76 Gremmie with Prestolite

amcenthusiast wrote: -Incredible they were still trying to keep remnants of the old 6 volt systems (like the coil)... but the higher amp flow in the coil with 6 volts is obviously better for shooting a hot spark to the plugs... Maybe this is partly why the modules seem to have a short lifespan because they modulate (with a variable resistor function as expressed above) 12 volts to make the higher amp flow of a 6 volt system. -how to say it right... this does seem like the module would create heat (by technically overworking the 12 volt to amp to resistance relationship) and create an early end-of-life scenario...

The whole point of using a 6 volt coil on a 12 volt car is so the coil can be temporarily overloaded with 12 volts with a bypass system for hotter spark while starting. There is always some kind of ballast to drop the coil back down to 6 volts operating voltage while running. Otherwise the coil would overheat and fail. Points ignition does this with a ballast resistor between the ignition switch and the coil, and Prestolite ignition does it between the coil and ground, which is all located inside the control module. The control module itself contains the ballast.

The Prestolite module contains several resistors and transistors working as a current regulator (fancy name for a variable resistor). It is able to regulate the coil's operating voltage at 5-8 volts under all conditions regardless of battery voltage by varying its own resistance, hence why a ballast bypass system is not needed. Because the module is operating as a ballast resistor, it will get hot. It dissipates 8 watts of RMS power, the same as an external ballast resistor on a points ignition car. It's like having an 8 watt light bulb inside the module. If the ignition switch is left on, the power jumps to 24 watts constant and will cook the module. That is why you never leave your key in the run position. On top of that, the current regulating and coil switching circuitry inside the module are subjected to up to 250 volts when the switching transistor breaks the coil current to fire a spark. It is also located under the hood of the car. All these things contribute to thermal cycling, which eventually causes the cracked solder joints that often cause a Prestolite module to become flakey as it gets hot. It heats up, parts expand, the cracked joints that were making contact when they were cold pull apart and break contact, and the ignition shuts off until the module cools again. 

So you are correct. The module acting as a power-absorbing ballast for the coil eventually contributes to its failure. The case acts as a heat sink, but it may not be sufficient. It is also Apollo-era technology using discrete components aside from one microchip. The heat, vibration, and high flyback voltage from the coil eventually kill it.