Originally Posted by EOD Guy
F = not used
L = a small gauge wire that comes from the idiot light and energizes the alternator
Actually, the F terminal is used if you DON'T have an idiot light. The alternator has a built-in resistor connected to this terminal that replaces the resistance of the idiot light. When the F terminal is connected to a key-switched 12V source, this will also energize the alternator. Use this terminal instead of the L terminal if you have a voltmeter instead of an idiot light. Be sure to use only one of the two (either the F or L terminal), not both.