The Stovebolt Site Forums The Shop Area Engine Shop 12 volt Coil Questions

I just got a 54 motor that was converted years ago to 12V with one with alternator. It's in a 47 car converted to 12 volts with some aftermarket street rod wiring. Do I need a external ballast before the coil? Am I correct in assuming that the wiring harness-if equipped with a ballast/resistor will only deliver 6 volts to the coil? I need to get a new coil and can't figure out reasons for coils with or without resistor/ballasts. I know my flat head 6 volt Fords have resistors under the dash cutting the load to 3 volts at coil. Thanks in advance

I doubt that the wire harness would have a ballast resistor in it. I would think that you would have to use a ballast resistor mounted to the fire wall and a coil for a 1955 and later GM vehicle.

There are 12v coils that do not have an internal resistor - these need a ballast resistor.

There are 12v coils with internal resistor - these do not have an external ballast resistor.

Just to confuse us, there are also different resistor ratings.

Also, to further confuse someone who has bought a truck converted to 12v, there are "resistor wires" that provide resistance - this might have been used by someone who wanted "neater" wiring.

These 12v systems provide more that 6v to the coil (often 8v-9v).

http://randysrepairshop.net/general-motors-breaker-point-ignition.html

I understand the differences in the styles of coils-I'm trying to determine which style I should use. The one on the car now is 12V but not marked either way. In a 12V application do I want to deliver less voltage to the coil thru the use of a ballast-or use a coil with a ballast or do I need to worry about 12 volts at the coil.

The advantage to having an external resistor is that the coil can get a full 12v during engine start-up via the by-pass wire, then once it's running it gets reduced voltage through the resistor.

If you use a internal resistor coil the coil voltage will always be reduced, even during start-up, which could lead to hard starting.

Hope this helps,

Mike B

Like Mike B. Said, start on 12V and run in 9V. GM has done that for years.
Improved starting was their goal-

What is missing here is the type of 12v coil.

The 12v coil in the first post was not original (on the originally 6v 1954 truck).

Depending on the type of 12v coil found in that 1954 truck, you may or may not need a ballast resistor or resistor wire.

The thing most folks miss (or ignore) is the fact that battery voltage is not constant. It can vary from around 9V on a cold (or very hot) start due to the current draw of the starter, and can be a maximum of 14.5 V or a little more with the charging system working against a fully charged battery.

A coil designed to protect the points from overcurrent at 14 volts will produce a VERY weak spark, or none at all with low battery voltage during a hard start. GM compromised by installing a coil that will work at 9V with the resistor bypassed during startup, and installed a resistor for point protection during cruise with a fully-charged battery and 14.5 volts from the charging system. Smart guys, those designers!

Then some self-appointed "expert" comes along and declares himself smarter than the guys who spent millions of dollars on research and development and declares the resistor isn't necessary.- - - - -Go figure!
Jerry

Just ohm the coil with a multimeter. Across the terminals (+) to (-) it should read 1.5 ohms. If it reads 1.5 ohms, you need a 2 ohm ballast resistor on the firewall in line with the ignition switch.

If it reads 3 or 4 ohms, it is what they call a Resistor Coil and has that ballast resistor incorporated in the coil.

Hope that helps. Either way, you need 3.5 or so ohms so that you have approx 6 volts to the points. Like Jerry said, it approximate depending on charging, etc.

Another thing.. most 12v conversions in these trucks do NOT have a secondary wire for better starting during cranking. That is because we use foot operated solenoids and an ON/OFF ignition switch. So there is normally only one wire to the coil that goes through a ballast resistor. It seems to work just fine even during starting.