Using a 12 volt Nappa coil with a 6 volt electrical system on Model A

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What potential problems can this cause?


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[eagle]

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?

[Fullraceflathead]

If the 12 volt coil doesn't have an Internal Resistor it will be fine.

[duke36]

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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Originally Posted by duke36

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 don’t break down due to heat. This is for touring A’s.


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[BRENT in 10-uh-C]

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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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Originally Posted by BRENT in 10-uh-C

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

.

I don’t know. What would happen if you use a 6 volt coil for a 12 volt system? I’m 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 don’t 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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[Patrick L.]

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.

[BRENT in 10-uh-C]

Originally Posted by Patrick L.

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.

[duke36]

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 ?

[Ronald]

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 !

[eagle]

I have used the 3 ohm coil on 6 volt and it throws a nice hot spark.

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No resistor.


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[Tom Wesenberg]

Originally Posted by duke36

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.

[duke36]

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.

[Mike V. Florida]

Originally Posted by duke36

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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Very good info. Thanks Mike


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[Fullraceflathead]

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.

[duke36]

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.