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Old 10-30-2017, 02:59 PM   #1
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Default Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

What potential problems can this cause?


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Old 10-30-2017, 03:05 PM   #2
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

I have done the same thing before, although not on an A. Would throw a 1/2 inch spark. I figure if it throws a good spark, why not?
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Old 10-30-2017, 03:45 PM   #3
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

If the 12 volt coil doesn't have an Internal Resistor it will be fine.
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Old 10-30-2017, 04:37 PM   #4
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

Good question johnr.
How about a car with a 12 volt system (12volt coil , electronic ignition, etc.) and reverting back to points/condensor distributor. Assume no issues?
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Old 10-30-2017, 05:32 PM   #5
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

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Good question johnr.
How about a car with a 12 volt system (12volt coil , electronic ignition, etc.) and reverting back to points/condensor distributor. Assume no issues?


That should work. The reason for using the Nappa 12 volt coil is that they donít break down due to heat. This is for touring Aís.


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Old 10-30-2017, 06:08 PM   #6
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

Not being a smart aleck, but what is a 12 volt coil? Isn't it really about the ohms??

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Old 10-30-2017, 06:17 PM   #7
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

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Not being a smart aleck, but what is a 12 volt coil? Isn't it really about the ohms??

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I donít know. What would happen if you use a 6 volt coil for a 12 volt system? Iím using the 12 volt with a 6 volt system. It works but there are some strange things happening. When I start the car the lights and horn work. After I take a nice ride, the lights and horn donít work. Later, say the next day, the lights and horn work again. This has happened several times. The engine is not affected.


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Old 10-30-2017, 06:50 PM   #8
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

Its about the internal resistor, does it have one or not ? Automotive coils are either 1.5 or 3.0 ohms. If it has a resistor, it'll generally say so.
As long as yours is a 1.5 ohm and is epoxy filled it'll be fine.

I'm thinking your light and horn issue are probably steering column/light switch/ wiring issues.
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Old 10-30-2017, 07:49 PM   #9
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

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Its about the internal resistor, does it have one or not ? Automotive coils are either 1.5 or 3.0 ohms. If it has a resistor, it'll generally say so.
As long as yours is a 1.5 ohm and is epoxy filled it'll be fine.

I'm thinking your light and horn issue are probably steering column/light switch/ wiring issues.

Yes, that is correct. There really is not a 6v or 12v coil, but more about the ohms. John's comments about other peculiar wiring issues leads me to believe the wiring has issues, one of which may be the culprit why the original coil is getting warm and breaking down.
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Old 10-30-2017, 08:20 PM   #10
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

Yes concur; our car came with a 12 volt coil installed with the FS system. We haven't checked the coil resistance yet. What ohms value is desired with a stock distributor ?
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Old 10-30-2017, 08:23 PM   #11
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

Keep in mind that in a 12 volt system the 12 volts from the battery (or more like 14 volts ) is only used to operate the starter and then when you release the key a ballast resister reduces the voltage to 8 volts to be used for running. So by using a 12 volt coil without a resister in a 6 volt system your battery would only provide about 7 volts. not much difference !
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Old 10-30-2017, 08:58 PM   #12
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

I have used the 3 ohm coil on 6 volt and it throws a nice hot spark.
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Old 10-30-2017, 08:59 PM   #13
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

No resistor.


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Old 10-30-2017, 09:06 PM   #14
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

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Originally Posted by duke36 View Post
Yes concur; our car came with a 12 volt coil installed with the FS system. We haven't checked the coil resistance yet. What ohms value is desired with a stock distributor ?
1 1/2 ohms, so if you turn the key on with the points closed, the ammeter should read about 4 amps.

The Model A coil feed wire was moved to the opposite terminal, in the firewall terminal box, in late 1929, so the coil draw would show on the ammeter. On my 1928, I moved the black feed wire to the other side, so it also shows on the ammeter. This makes a good diagnostic tool to check for current draw while cranking the engine.
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Old 10-31-2017, 12:20 AM   #15
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

OK in summary, for 12 volt system with stock distributor, a 3 ohm primary coil will yield 4 amps. For 6 volt applications, a 1.5 ohm reading would produce 4 amps.
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Old 10-31-2017, 12:47 AM   #16
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

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Originally Posted by duke36 View Post
OK in summary, for 12 volt system with stock distributor, a 3 ohm primary coil will yield 4 amps. For 6 volt applications, a 1.5 ohm reading would produce 4 amps.
Yup

12/3 = 4
6/1.5 = 4
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Old 10-31-2017, 01:06 AM   #17
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

Very good info. Thanks Mike


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Old 10-31-2017, 10:41 AM   #18
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

A 12 volt coil is actually a 6 volt coil with the intention of using a Ballast resistor.
If the 12 volt coil doesn't have an internal resistor then its basically a 6 volt coil but in a 12 volt application it is intended to be used with a external ballast resistor.
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1930 Gordon Smith Air Compressor
1941 Willy's Pickup
1960 Thunderbird-For Sale
1964 Buick Riviera 2x4 425
1965 Pontiac GTO, 455 Super Duty
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Old 10-31-2017, 11:02 AM   #19
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Default Re: Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

We checked our (12v coil ) that came with the FS zipper we're removing in favor of points and it reads about 3 ohms. Appears to be oil filled. We assume it is internally regulated. We also assume it's OK to reuse with modern points. I guess we could check the output voltage..

Further research from our supplier:
For epoxy filled coils that can be placed with leads down, etc.
Pertronix 40611 black (origin?) 3ohm
Bosch 022 1119 027 (Brazil) 3ohm
Would appreciate any other products or feedback re; the above.

Last edited by duke36; 10-31-2017 at 12:49 PM. Reason: supplier suggested products
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