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In the dash there is usually a bulb on older cars like pre-02 ish. One side of the bulb gets 12v switched power. the other side goes to the alternator plug. When the alternator is turning making a/c power the bulb stays off because the diode trio prevents a/c current from entering that wire that leads to the bulb. When the diodes in the alternator go bad, the bulb will light because a/c current looks like a ground to 12v, thus completing the 12v circuit.
In many applications this wire acts to 'sense' the voltage supplied to the bulb, in order to determine charging voltage. Without a 12v source with a resistor as a bulb provides (or just a resistor I guess) the alternator may not charge correctly. Some computers directly control alternator voltage through this wire, so a 'dummy bulb' can be used in those applications to bypass using the computer to control alternator voltage. Maybe someone could elaborate on the field wire now...
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