Coil & Ballast Resistor

[mccsix]

I have 2 coils how can I tell if a BR is necessary for either one?

[Will N]

If you have a 6v system in your car, you do not need a BR for any coil. The only time you need an external BR is if you run a 12v system and the coil does not have a BR built into it. The BR is needed to step the voltage down from 12v to 6v in the primary circuit because 12v would arc across the points too much, burning and pitting them. Do not use a coil with a built in BR in a 6v system.

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The purpose of a ballast resistor is that when the points open and close faster on a 12 V car voltage will rise and as you slow down and go along and then idle you’ll get down to around 6 to 9 volts

[Patrick L.]

If there are no markings on the coils then take an ohm reading.

1.5 ohms for 6v

3.0 ohms for 12v.

A 12v coil can come with or without a resistor. There are usually markings if a resistor is in it. If it doesn't have an internal resister then just hang one in-line somewhere.


You can also take a volt reading in the primary circuit to the points. The voltage, as mentioned, should not be over 8-9 v.

[katy]

So, if it reads 3.0 then add the ballast.

Methinks you have it backwards,.

[Crankster]

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It helps a lot to understand first what the purpose is and what the numbers are ‘sposed to be, what we’re always going to be looking for, on most any engine. The shop manual (I know, I know) ‘splains this. But ignition coils will have a different resistance across the primary winding depending on the application. What you’re really interested in overall is what the total or combined primary ignition circuit ohms resistance is. This incorporates the wiring, ballast (if any) primary coil winding, wiring connections, points, ignition switch, ground connections etc.

In a 12 volt V8 contact point ignition system, about ~ 3 ohms when running is the magic number to keep things from burning up. Take care of the ohms, and the voltage will kind of take care of itself. It’s kind of a close run thing though, this is why if the ignition switch is inadvertantly left to “Run” position without the engine actually running, things can go ‘splodey, if the points just happen to be closed in this situation, about 5+ amperes will be flowing through the primary ignition circuit on a constant basis, and it don’t like that.

Remember too, an ignition coil isn’t always “On” when the engine is actually running, so the current flow is reduced substantially. As it happens about 3 amperes of current is the limit for a 12 volt contact point ignition, Ohms Law is your friend here. A little quick Arithmetic will tell you whether your combo and setup is OK.

[nkaminar]

On newer 12 volt cars, not modern cars that do not have the traditional coil, the ballast resistor is bypassed for starting. The battery voltage is usually lower when starting so bypassing the resistor helps produce a hot spark for starting. On the Model A you would have to access the button on the top of the starter that is contacted by the starter switch to bypass the resistor. If you have push button starting with a solenoid then bypassing the resistor is easier.