So the L terminal is all that is needed to excite, or "turn on" the alternator. A small amount of amperage and voltage is applied here to excite, or magnetize the rotor. This allows the alternator to produce voltage when spinning. A full battery voltage/amperage here will destroy the unit. So one factory method was to pass an ignition circuit thru a light bulb, in series, to this terminal. So the voltage had the resistance of the bulb to lower it once the power was applied to the rotor. If the alternator failed to make voltage, this circuit was then LOWER voltage than the ignition circuit passing through the light bulb, so it caused the light to illuminate. This only works with incandescent bulbs. An LED does NOT have enough resistance to achieve the need goal.
But now days, we don't like to run a big dummy light in the dash, so we instead replace the light bulb with a resistor. The resistor is doing the EXACT same thing as the bulb, but we just don't have a visual aid if it fails. NOW, this is not a 5v circuit like everyone says. If you put a multi-meter on the circuit, with bulb or resistor in line, you will NOT see a drop in voltage. You will only see this if the connector is also hooked to the alternator. A resistor is a CURRENT LIMITING DEVICE, not a voltage limiting device. A single resistor will never have a large voltage drop.
So now we know to put a resistor inline to an ignition on power source. The most common resistor used for this is 470 ohm. Not 470 kohm, as this is a hugely different value. You can get away with a bit more resistance, such as 500-600ohm and be safe.
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