknown age, 800 cca
     
  15. gnathv

    gnathv Full Access Member

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    13.45 should charge the batteries satisfactorily and not overheat them. My IDI (93) ran in that range, but my powerstroke charges at 14.3 consistently. Maybe the set voltage does have to do with more electronics.
     

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