A plain 12 volt coil only runs on 6-8 volts. That's because a resistor (external or internal) is designed into a 12volt system. The resistor is used to drop the voltage to around 6 - 7.5 volts and in turn lower the amperage through the points so they don't crystalize with the higher voltage/heat. A 6 volt coil has no resistor whatsoever.

Electronic:
You also have to reduce the voltage from the coil to the Ignition switching module below 8 volts for it to survive also. The primary resistance of the coil is key to this. Use coil test method to find resistance.

Oct 2009
6 Volt coils - Require using an External Resistor For 12 Volt Systems...

Part: ECH IC7 Product Line: Echlin Ignition Price:$51.49
Part: MPE IC7SB Product Line: Mileage Plus Electrical Price: $18.29
Part: ECH IC10 Product Line: Echlin Ignition Price: $49.99 Each
Part: MPE IC10SB Product Line: Mileage Plus Electrical Price: $17.69
Part: ECH IC12 Product Line: Echlin Ignition Price: $45.49 Each
Part: MPE IC12SB Product Line: Mileage Plus Electrical Price: $16.69 Each
Part: ECH IC64 Product Line: Echlin Ignition Price: $35.99 Each
Part: MPE IC64SB Product Line: Mileage Plus Electrical Price: $ 26.99 Each
Part: ECH IC676 Product Line: Echlin Ignition Price: $58.99 Each


12 Volt Coils

Part: ECH IC14 Product Line: Echlin Ignition Price: $51.49 Each. Has "Use with External resistor Label

Part: MPE IC14SB Product Line: Mileage Plus Electrical Price: $18.69 Each. Has "Use with External resistor" Label

Part: ECH IC21 Product Line: Echlin Ignition Price: $42.99 Each Indicated 12 volts, Has "Use with External resistor" Label

Part: MPE IC21SB Product Line: Mileage Plus Electrical Price: $17.29 Each Indicated 12 volts, does not indicate if External Resistor is Required.

Part: ECH IC27 Product Line: Echlin Ignition Price: $54.49 Each, Indicated 12 volts, has "External Resistor is Required" Label.

Part: MPE IC27SB Product Line: Mileage Plus Electrical 2020 Price: $27.99 Each Indicated 12 volts, does not indicate if External Resistor is Required.

Part: MPE IC277SB Product Line: Mileage Plus Electrical 2020 Price: $31.99 Each No External Resistor Required ( Use with Electronic Ignition)