You should have the following voltages coming from these terminals with the key in the "on" position. Zero volts at "S" and about 8 volts at "I". Th "S" terminal is nothing more than the positive side of a wire wound electro magnet. The negative side of the winding is internally attached to the mounting bracketof the solenoid and becomes a ground when the bracket makes contact with chassis ground by the bracket's contact with the inner fender when it's bolted down, completing the circuit.
When terminal "S" gets energized, (12 volts, key in "start" position), it creates a magnetic field that draws the contacts of the large terminals together and the "I" terminal also gets energized and delivers a full 12 volts at the same time. You will only have the 12 volts there when the solenoidis energized and voltage to the starter side, (large terminal), is present.
Again, the only time there should be voltage at the "S" terminal is when the key is in the "start" position. The "I" terminal wire will have 12 volts at the same time. This wire is connected to the coil circuit and provides a full 12 volts to the coil to make it easier to start the engine. It by-passes the coil's ballast resistorwire, which when in the "on" position, delivers a lower 8 volts to the coil (the voltage is now routed through the ballast? resistor) to prevent the coil from overheating and to help the ignition points to live longer.
If you were to disconnect the wire to the "S" terminal with key in the "on" position and take a voltage reading at the wire itself, it should show zero volts. At the "I" terminal, again disconnected and checked the same way, there should be approximately 8 volts present.
The 8 volts at "I" will have no effect on the solenoiditself in the "on" position because it is now an open circuit, at this point, unless thesolenoid
is energized and the key is in the "start" position.
hope this helps!