12v coil vs 6v coil

chuck
436244

The Original Equipment Mfg (OEM) 6 volt frontmount coil is designed from the get-go to work with 3.5 volts, ok?. Except there is NO 3.5 volt battery, ok? So you have to reduce what ever the battery voltage is, by an electronic trick called a resistor. This trick is governed by Ohms Law and Kerchoffs Law.

This resistor is carefully chosen to operate with the coil and the battery so that the coil always gets its designed voltage of 3.5 volts. Whether that is the OEM 6 volt battery or the 12 volt conversion battery, that OEM 6 volt coil needs to see about 3.5 volts for long life and eazy starting.

Infact, Ford chose a very special resistor that automatically changes resistance with tempature, and it has a special name, "ballast resistor". The colder there resistor, the less resistance. The less the resistance, the more power is available for the coil to make hotter sparkies. Just when you need hotter sparkies to start that cold engine. Isn't that amazing?

But wait, there's more. Too much power in the sparkie coil will cause it to over-heat and melt its insulative tar and short out the sparkie coils and it will loose its sparkies and stop running. Oooh bad news.....

No wait, theres good news..... the ballast resistor heats up too and automatically increases its resistance value and that reduces the power into the ignition coil and prevents the coil from overheating and melting its tar and loosing its sparkies. And that takes just about 2 minutes. And you don't have to do a thing. Isn't that amazing?

So here is where it really gets tricky for the electrically challenged. If the OEM 6 volt coil is used in a 12 volt conversion and always requires 3.5 volts, it needs BOTH the infamous ballast resistor AND a 12 to 6 volt converting resistor. Always will.

BUT 50 yrs later someone said, "ya know, that 12 to 6 volt converting resistor scheme is inefficent. What if we designed a NEW frontmount ignition coil to work on 12.6 volts instead of 3.5 volts?" And they did. 12.6 volts with 10% headroom, thats 13.8 volts max. Ooops..... they forgot that the 12 volt alternator actually outputs 14.7 volts to charge the 12.6 volt battery.

Guess what happens to that nice 12 volt coil? Too many volts, huh? Guess what happens when that coil gets too many volts? It melts it insulative tar and looses its sparkies. So now back to the electrical drawing board.

A special trick 1/2 ohm current limiting resistor is used to save the 12 volt coil from meltdown. Its not a ballast resistor because it doesn't change value with tempature. And NO you can't use the OEM 6 volt ballast resistor eather, it has too much resistance (which makes for low sparkies).

Make a 20 watt 1/2 ohm current limiting resistor with 2 1-ohm 10 watt resistors in parallel. Get a RadioShack 2-pack #271-131 and make your own custom 1/2 ohm current limiting resistor to use with the 12 volt frontmount coil.

As for your question, are the 6 and 12 volt coils interchangeable. Physically yes, electrically NO!!!!! Understand?

As for your positive vs negative ground question. The 12 volt alternator DEMANDS negative ground, its the internal solid state voltage regulator, ok?

Hope this helps..... .....respectfully, Dell..... a 12 volt advocate for the right reasons

To quote an earlier post by Dell, "Unfortunately, not all 12 volt ignition coils are real 12 volt coil which is where and why you were told "that if I had a 12v coil, to use the "proper" ballast resistor"...since we're not standing by your shoulder at the parts counter, we don't know what the parts person is selling you and often times the parts person doesn't know anything eather. This is why we WARN YOU and explain to you not all 12 volt ignition coils are real 12 volt coils and that you may need the matching (proper) ballast resistor."

This leaves be with one last question. Are the 12v coils different than the 6v coils because the ground is reversed? Just curious.

Thanks

PS Dell - Have you ever considered Pinball machines for a hobby? I have just one but the amount of circuts inside that thing can provide some real entertainment.

Incorrect coil polarity (to match battery polarity) can "cost" you up to 40% of your sparkies. Fortunately, the whole sparkie system is originally designed to generate excess sparkies because of system wear and consumption. ...ie... points gap, plug gap, plug points errosion, plug points surface contamination, plug insulation carbon contamination.....etc.....

Remember, the sparkie fires when the sparkie volts reach the ionization potential. Any excess sparkie volts dissapate. This is why when you replace your sparkies with new ones, everything seems to run better, well guess why? Lower ionization requirement because of clean and sharp metal edges and correct gap.

There are also some obscure technical reasons for "positive ground" called the "Edison Effect" but lets ignore that for now and if you really want to know about the "Edison Effect", do a search..... ....Dell

Thanks again!

Heres what I'd do. I'd measure the square frontmount coil primary resistance by connecting one ohmmeter lead to the top terminal and then measuring to both bottom terminals (top terminal is common to both coil windings)

An OEM 6 volt primary should read about 1 ohm (exact value not important) and the OEM secondary about 7000 ohms, a major difference between primary and secondary.

The 12 volt primary should read about 5 ohms, (exact value not important) a major difference from the OEM 6 volt, understand? The secondary should again read about 7000 ohms.

Then based upon the certain knowledge of coil voltage design application, I'd use my electrical knowledge base, and use the correct resistances as previously instructed. As far as I know, they have NOT changed the 12 volt coil to use the OEM ballast resistor. Use my trick 1/2 ohm current limiting resistor without the ballast resistor if its a 12 volt coil..... ....Dell