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Checking for a 6 volt or 12 volt coil

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Smokeys427

07-12-2007 08:10:57




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Ok, I think I may have asked this before but with my almost worn out gray matter in the ole noggin I have forgotten. My 1948 year old 8N tractor that has been converted to 12 volt system (Integral regulator three wire) that is running great thanks to all you here on the board

I am buying some extra parts before things happen such as points, plugs, condenser, cap, rotor and the Autolite AL437's.
I have got almost everything except a new coil. Here's the question.
I do not know for sure if it's a 6 or 12 volt coil so to find this out and WITHOUT taking it off, can I just check the voltage (key on) at the wire at the existing coil to see how many volts it has going to it? I am assuming here that if I have 12 volts going to it it's a 12, and 6 volts to it it's 6 volt coil.
would I be correct doing it this way?

Thanks again for all the help you guys give me

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Smokeys427

07-12-2007 16:51:05




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 Re: Checking for a 6 volt or 12 volt coil in reply to Smokeys427, 07-12-2007 08:10:57  
Thank you very much Bruce for the help. I just keep going back to my automotive knowledge on these tractors and I have to learn things all over again.

Why I did mention this though, is I had a '40 Ford coupe and as you know they had a front mount distributor too (but without coil mounted to it) and it had been changed out to a 12 volt system. It had a coil problem and I had the same delima as these 8N's do. Back then I checked the voltage to the coil and it had twelve volts. So.... I just put a 12 volt coil on it and never had a problem since. The cars gone now and as I said this is a new learning experience for me on these 8N's and I just have to keep remembering this
Again thanks for the help

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Bruce (VA)

07-12-2007 11:18:25




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 Re: Checking for a 6 volt or 12 volt coil in reply to Smokeys427, 07-12-2007 08:10:57  
The short answer is no. Take it off & check resistance. A 6v coil should have 1.5 ohms & a 12v coil will have 3 ohms.

Phil posted the 'long answer' a week or so ago:

All ignition resistors are not EQUAL and Front Mount Coils will not Mix/follow Automotive Specifications.

The Rule of Thumb for a Front Mount Starts with the OEM Ballast Resistor. This is a PTC device that Limits current to the required 3 AMP max thermal load. This Protects your COIL!

A) The OEM Ballast is the Only Resistor required for OEM 6V coils operated on OEM 6 Volts. Those PPL that use Ceramic Devices as a substitute will suffer Under and/or Over Current.

B) The OEM Ballast is the Only Resistor required for 12V Coils operated on 12 Volt Systems, these have a "Pure" resistance value of 3.0 Ohms (-.5 to + .25) or dry rated @ 2.5 -3.25 ohms

C) The only 12V coil that does not use the OEM ballast is the 12V coil that has a "Pure" Resistive value of 4.0 Ohms @3 Amps

Automotive Ignition Resistors do NOT Operate or function the same as the original OEM Ballast. If you use these you will have UNDER and/or OVER current conditions. The Automotive industry uses both PTC (positive temperature coefficient), and NTC (negative Temperature coefficient), and Fixed resistive devices. These are designed to function with the side mount Coil (more than)> 4 amps

If you want to use an OEM 6 Volt coil on a 12 Volt system the OEM Ballast MUST have an additional resistor. This Resistor value can be an Automotive Type (fixed) of ~ 2.25 Ohms rated @25 watts or made from RAT Shack Resistors with Equivalent Wattage ratings. The challenge is meeting the Wattage rating. The solution may look Ugly ( 2 R’s in series or Parallel) but function is important

The conclusion: Use the OEM Ballast in ALL cases. If the OEM Ballast Burns out you will need to add Resistance but the OEM Ballast must be Part of the Circuit with only one exception.

I hope that Helps,

BTW: Measuring Coil resistance will depend on current(heat). The OEM 6V coil meaures .64 ohms (dry), .76 ohms @ 2 Amps, and .98 ohms @ 3Amps. The value to use is .98 or 1.0 Ohm

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don b

07-13-2007 10:42:36




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 Re: Checking for a 6 volt or 12 volt coil in reply to Bruce (VA), 07-12-2007 11:18:25  
Bruce.....just thoughtI would add my two cents worth.
I have measured so called 8ne10306 resistors from diff manufactors.There seems to be no standard.The same goes for front mount coils,6 or 12v.While I agree that the oem type ballast resistor should always be used in a 6 or 12 v system,diff manufactors make them different and resistance varies by manufactor.I guess what I'm trying to say is that every resistor and coil should be measured for resistance that goes into the system.I know of no blanket set of parts to go into these front mount systems.Most of the so called front mount 6v coils I have measured are about 1.0 ohm and most of the 12 v coils are 2.5.BUT,there is no standard.I have measured so called 12v front mount coils and they vary from 1.2 ohms to 3.0 ohms.The 8ne 10306 fixed resistors measure anywhere from .1 to whatever and the same goes for the A8nn 12250a.Here again,no standard.
I think that most agree that we should have between 3 and 4 amps going to the points.The only blanket of parts that I can think of, would be is,for a so called front mount 6v system,you would need about 2.5 ohms resistance,including the coil resistance, and about 4.5 ohms resistance,including the coil resistance,in a front mount 12v system. don b

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Smokeys427

07-12-2007 10:35:53




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 Re: Checking for a 6 volt or 12 volt coil in reply to Smokeys427, 07-12-2007 08:10:57  
Anybody know???third party image



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Bob

07-12-2007 11:15:33




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 Re: Checking for a 6 volt or 12 volt coil in reply to Smokeys427, 07-12-2007 10:35:53  
A "6-Volt" square-can coil will have about 1 Ohm of primary winding resistance.

A "12-Volt" square-can coil will have 2.5 Ohms, or more of primary winding resistance.

This is measured from the terminal post at the coil's top to the coiled "spring thingy" at the base of the coil.



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