many round can coils today will say something like, 'needs no external resistance' and then list, for instance, 12v. a napa ic14sb ( round can ) coil is printed like this. this tells you it is a native coil for a 12v application with no series primary resistance.
on the other hand.. some companies get a lil deceptive, kinda... they will print '12v' then in flyspec printing say 'with serial resistance' telling you it is basically a '6v' or '8v' coil that needs serial primary resistance to function in a 12v application.
Many do that if it is in a bypass ignition system ( an N does not have a bypass system .. close.. but no cigar ). in a bypass system, the 6v coil lives in series with a resistor on a 12v system, during startng when battery voltage is taxed, the resistor is out of circuit and full 'available' bat volts hits the coil, making as hot as possible sparks with respect to air gap.. a N uses a ballast resistor with a thermal coeficient. when it is cold, it is low resistance... thus giving some extra sparkies.. it quicklyu heats up as current flows like the wire in a toaster, and the resistance goes up, lowering the current. i won't get into the electronics definition of voltage drop and current flow.. but suffice it to say it works according to ohms law among others.
now.. on the 9n.. they don't have those nifty round coils.. they have square coils... the originals were more like a 4.5v coil in a 6v system, which is why a resistro was needed. those original coils had a primary impedance of about 1 ohm 'or so' I have measured units from .9 to 1.25 depending on age and country of origin. the 12v application square coils typically measure out to about 2.5 ohms.. though again. i have seen rare variants down to 2.0 ohms.. and some as high as 2.8... 2.5 is the commonly supplied chinese origin one.... if you are real lucky, the coil may say what it's primary resistance is. a square cold sold as 12v can be assumed to be a 2.5 ohm coil... add that tot he hot ballast resistor that stabilizes around a half an ohm or a lil more.. and you have a safe primary current.
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Today's Featured Article - 12-Volt Conversions for 4-Cylinder Ford 2000 & 4000 Tractors - by Tommy Duvall. After two summers of having to park my old 1964 model 4000 gas 4 cyl. on a hill just in case the 6 volt system, for whatever reason, would not crank her, I decided to try the 12 volt conversion. After some research of convert or not, I decided to go ahead, the main reason being that this tractor was a working tractor, not a show tractor (yet). I did keep everything I replaced for the day I do want to restore her to showroom condition.
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