I assume the light is hooked to the #1 terminal? I also assume #1 is what you are calling the "excite terminal."
The colors of your wires don't help. When you are facing the alternator from the back - the two spade connectors on top are #1 (excite) to the left and #2 (sensing wire) on right.
If I read you correctly, you say your light goes on when hooked to #1 but no charge. You then jump 12 volts positive to #1 and it charges? If so, obviously you're not getting enough power through the light wire to excite.
Since it charges when you jump, RPM cannot be the issue. But do keep in mind that the 10SI or 12SI needs to spin 1600 RPM to charge. If the engine idles at 800 RPM, the pulley ratio must be at least 2 to 1.