what voltage to coil

Discussion in 'General IH Tech' started by binders4ever, Jan 18, 2018.


  1. binders4ever

    binders4ever Farmall Cub

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    i am trying to start newly rebuilt 304.there is a resistor between my ign. switch and coil.12v going in and 6v going out to coil.engine is in 1966 1100a. I have replaced all of burnt wires and connectors, this is the way it was when i got it. Dont you run 12v to coil. prestolite ei distributor. Thanks
     
  2. scoutboy74

    scoutboy74 Lives in an IH Dealership

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    Your question/slash description is a bit muddled. If you have a points distributor, you need an external resistor. If you have an electronic ignition, then you don't.
     
  3. binders4ever

    binders4ever Farmall Cub

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    sorry,i have electronic,thanks
     
  4. Greg R

    Greg R Lives in an IH Dealership

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    Trevor is spot on. A Prestolite distributor with the electronic module needs 12V to coil + The module does the current regulation, no resistor needed.
     
  5. scoutboy74

    scoutboy74 Lives in an IH Dealership

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    I'm not sure where you've come by this information, but it is factually incorrect for the most part. Pretty much every ignition coil, even the ones that say "use with external resistor", have at least a little measurable primary resistance across the polarity terminals. Let's use a stock Scout II V8 engine coil as an example. That coil will have @ 1.5 ohms of primary resistance across the terminals. It might say to use with external resistor, or might not say a damn thing about an external resistor. This same coil can be used no matter whether the distributor has breaker points or an electronic ignition module inside. Now, if the distributor has points, in order to protect the points and the coil during engine use, an additional amount of resistance must be provided. In some IH vehicles this was accomplished via a 72 inch long resistor wire which was good for @ 1.8 ohms of resistance. It can also be accomplished via a ceramic ballast resistor block with a value of 1.8 ohms interrupting the switched feed to the coil POS terminal. Either way, 1.5 primary in the stock coil and 1.8 ohms through the resistor wire or the ballast block makes a total of @ 3.3 ohms resistance. That figure is perfect for keeping the points and the coil on a V8 engine happy for a long time. And yes, that much resistance will reduce the voltage going to the coil somewhat while the engine is running, but not by half. Really, the voltage figure there isn't what matters. Ensuring correct resistance to match the number of engine cylinders and more importantly the type of ignition system in place is what really matters. As has already been answered for the original poster...with his electronic ignition, no external ballast is required, nor is it recommended. Full voltage to the coil having a primary resistance of 1.5 ohms across polarity and he's good to go.
     
  6. scoutboy74

    scoutboy74 Lives in an IH Dealership

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    Re-read my previous post and make a better effort to comprehend it this time. It isn't what I think. Its what I know. And I learned it the hard way by frying a coil just as you did. I'm not arguing with you that in certain situations, additional ballast is required. That's undisputed fact. I explained quite thoroughly when additional resistance is required for protection when points and coil are combined. But the subject ignition system in this thread is electronic...or have you not picked up on that detail yet? A person needs to have a basic understanding of what ignition components are in place with the vehicle they are working on. And they should also have at their disposal a multi-meter capable of measuring resistance in ohms. Then if they know what resistance factor is required for a V8 with points versus electronic, or an inline six with points versus electronic, or an inline 4 with points versus electronic, all that's required is to measure the resistance of the ignition components in place and either add ballast or remove it as necessary to hit the right number. This isn't a one-size-fits-all scenario. For a V8 with points, that range is 3.0 to 3.5 ohms. For a V8 with original electronic system, such as the subject Prestolite above, a standard coil having measurable primary resistance of @ 1.5 ohms is satisfactory. Adding ballast resistance to that system as if it were breaker points will needlessly hamper spark energy production. My main point in all of this is, forget about the words you may or may not see printed on the exterior of a coil, and actually measure the primary resistance with a meter so that you know what you're dealing with.
     
  7. scoutboy74

    scoutboy74 Lives in an IH Dealership

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    Of course that would only do him good if he still had the original points distributor in his engine, which he does not. Either he or someone else at some point has swapped in a Prestolite solid state distributor. If you've never heard of or seen one, they don't have points and condenser. They have a reluctor wheel and a pickup module instead. It's an entirely different critter. That's why actually knowing what bits and pieces are involved matters. But do go on with your head buried in the sand if you must. Just try not to drag anyone else along with you if you can help it.
     
  8. Greg R

    Greg R Lives in an IH Dealership

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    Here's what IH sez: img019.jpg
     
  9. scoutboy74

    scoutboy74 Lives in an IH Dealership

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    Offering correct advice or information isn't a contest.
     
  10. scoutboy74

    scoutboy74 Lives in an IH Dealership

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    I didn't pee in your cornflakes, dude. Nobody told you to eat those urine soaked oats anyway. You can always dump 'em out. They make more every day.
     

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