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Old 10-17-2011, 03:30 PM   #1
james.heider
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Default 12 volt or 6 volt coil how to tell difference

I have several coils that are either 12 volt or 6 volt. They are not marked and I was wondering how you can tell the difference?
There must be an ohms reading that would give some indication.
Any help would be appreciated. I don't wanrt ot just hook them up to a 12 volt battery and burn them out if they are 6 volt.
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Old 10-17-2011, 03:57 PM   #2
Dave in Boise
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Default Re: 12 volt or 6 volt coil how to tell difference

You won't burn out the coils, but here is an easy way to tell..

Hook 12v power to 1 side of the coil, and then check the voltage on the other side.. H

(hook to + and check output on - for instance, the polarity for the test is not important and won't hurt either way)


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12v through a 12v coil will still show 12v when it comes out the other post of the coil, however a 6v coil will drop the voltage to 6v.. I have used old 6v coils as voltage reducers before for intermittent items in a 12 v system, like a horn or other item

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Old 10-17-2011, 06:01 PM   #3
james.heider
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Default Re: 12 volt or 6 volt coil how to tell difference

Thank You Dave.
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Old 10-18-2011, 12:28 AM   #4
Tom Wesenberg
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Default Re: 12 volt or 6 volt coil how to tell difference

You need to connect an amp meter in series with the coil and 6 volt battery. A 6 volt coil will draw 4 amps.
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Old 10-18-2011, 04:15 AM   #5
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Default Re: 12 volt or 6 volt coil how to tell difference

If you have an accurate ohm meter, reading across the posts of a 12 volt coil should be about 3 ohms. A 6 volt coil is about half of that. It is not an exact number, but close enough that you can tell 6 from 12 volt.

Or, doing what Tom says, a 12 volt coil will flow about 2 amps if connected to 6 volts. Instead of an accurate ohm meter, you need an accurate ammeter (that can tell the difference between 4 amps vs. 2 amps).
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Old 10-18-2011, 08:16 AM   #6
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Default Re: 12 volt or 6 volt coil how to tell difference

There will be a difference in test results in a 12v coil that is to be used with a resister than one that doesn't need a resister. Also a 12v coil that doesn't need a external resister will test very close to a 6v coil. Would that 12v coil be ok on on a 6v car?
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Old 10-18-2011, 08:49 AM   #7
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Default Re: 12 volt or 6 volt coil how to tell difference

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Originally Posted by Tom Wesenberg View Post
You need to connect an amp meter in series with the coil and 6 volt battery. A 6 volt coil will draw 4 amps.
I would speculate this is the only valid test I've seen so far. Testing a coil that has a resistor in series with it without one end connected should always show whatever the input voltage is as there will be no drop across the resistor or coil.

That and Tom usually knows his stuff
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Old 10-18-2011, 09:16 AM   #8
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Default Re: 12 volt or 6 volt coil how to tell difference

12 volt coils are actually 6 volt coils with a resister connected in series within the primary circuit usually internal to the coil. That fact allows you to check the ohms across the primary terminals to see what the resistance is of the coil. As mentioned above a 6 volt coil should have less than 2 ohms and a 12 volt coil will have more closer to 2.5 to 3 ohms of resistance
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Old 10-18-2011, 09:38 AM   #9
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Default Re: 12 volt or 6 volt coil how to tell difference

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12 volt coils are actually 6 volt coils with a resister connected in series within the primary circuit usually internal to the coil. That fact allows you to check the ohms across the primary terminals to see what the resistance is of the coil. As mentioned above a 6 volt coil should have less than 2 ohms and a 12 volt coil will have more closer to 2.5 to 3 ohms of resistance
I agree however, how many people here own an ohm meter which is known to be accurate within 1 ohm? Sure a cheap analog meter will show a difference between a 6v coil and a 12v one, but what if someone has no reference?
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Old 10-18-2011, 09:58 AM   #10
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Default Re: 12 volt or 6 volt coil how to tell difference

You need to connect an amp meter in series with the coil and 6 volt battery. A 6 volt coil will draw 4 amps.


How many amps does a 12 volt coil draw using a 12 volt battery?
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Old 10-18-2011, 10:38 AM   #11
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Default Re: 12 volt or 6 volt coil how to tell difference

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I agree however, how many people here own an ohm meter which is known to be accurate within 1 ohm? Sure a cheap analog meter will show a difference between a 6v coil and a 12v one, but what if someone has no reference?
That would be a problem. I assume we are talking about old coils laying around of questionable conditon and you don't want to buy a decent ohm meter just to see if you have 6 or 12 volt coils? In that case I would recommend tossing the coils in the trash and buying a new one 6 volt coil at autozone for only $16.00.
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Old 10-18-2011, 10:41 AM   #12
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Default Re: 12 volt or 6 volt coil how to tell difference

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Originally Posted by james.heider View Post
You need to connect an amp meter in series with the coil and 6 volt battery. A 6 volt coil will draw 4 amps.


How many amps does a 12 volt coil draw using a 12 volt battery?

The amps would be the same. 4 amps
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Old 10-19-2011, 12:42 AM   #13
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Default Re: 12 volt or 6 volt coil how to tell difference

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Originally Posted by ldj1002 View Post
There will be a difference in test results in a 12v coil that is to be used with a resister than one that doesn't need a resister. Also a 12v coil that doesn't need a external resister will test very close to a 6v coil. Would that 12v coil be ok on on a 6v car?
That coil should give you enough spark to run the car, but it may be a bit weaker since you mentioned that it has an internal resistor. A better choice would be a 12 volt coil that needs an external resistor. Then when it's run on 6 volts you'll get a hotter spark than the 12 volt coil with the internal resistor.

I find 6 volt coils at most swap meets and don't recall buying a bad one yet. That's because so many coils are needlessly replaced.
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Old 10-29-2011, 08:47 AM   #14
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Default Re: 12 volt or 6 volt coil how to tell difference

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Originally Posted by Tom Wesenberg View Post
That coil should give you enough spark to run the car, but it may be a bit weaker since you mentioned that it has an internal resistor. A better choice would be a 12 volt coil that needs an external resistor. Then when it's run on 6 volts you'll get a hotter spark than the 12 volt coil with the internal resistor.

I find 6 volt coils at most swap meets and don't recall buying a bad one yet. That's because so many coils are needlessly replaced.

OK, I didn't make myself clear. If I have a coil and all it says is 12v or nothing, How can I check it to determine if I use it on a 12v as is or put a resistor in line?

Last edited by ldj1002; 10-29-2011 at 08:55 AM.
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Old 10-29-2011, 09:11 AM   #15
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Default Re: 12 volt or 6 volt coil how to tell difference

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Originally Posted by ldj1002 View Post
OK, I didn't make myself clear. If I have a coil and all it says is 12v or nothing, How can I check it to determine if I use it on a 12v as is or put a resistor in line?
Read across the primary posts with an ohm meter. If it reads about 3 ohms, you can use it on a 12 volt car without an external resistor. If it reads about 1.5 ohms, you need an external resistor.
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Old 10-29-2011, 10:52 AM   #16
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Default Re: 12 volt or 6 volt coil how to tell difference

Thanks very much
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Old 10-29-2011, 04:50 PM   #17
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Default Re: 12 volt or 6 volt coil how to tell difference

Is the mallory available for 6 volt? I'm about to put a B motor w OHV & t-5 in my '31 roadster. It's hard to sleep nights !
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Old 10-30-2011, 08:15 AM   #18
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Default Re: 12 volt or 6 volt coil how to tell difference

Oops! Wrong thread.
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Old 10-30-2011, 10:35 AM   #19
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Default Re: 12 volt or 6 volt coil how to tell difference

you can delete any of your posts. click on the EDIT button and then DELETE.
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Old 10-31-2011, 09:49 PM   #20
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Default Re: 12 volt or 6 volt coil how to tell difference

NO such thing as a 6 volt or 12 volt coil. You need to match the impedience of what your working with. Iceman
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Old 11-01-2011, 11:41 AM   #21
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Default Re: 12 volt or 6 volt coil how to tell difference

Ok you lot, Slow witted limey here... most of us have kicking around our workshops a variety of coils like this....
a) coils for 12 volt non ballasted ignition circuits, b) coils for 6 volt ignition circuits, c) coils for 12 volt ignition circuits with a ballast resistor. d) so-called sports coils for 12 volt circuits.
Can we get a definitive answer between us as to how to identify each of them?
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Old 11-01-2011, 11:59 AM   #22
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Default Re: 12 volt or 6 volt coil how to tell difference

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Originally Posted by johnbuckley View Post
Ok you lot, Slow witted limey here... most of us have kicking around our workshops a variety of coils like this....
a) coils for 12 volt non ballasted ignition circuits, b) coils for 6 volt ignition circuits, c) coils for 12 volt ignition circuits with a ballast resistor. d) so-called sports coils for 12 volt circuits.
Can we get a definitive answer between us as to how to identify each of them?
Here's my take.. Very simple

a) coils for 12 volt non ballasted ignition circuits ---- will show 12 v out when put from one terminal to the other
b) coils for 6 volt ignition circuits ---- will show 6v out when 12v is put in one side
c) coils for 12 volt ignition circuits with a ballast resistor ---- will show 6v out when 12v is put in one side due to the internal resistor
d) so-called sports coils for 12 volt circuits ---- will show 12 v out when put from one terminal to the other.

Hope this Helps.

Dave
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Old 11-01-2011, 10:22 PM   #23
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Default Re: 12 volt or 6 volt coil how to tell difference

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Originally Posted by Dave in Boise View Post
Here's my take.. Very simple

a) coils for 12 volt non ballasted ignition circuits ---- will show 12 v out when put from one terminal to the other
b) coils for 6 volt ignition circuits ---- will show 6v out when 12v is put in one side
c) coils for 12 volt ignition circuits with a ballast resistor ---- will show 6v out when 12v is put in one side due to the internal resistor
d) so-called sports coils for 12 volt circuits ---- will show 12 v out when put from one terminal to the other.

Hope this Helps.

Dave
You need current flow to get a voltage drop. If you apply x volts in you will get x volts out without current flow.

I find 12 volt coils are marked 12 volts on the case, but this still doesn't tell you if it should have an external resistor. I'd connect the voltage across the 2 primary termianls to determin if it needs an external resistor. If the coil draws about 3 to 4 amps then it shouldn't need an external resistor. If it draws more amps, then add a resistor to bring the current draw down to 3 to 4 amps.

BTW, the markings "DIST" and "BAT" on the coil primary termianls are useless unless you know what the coil was built for as far as battery ground.
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Old 11-02-2011, 08:21 AM   #24
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Default Re: 12 volt or 6 volt coil how to tell difference

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Originally Posted by Tom Wesenberg View Post
You need current flow to get a voltage drop. If you apply x volts in you will get x volts out without current flow.

I find 12 volt coils are marked 12 volts on the case, but this still doesn't tell you if it should have an external resistor. I'd connect the voltage across the 2 primary termianls to determin if it needs an external resistor. If the coil draws about 3 to 4 amps then it shouldn't need an external resistor. If it draws more amps, then add a resistor to bring the current draw down to 3 to 4 amps.

BTW, the markings "DIST" and "BAT" on the coil primary termianls are useless unless you know what the coil was built for as far as battery ground.
So does this all mean its just easier for most people to throw out unknown coils and simply buy a coil for the proper application?
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Old 11-02-2011, 09:34 AM   #25
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Default Re: 12 volt or 6 volt coil how to tell difference

Succintly.... YES!
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Old 11-02-2011, 10:40 AM   #26
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Default Re: 12 volt or 6 volt coil how to tell difference

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So does this all mean its just easier for most people to throw out unknown coils and simply buy a coil for the proper application?
No, as it's easy to check coil polarity, as has been posted on the internet by the Chicagoland MG club.

I know a lot of people consider changing parts the easiest way to go, but it isn't the cheapest way to fix the problem and often leads to more problems.

http://www.chicagolandmgclub.com/tec...neral/574.html

Last edited by Tom Wesenberg; 11-02-2011 at 11:04 AM.
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Old 11-02-2011, 11:47 AM   #27
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Default Re: 12 volt or 6 volt coil how to tell difference

Aaaaah Tom, you've started us coil polarity again, you cad!
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Old 11-02-2011, 12:03 PM   #28
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Default Re: 12 volt or 6 volt coil how to tell difference

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So does this all mean its just easier for most people to throw out unknown coils and simply buy a coil for the proper application?

A brand new 6 volt coil at autozone costs only $16.00. I guess it depends on what your time is worth expecially if that kicking around coil quits during your next tour.
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