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Ford Alternator w/ External Regulator...?

Discussion in 'The Hokey Ass Message Board' started by CRH, Feb 22, 2009.

  1. CRH
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    CRH Member

    Hey guys, I hope my question isn't stupid! I made the entire harness for my little '27 Roadster, and everything seems to be working fine except the charging circuit.
    The car is running a little 302, and it used an external voltage regulator. I wired the regulator terminals correctly, but didn't use the "I" terminal, since I always assumed that is only for a charge indicator light on the dashboard. I just have the other three terminals properly hooked up (A to BAT, S to Stator, F to Field). Do I need to have the resistance of an indicator light connected to the terminal "I" on my regulator in order for proper charging, assuming all other parts and pieces are working and connected right?
    When I test the new alternator, it only raises the charging to about 12.6 volts, and that requires at least 1200 RPM's. Any help appreciated.
  2. coupster
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    coupster Member

    The "I" has to be hooked up to excite the field. Did you try to full field the Alt to see if it is working?
  3. coupster
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    coupster Member

    To give you a little more info about that circut, Ford used a 15 ohm resistor in parallel with the indicator light. It was there in case the bulb burned out. You need to use ignition switch voltage for this as the battery would run down if you left voltage at "I" when the engine is not running. Hope I didn't muddy up the explination to bad.
  4. 58custom
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    58custom Member

    Coupster is exactly right. Run 12v from the ignition or running accessory supply through a 15 ohm resistor to that terminal whether or not you use a charge indicator light. Use a 1 watt resistor or greater. This works the same when you use a Ford 3G internally regulated alt. I found that out when adding a 3G to my '58 Ford.
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  5. 24riverview
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    24riverview Member

  6. butch27
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    butch27 Member

    I put a diode in and ran the wire to ACC on the switch.
  7. CRH
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    CRH Member


    Holy Moly, thanks for this helpful, straightforward info, people! I'm very grateful, and the reason-why type explanations I like best are what you people have provided. Thanks!! I'll wire the '27 correct tomorrow...
  8. CRH
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    CRH Member

    Hey guys, one more question: Is it bad to simply have the "I" terminal run straight from a switchable power source, like the ACC part of my ignition switch? I tried this for fun and it did as stated previously; it excited the field. I'm just wondering if and why a resistance must be part of that circuit...? Is it too much amperage, hence the use of 15 ohm resistor by Ford?
  9. 58custom
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    58custom Member

    Well, first, I must correct my previous post. I took a look at my factory data and the resistor is 560 ohms, not 15. Sorry for the bad cheese. The resistor is there to simulate the DC resistance of the charge indicator lamp should that lamp burn out. I have heard of people wiring it up straight without either lamp or resistor but I think I would use a resistor anyways. Another poster mentioned a diode. You have a 50/50 chance of getting that one right. Just get a 470 to 560 ohm 1 watt resistor or a pair of 1K 1/2 watt resistors and put them in parallel, solder on the wires, use two pieces of shrink wrap (shrink one at a time) and there you have your inline resistomathingie for much charging goodness.
  10. CRH
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    CRH Member

    That makes sense, no worries. I'm just too "in-the-dark" about why there should be the resistance. I'm scared that maybe it lessens the load on the "I" terminal for a good reason, like protection or something...? I have driven the '27 for the past two days with just a straight wire from the ignition ACC side to the "I" on my voltage regulator, and the alternator charges great. But, does this mean it is overcharging or that my un-resisted wire will eventually burn out the voltage regulator?
  11. butch27
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    butch27 Member

    Put a diode in. You just don't want the voltage to back up to the reg.
  12. hrspwrranch
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    hrspwrranch Member

    Do you put the diode on the I terminal?
  13. Kenneth S
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    Kenneth S Member

    This is how I wire a Ford alternator w/ external regulator, except I delete the amp guage (fire hazard), and use a volt guage. Been wiring then this way for over 30 years with no problems.
    [​IMG]
    Last edited: May 24, 2014

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