| The bulb limits the current draw from the battery. When the alternator is starting up, the field coil current comes from the battery. Simply connecting the battery across that coil with no additional resistance would draw to much current from the battery. Alternators have a circuit such that when the battery current is no longer needed (the alternator has spun up and is putting out more voltage than the battery) an arrangement of diodes cuts off the battery field coil circuit and then the light goes out. When the alternator is self-energizing, there is a feedback mechanism then at play which controls the field current, keeping the alternator voltage level: without it, the voltage would rise uncontrollably: more voltage from alternator -> more current through field coil -> greater induction of voltage. Before the alternator is self-energizing, what regulates the field current is that bulb. So that is to say, if you see the light on all the time when the alternator is spinning, that warns you that it's not actually working. It is not it's not putting out enough voltage (perhaps none at all) to overpower the battery circuit and become self-energizing. This could be because the rotor coils are burned out, or other problem: wiring or whatever. Since that bulb is off most of the time, we don't expect to have to change it very often. If an alternator bulb has burned out, and the unit is not very old, that could indicate a problem. |
Going back and forth with alternator mechanic he asked me to pull the instrument panel and inspect the bulb.