Why the Resistor in 12V Conversions

Would you please clarify the function of the resisters in the electrical system of a Ferguson system, specifically one that has been converted to 12 volts. And, what polarity adjustments are made in the system? June 2, 2015.

First, the matter of internal and external resistors for ignition coils. You know how the lights dim on a car, truck or tractor when you engage the starter while the lights are on.? The starter is demanding all the electricity it can get to turn over the engine. That leaves everything else starving for electricity. The ignition coil needs electricity to fire the spark plugs and that need is critical when starting the engine.

Early automotive engineers determined that an ignition coil requiring less current would provide adequate spark to fire the spark plugs. However, once the engine started and the generator began producing current, and the starter no longer required current, the coil would overheat and burn out as it began receiving more electricity/current.

The solution was to install a regulating device in the primary side of the coil wiring to reduce the current going to the coil after the engine starts, and yet allow adequate current flow to the coil when the starter was engaged. That regulating device is the ignition coil resistor. The resistor acts as a regulator, allowing adequate current to flow to the ignition coil to provide a good spark while the starter is engaged and limit the amount of current flowing to the coil once the engine starts.

Some companies, Delco Remy for example, built resistors into their coils to eliminate the need for an external resistor. Most Ferguson tractors, TO-20, TO-30, TO-35, MF-35, F-40, etc. have Delco Remy ignition systems using ignition coils with built-in resistors.

Most Ford tractors Ford 9N, 2N, 8N, NAA, etc. have Autolite ignition systems utilizing ignition coils that require external resistors. (The ignition coil resistor on the 9N/2N is located on the back of the dash above the ammeter.) Therefore, when an ignition coil that does not have an internal resistor is installed on a Ferguson tractor, an external resistor must be added to the ignition circuit.

Second, regarding 6 volt ignition systems converted to 12 volts. When a 6 volt ignition system is converted to 12 volts either a 12 volt coil must be installed or a resistor installed in the circuit to reduce the coil voltage to 6 volts. The resistor regulates the voltage, keeping it at 6 volts, allowing the original 6 volt coil to be used in a 12 electrical system. Those two situations are independent of each other however, when a 6 volt ignition system utilizing a 6 volt coil with an external resistor is converted to 12 volts, two different resistors are required. The original coil resistor for the ignition coil requiring an external resistor and the resistor to drop the voltage from 12 volts to 6 volts for the 6 volt coil.

If a 6 volt ignition system is converted to 12 volts and has an original Delco style 6 volt coil with an internal resistor, the original 6 volt coil with internal resistor can be used if a voltage reducing resistor, reducing 12 volts to 6 volts is installed in the coil circuit. If a 6 volt ignition system is converted to 12 volts and a Delco style 12 volt coil with an internal resistor is installed as part of the conversion, no external resistors are required.

Regarding polarity. Most 12 volt ignition systems are negative ground systems, meaning the negative (-) battery terminal is connected to ground (the frame/castings/engine.) The positive terminal on the ignition coil primary is connected to the ignition switch. Most 6 volt ignition systems are positive ground systems, meaning the positive (+) battery terminal is connected to ground, and the negative terminal on the ignition coil primary is connected to the ignition switch. When a 6 volt ignition system is converted to 12 volts, the coil must be wired for a 12 volt system, regardless of the voltage rating of the coil. In other words, the coil wiring must be changed so the positive terminal on the ignition coil primary is connected to the ignition switch. The 12 volt battery must be connected so the negative (-) battery terminal is connected to ground.

There are alternators that require external voltage regulators. The first alternators used on cars trucks and tractors used external voltage regulators. The MF 100 series tractors were the first ones to use Delco alternators with external regulators. The Delco regulators used with these alternators used a “harmonica connector” on the voltage regulator wiring. The voltage regulators used with alternators are not the same as those used with generators.

Those 12 volt Delco electrical systems, including the alternator and voltage regulator, were all negative ground systems. That being said, if you have a 12 volt alternator on a tractor, it is most likely negative ground and the system would use a 12 volt battery wired with the negative (-) terminal connected to ground.