"What does that little red light that says ALT mean when it
comes on?" Very basically, it means that either the alternator
output voltage is lower than the battery voltage, or the battery
voltage is lower than the alternator output voltage. If the
light gets dimmer as you rev up the engine, then you most likely
have a problem with the alternator. If it gets brighter, then
the battery is most likely bad. That's all well and good, but
just exactly what does all that mean? To get a good idea, it is
first necessary to understand how an alternator works. You don't
need an engineering degree, just a basic understanding of the
general principles. Figure 1, below, is a block diagram, or a
"functional" diagram, of an alternator, and its connections to
the remainder of the automobile electrical system. Following the
figure is a description of the various components that make up
an alternator, and a description of how each operates to keep
the battery charged in your car. We'll start our tour of the
alternator where it