The questions about 6 Volt coils and 12 Volt coils are fertile ground for confusion. So may I add to it? The difference in the ignition system between 6 and 12 Volts comes down to just one thing. Points. Some bright Engineer, very smart person, came up with a switch(that is what points are). The switch had to open and close many thousands of times per minute and...last for thousands and thousands of cycles. A lot of thinking went into this little often under appreciated part. So what do we need to know about this part? It has a limit. The limit is..current. yep them pesky Amps. It takes so much current to make the coil work(Volts be damned). Turns out the current is about 4 Amps. The switch(points) is made to handle those 4 Amps. So when the big change came to 12 Volts...The one part the "Company" did not spend money on...was the points. No I won't talk about HEI(different beast). Just how to use the 12 Volts and keep the points that worked so well? Limit the current across the contacts. So it came down to two simple fixes. 1, put a Resistor in series with the points. And 2, put a Resistor in series with the points...sound familiar? The Resistor is outside the coil if it is a "Ballast" Resistor. Common place is on the firewall and out in the air to cool(4 Amps through a little Resistance is gonna make some heat). The Resistor is inside the Coil...and presto changeo it is now a "12 Volt coil". That is how it was done. Inside equals a 12 Volt coil. Not inside...well that could still be a "12" volt coil if the Ballast Resistor was added. Or a 6 Volt coil if no Resistor is needed. Sneaky company trick? No just simple current management. Still use 4 amps to make the coil work. Still run 4 Amps through the points. Yep for those paying attention so far, the 12 Volt coil will get hot. Still have the "Resistor" to cool and the coil as well. So the question of the day? How to tell a 12 Volt(internal Resisted) coil? Measure the Resistance of the primary circuit, those funny + and - on the coil. Here is a simple to use calculator for just this kind of question:
http://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.htm So with 12 Volts and 4 Amps we get..3 Ohms at the coil(internal Resisted). And for the coil without an internal Resistor? The current stays the same, and the Volts goes to 6 Volts and the Resistance at the primary(remeber the + and - on the coil) we get 1.5 Ohms. Want to use it in a 12 Volt system? No problem, just add a 1.5 Ohm Ballast resistor. Want to use the 1.5 Ohm coil on a 6 Volt truck? No Resistor needed. Want to use that 3 Ohm coil on a 6 Volt truck? Nope, ain't gonna work(too little current to make the coil happy).
