QuestionI have been searching on the net for a hour or so now. I'm not sure what to believe. I know that my 1972 spitfire 4 was originally fitted with a 6 volt coil. I see it in the haynes Manuel too. Is it going to be ok to put in a 12volt coil with a external resister in my car? the advance auto parts man says its ok. Says 12 volt to start the resistor will kick it down to 7-8 when running trust me I worked for British Leyland. Ok then I read other people say 12v upgrade its going to destroy the points more often. is this true? other say its completely fine your car will have no troubles with it.. What am I supposed to believe Howard? I have a 12v coil w/ external resistor in my car now. and I bought all new complete tune up parts for the car and a 12v coil w/ resister, . Because its really time the parts are over 18 years old and show it. but it does run but loosing spark at times. help Howard what am I to believe?
AnswerHi Mark,
When any car uses a 6v coil, they must be used with a ballast resistor. The reason they are used is that the starter draws so much current that it drops the available voltage when engaged. If a 12v coil were used straight, there would not be enough voltage available to operate the coil when the starter was engaged. So they wire in a bypass circuit and use a 6v coil. Here is the way it works.
When you just turn the ignition to "ON" you run current to a resistor and through the resistor to a 6v coil. The resistor drops the voltage from 12v to aprox. 6v to power the 6v coil so no damage is done to the wiring or points.
When you turn the ignition to "START" position, the starter is engaged and the starter draws so much current that there is no longer 12v available to the coil and resistor. Thus the coil would not operate, so a bypass wiring is added into the ignition switch and harness to rout the current directly to the 6v coil, bypassing the resistor and applying what voltage that is left over that the starter motor didn't use. This voltage can be as low as 9 volts. Nine volts applied to a 6v coil for a short time will not hurt the wiring nor the points and give a great spark for starting. As soon as the key is released back to the "RUN" position, the current is routed to the resistor to drop the voltage available to 6v for the coil. The 6v coming from the resistor to now operate the 6v coil works fine and will not over heat the wiring nor the points.
Your car, the 72 Mk IV Spitfire used this bypass system that powered the coil via a white wire to the resistor and on the the coil from the resistor via a white w/yellow tracer wire. The white wire was powered form the ignition switch. When you are in the "START" position another white w/yellow tracer wire from the starter solenoid routs direct battery voltage (about 9v) to the other white w/yellow wire bypassing the resistor and going directly to the 6v coil.
You can use a coil that is marked ("12v but must be used with external ballast resistor") If it does not say that on the coil, the only way you can use a straight 12v coil is to do away with the ballast resistor. This will run fine that way but may have starting problems.
I hope this clears it up for you.
Howard
