[Excaliber]

I find it hard to believe you even need the I terminal at all with todays modern electronic ignition systems. MSD doesn't use it, for instance.

The I terminal was for "back in the day" with POINTS and CONDENSERS and RESISTORS in the circuit. Does anybody even run points anymore? Certainly not a single modern ignition system.

Voltage, with a POINT system, was reduced from 12-14 volts to 6 or 7 volts to keep the points from burning up. It had nothing to do with running the starter itself. The I terminal by-passed the resister and hit the points with full battery voltage ONLY when the solenoid was actually engaged. That is, when the starter was actually cranking the motor over. Thats why the I terminal is ON the solenoid, which could be mounted on the firewall (typical of Fords) or the starter (some Fords) or in BOTH places, some "special" applications (big Lincoln with a 460 for instance).

The S stands for solenoid (a mechanical switch drawn in by a strong magnetic force) but it could just as easily stand for "starter" as it regards a typical Ford.

You needed to by-pass the resister when cranking because battery voltage falls to about 10 volts when the starter is engaged. Running only 10 volts THROUGH the resister meant the POINTS were only getting 4 or 5 volts, which was not enough to deliver a good hot spark from the coil. Hard starting was the end result if the I terminal was not employed.

CW's drawing is very accurate, by the way.