More on ignition resistors

Question:

I thought I would comment on Mr. Ward’s question about ignition resistors and why he has one between the ignition switch and the coil on his ’48 Ford Flathead (August).

I’ve been a mechanic since before electronic ignition was common, and Mr. Prince got it partway right. The real reason for a resistor in the coil circuit was to provide full battery voltage on engine cranking, since 12-volt systems of the time typically dropped to about 8.5 volts on cranking, the coil was designed to have full output at 8.5 volts and was fed this voltage on cranking with a circuit that bypasses the resistor. After the key is released to the “run” position, the circuit runs through the resistor to provide the 8.5 volts that the coil was built to operate on with the rest of the system running at charge voltage of approximately 13 to 14.5 volts. The six-volt systems are the same with about 4 to 4.5 volts on crank.

The common thought that the resistor is to protect the points grew from the fact that the first thing to fail is the points if the resistor is removed and the coil is fed with full-system voltage.

Mr. Ward was asking about Fords. Almost all battery ignition systems operated this way, even 1960s and early 1970s GM that did not appear to have resistors.

The needed resistance was built into the ignition lead between the switch and coil.

Answer:

Thank you for taking the time to share your thoughts with your fellow readers.

Though I agree with your basic premise I wouldn’t say, as you did, that “the real reason for a resistor in the coil circuit was to provide full battery voltage on engine cranking.” The provision of the ignition system that bypasses the resistor during cranking is what allows full battery voltage to reach the coil. The purpose of the resistor is to reduce full battery voltage while the engine is running. The reduction in voltage protects both the ignition points and coil. Excess voltage will burn the contact surfaces of the points and cause the coil to operate at elevated temperatures. The rise in operating temperature is particularly dramatic because heat generated in the coil is proportional to the square of the current flowing into it. Excessive heat degrades the insulation in the coil’s secondary winding, leading to short circuits and, ultimately, complete coil failure.