Someone asked about the function of a ballast resistor. (I can't locate the question now.) A few years a go, I had a starting problem on a Ford V-8 in a '73 Century and discovered it was in the solenoid that switched the BR in and out of the circuit. Some of the following was found on the www.

Cranking an engine during starting requires more electrical energy than is required to run the engine. The engine might be cold, and is turning at very low RPM when being turned only by the electric starter. The starter is drawing a lot of current from the battery, so less is available for ignition.

Engine designers could design a high powered "starting system" to provide the extra energy needed, or they could design the engine to RUN on less power than is needed to START the engine.

An operating engine (started, running, hot) does not require the higher energy levels required for starting. In fact, if more energy is provided to the ignition system than is needed, the ignition system would wear prematurely.

So what to do? Provide a "high energy" starting system? Or use the standard battery (these days usually called "12 volts") to start the engine, then run engine with less than "12 volts." Maybe 8 or 9 volts.

Enter the "ballast resistor."

During staring the ballast resistor is "out of the circuit," and when running, the ballast resistor is "in the circuit."

Stated another way: In order to increase the coil voltage at startup some ignition system designs incorporate a "ballast" resistor. The resistor is switched in and out of the supply voltage to the coil.

Once running, the resistor is switched in place and the coil is actually getting less than 12volts.

While the engine is being started, the resistor is removed and the coil gets the full "12volts."

This scheme provides a much better spark at startup to compensate for reduced battery voltage drawn by the starter. When starting a cold engine, the plugs and the air are cold, the cylinder pressure is up, and the fuel/air mixture is poorly controlled. The oil is thick, the battery is cold and its voltage drops as much as 60% because of the high current drained by the starter motor.

-=Grant MacLaren=-
Retired Expert

(And this) My ONLY experience with a Ballast Resistor was on a Ford MercCruiser in a 1973 Century Resorter 16. It was shown in engine's manual -- in a schematic. The only way I could get that engine to start was with a jumper "around" the Resistor. I'd remove the jumper as soon as it started. When I get my Skier back from the trailer maker, I'll look for a BR. Now I wonder how (or if) this is handled on the HEI systems. I have a friend who will know.
-=Grant=-

Here is a photo of the Crusader Conqueror engine in MacSkier. The arrows point to the ballast resistor. BTW, the ballast resistor in the '73 Resorter did not look like this. It was a long wire of high resistance.

Ballast resistor on another boat: