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The Delco 10SI alternators were designed to have an "idiot lamp" or resistance in the lead to the #1 "switched excite" wire, and a diode works, too. Without one of the 3 devices, once the engine is running, and the alternator begins to charge, when the switch is shut "off", current from the alternator's internal diode trio will "backfeed" out the #1 "switched excite" terminal, and provide current to the ignition coil, preventing the engine from shutting down. The engine continuing to run when you want it to shut down is obviously undesirable, AND the current backfeeding OUT the #1 "switched excite" MAY damage the alternator's diode trio and/or internal voltage regulator. Using a 10 Ohm resistor or a #194 lamp will limit the backfed current to a value too small to power the ignition, and the engine will shut down. Using a diode prevents the backfeed of current altogether. So, make it easy on yourself and the alternator's internals, and use a resistor, a #194 lamp, or a diode, such as a 1N5406 in the circuit to the alternator's #1 "switched excite" terminal. I see someone has already linked to one of my photos, but I will repeat it in this post. 
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