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Technical Understanding alternator wiring

Discussion in 'The Hokey Ass Message Board' started by Ian Olmsted, Feb 2, 2019.

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  1. Wiring

    4 vote(s)
    100.0%
  2. Alternator

    0 vote(s)
    0.0%
  1. Ian Olmsted
    Joined: Feb 2, 2019
    Posts: 10

    Ian Olmsted

    Sorry if this has been discussed before....I just added an HEI system and decided to add an LED idiot light to the alternator wiring while I was at it. The idiot light lights up when I start the engine and stays on until I rev the engine. I know the alternator is designed to start generating power once revved (this is how guys with 1 wire alternators do it). If I don't rev the engine, it will fire up but then run off the battery. Question is, is the issue with my wiring or with the alternator? With key on, engine not running, the idiot light is on but there is no voltage at the exciter wire where it plugs into the alternator. That makes no sense to me since they are on the same circuit. Is it possible I have the LED wired backwards?? Thanks in advance for the help. I read this forum all the time, figured it's time to join and make it official.
     
    Last edited: Feb 2, 2019
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  2. Ian Olmsted
    Joined: Feb 2, 2019
    Posts: 10

    Ian Olmsted

    I should probably mention that the idiot light/exciter circuit is getting power from the same wire that powers the HEI distributor. It is definitely 12v switched power.
     
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  3. BJR
    Joined: Mar 11, 2005
    Posts: 9,814

    BJR
    Member

    The LED would not light if wired backwards.
     
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  4. TrailerTrashToo
    Joined: Jun 20, 2018
    Posts: 1,289

    TrailerTrashToo
    Member

    Sounds like the alternator is not getting reference voltage when the ignition switch is in the "RUN" position. LED is Light Emitting Diode. Current flows one way and not the other way - thus the alternator is not getting the reference voltage. Replace the LED with a small indicator light (conventional filament bulb). If that does not solve the problem, something else is going on.
     
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  5. theboss20
    Joined: Dec 30, 2018
    Posts: 274

    theboss20

    The LED won’t pass enough current to excite the alternator...


    Sent from my iPad using Tapatalk
     
  6. This ^^^^^
     
  7. Ian Olmsted
    Joined: Feb 2, 2019
    Posts: 10

    Ian Olmsted

    Makes sense. The reason I initially added the diode was because having the HEI distributor connected to the exciter wire was causing me to not be able to shut the engine off due to the direct alt-HEI flow.

    I'll try a regular bulb and see if that allows sufficient voltage to the exciter.

    Any chance it's a bad alternator component, or do we all agree the issue is the diode?
     
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  8. TrailerTrashToo
    Joined: Jun 20, 2018
    Posts: 1,289

    TrailerTrashToo
    Member

    Heads or tails. Try the conventional light bulb and see if the problem goes away.
     
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  9. Mr48chev
    Joined: Dec 28, 2007
    Posts: 33,861

    Mr48chev
    ALLIANCE MEMBER

    Here are a couple of diagrams I saved over the years. I have used the one in the second photo a number of times and it works good.
    [​IMG]

    [​IMG]

    I'd say your LED bulb doesn't have near the resistance that a regular light bulb would.
    The 1N5408 diode is a Massey Ferguson tractor number. They aren't hard to find though.
    You can replace that with the gen light though.
     
  10. Ian Olmsted
    Joined: Feb 2, 2019
    Posts: 10

    Ian Olmsted

    I'll give the standard bulb a try. I am definitely getting resistance because the engine shuts off when I turn the key to off (didn't before the LED). It seems the LED doesn't allow enough voltage to pass to the alt. I have 12v going into the LED, almost zero coming out.
     
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  11. MAD MIKE
    Joined: Aug 1, 2009
    Posts: 772

    MAD MIKE
    Member
    from 94577

    Purpose of the 'ALT/GEN' light is two fold.
    One is to excite the generator immediately(its a resistor), thus begin charging after startup.
    Two is to warn the operator/driver that there is a fault in the electrical system if/when a no charge(alt/gen) or no storage(batt) condition occurs.

    ALT/GEN light works due to difference of (electrical)potential. The light is not grounded. This form of lighting the bulb works like a see-saw. If there is higher voltage on either side of the lamp the lamp will illuminate indicating there is a problem in the charging system. When the voltage is equal on either side then there is no difference of potential, power will not flow through the lamp, and the lamp will stay off.
    This is why when the rpms drop excessively the alter/gen light will begin to glow as the power across the lamp is now uneven, allowing power to flow through the lamp and illuminate it.
    It is also a good idea to run a resistor in parallel with the lamp, if the lamp burns out the exciting function of the alternator will still work without having to rev the engine.
    A diode should not be used in this part of the circuit. Light emitting or not.

    When the ignition switch is turned to off, power should be removed from distributor(to kill engine) and from the excite circuit of the alt to turn off the self exciting circuit. Although both will derive power from the battery they should not be allowed to derive power from each other or you no longer have a STOP circuit(switch) in your system and the engine will continue to run.

    If you are using a 10/12SI alternator the wiring in the diagrams above are mostly correct, there are a few things that need to be addressed.

    T1 in the diagram is correctly wired, although a resistor should be run in parallel across the charge light to keep functionality of the system working if the lamp fails.

    T2 should not be run directly to the output of the alternator. It should be run to the main power distribution block or main power splice. T2 is the 'sense' wire, it monitors system voltage which will change from load requirements of the system. Running it directly to the battery or alternators Batt. terminal is kind of pointless as that will rarely be affected by system output.
     
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  12. Stueeee
    Joined: Oct 21, 2015
    Posts: 305

    Stueeee
    Member
    from Kent, UK

    The LED setup will work OK if you put a resistor across the two connections. a 70 ohm resistor will mimic the loading of the 2w bulb that would normally be in circuit; the LED still lights OK with the small amount of current that goes through it. I have had this setup working for the last 12 years on my '50 Chevy truck which has 10 SI alternator fitted.
     
  13. good thread guys.....
     
  14. Ian Olmsted
    Joined: Feb 2, 2019
    Posts: 10

    Ian Olmsted

    Ok, back to the drawing board....I replaced the LED with the incandescent bulb that was recommended and the alternator still requires a rev to start generating power and turn the light out. Should I look into replacing components in the alternator itself?
     
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  15. The37Kid
    Joined: Apr 30, 2004
    Posts: 30,627

    The37Kid
    Member

    With THREE votes cast even I answered it correctly.:)

    Bob
     
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  16. theboss20
    Joined: Dec 30, 2018
    Posts: 274

    theboss20





    Sent from my iPad using Tapatalk
     
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  17. BJR
    Joined: Mar 11, 2005
    Posts: 9,814

    BJR
    Member

    If you have to rev to get it to charge that is the regulator in the alternator. You need a different regulator, that is a feature of a one wire alternator regulator.
     
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  18. theboss20
    Joined: Dec 30, 2018
    Posts: 274

    theboss20

    Make sure that with the key on that both terminals on the alternator plug are hot...use a test-light or a multimeter. The multimeter will give you more info as to the voltage in the plug...then turn the key off and recheck...the #1 terminal (you will see the numbers embossed into the aluminum housing) should turn off with the key...the other #2 should be hot all the time. Confirm that and report back.


    Sent from my iPad using Tapatalk
     
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  19. theboss20
    Joined: Dec 30, 2018
    Posts: 274

    theboss20

    If you are truly using a one wire regulator then you don’t need an exciter wire...relocate the power to the ign. Module to the accessory side of the ign. Switch and use what the factory calls for for a resister. You can tell a self excited regulator by the web between both terminals when the alternator plug is pulled out. If no web it’s not a SE regulator.


    Sent from my iPad using Tapatalk
     
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  20. Ian Olmsted
    Joined: Feb 2, 2019
    Posts: 10

    Ian Olmsted

    It's a stock Jeep cj7 alternator. This issue only came up after changing to an HEI ignition. Before that, the exciter wire did it's job and no rev was required. I'll check voltage at the terminals and report back. Thanks so far for the help
     
  21. Ian Olmsted
    Joined: Feb 2, 2019
    Posts: 10

    Ian Olmsted

    Ok, interesting turn of events. The main red wire on the alternator harness/plug has power both on and off. The exciter wire had zero volts off, and only 0.8 volts on. I then checked the voltage at each of the wires coming from the idiot light. When the light is illuminated, I get 12+ volts coming in but only 0.8 volts coming out the other side of the light. This is the same thing I saw with the LED. It makes sense since the engine is not running (hence the light illuminated like it should) but turning the engine on, none of the conditions above change. Same 0.8volts at exciter terminal and light bulb "output" wire.
     
    Last edited: Feb 6, 2019
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  22. Ian Olmsted
    Joined: Feb 2, 2019
    Posts: 10

    Ian Olmsted

    Key on, Voltage at the dash light. Meter is grounded to the chassis.
     

    Attached Files:

    Last edited: Feb 6, 2019
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  23. Ian Olmsted
    Joined: Feb 2, 2019
    Posts: 10

    Ian Olmsted

    Key on, each terminal of the alternator plug. This makes sense since only 0.8 volts is coming from the light at the other end of the exciter wire.
     

    Attached Files:

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  24. Ian Olmsted
    Joined: Feb 2, 2019
    Posts: 10

    Ian Olmsted

    bump. Any recommendations? If I put 12 volts to the exciter terminal on the alternator with the key in the on position, will that circuit be grounded? Should I be reading 12 volts or close to 0?

    More importantly, do I need to replace my alternator? I have it wired exactly like the first photograph/diagram in this thread.
     

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