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Technical GM three wire alternator conversion

Discussion in 'The Hokey Ass Message Board' started by mayerst, Dec 10, 2021.

  1. mayerst
    Joined: Mar 3, 2008
    Posts: 23

    mayerst
    Member

    Hello all,
    I have a 1956 F250 w/272 and recently did a GM three wire conversion and have a question for anyone who may have had the same problem. I have alternator wired correctly and it still behaves like a one wire conversion. I have attached the exciter wire (with diode) to the coil, ign switch and tried wiring it into the gen light lead. The gen light stays on and the engine needs to be reved up before it starts to charge. I am getting power to the plug and when the plug is disconnected it won't charge at all. I have tried two different alternators (rebuilds) and still not working correctly. Does anyone have any suggestions?

    Thanks
    Tom
     
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  2. jimmy six
    Joined: Mar 21, 2006
    Posts: 11,210

    jimmy six
    ALLIANCE MEMBER

    Your Ford is rejecting a GM part….happens all the time.:oops:
     
  3. As I recall from 50 years ago, you need to put a resistor across the gen light to keep it from staying lit.
     
  4. Glenn Thoreson
    Joined: Aug 13, 2010
    Posts: 501

    Glenn Thoreson
    ALLIANCE MEMBER
    from SW Wyoming

    A lot of those GM alternators have to be revved up before they start charging. I have three cars I put them on and they all do the same thing but at different speeds. My cars are too old for a red light, though. I wouldn't worry about it to much. If the exciter wire sends voltage to the alternator and keeps it there, but turns it off when the engine is turned off your okay. Try running the exciter lead direct to the hot wire on the coil. You need a resistance in the wire and a small light bulb (like the idiot light ) will do the trick. My shriveled up old brain doesn't remember if the light turns on or off when charging, though.
    I don't understand why you would need a diode ? I've started using all one wire setups on my own cars. Much easier.
     
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  5. mayerst
    Joined: Mar 3, 2008
    Posts: 23

    mayerst
    Member

    Thank you all, I'll try the resistor. Aside from this issue it works and looks great.

    Thanks again
    Tom
     
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  6. I have 3 wire 10si’s on all my cars and the ones that don’t have a stock charge light I added a light. Some of the cars take a little more then cold idle rpms to turn off the light.
     
    Last edited: Dec 11, 2021
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  7. Mr48chev
    Joined: Dec 28, 2007
    Posts: 31,009

    Mr48chev
    ALLIANCE MEMBER

    I've wired up a lot of three wire alternators using the diode and have never used a charge light.
    I save every wire up a Delco 10 SI or 12 SI Alternator in something it didn't come in here.
    Photobucket I haven't got any diagrams for the newer ones yet but there are some pigtails out there to connect them where a 10 SI went.

    For the diode installed impaired this pigtail you can get from a guy in El Paso is pretty handy for the price. Ebay number 201247906131 I got one a while back and for the price it is hard to convince yourself to run all over town looking for diode even though the diodes are cheap.
     
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  8. Flathead Dave
    Joined: Mar 21, 2014
    Posts: 3,671

    Flathead Dave
    ALLIANCE MEMBER
    from So. Cal.

    Give it a rev to get it excited...
     
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  9. mayerst
    Joined: Mar 3, 2008
    Posts: 23

    mayerst
    Member

    Thanks again, I'll report back after trying your suggestions.

    Tom
     
    Last edited: Dec 11, 2021
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  10. connielu
    Joined: Apr 21, 2019
    Posts: 167

    connielu
    Member
    1. A-D Truckers

    The leads on the "gen" light are as follows: one to accy, not ign, one to the white lead on the alt plug. When the key is on, engine not running the circuit is grounded through the alt and the light lights. When the engine is running, the alt is excited (regulator turned on) both of the leads are then +, the light goes out (because the ground is lost). If the alt / regulator fails, the ground is restored and the gen light lights again.
    Does your gen light have two leads? it needs to be isolated from ground to work with a SI style alternator. My Falcon was wired this way and worked perfectly.
    Good Luck.




    http://madelectrical.com/electricaltech/onewire-threewire.shtml
     
    Last edited: Dec 11, 2021
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  11. dartracer
    Joined: Apr 18, 2009
    Posts: 280

    dartracer
    Member

  12. sunbeam
    Joined: Oct 22, 2010
    Posts: 5,641

    sunbeam
    Member

    Excite it from the ACC terminal of the switch
     
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  13. BJR
    Joined: Mar 11, 2005
    Posts: 7,520

    BJR
    Member

    The number 2 post on the flat plug, the one that everyone tells you to hook to the 12 volt + on the alternator is the load sensing wire. It should be connected to the junction block where all of the load, or accessories are connected, on older cars it is usually the ammeter or the headlight switch. This tells the alternator to up the charge rate when there is a large draw. The alternator does not work as well if the wire is jumped at the alternator to 12V.
     
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  14. connielu
    Joined: Apr 21, 2019
    Posts: 167

    connielu
    Member
    1. A-D Truckers

    Yes! The Mad Electrical link explains this in detail. Well worth the time to read.
     
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  15. Mr48chev
    Joined: Dec 28, 2007
    Posts: 31,009

    Mr48chev
    ALLIANCE MEMBER

    Last edited: Dec 11, 2021
    1Nimrod likes this.

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