In current SAE J1930 terminology, the "alternator" is now being referred to as a "generator." Of course, this isn't news for many technicians because many vehicles have always been equipped with "generators" for several years. By either name, the alternator's job has definitely become more complex during the past decade. Years ago, the typical vehicle could "live" very happily with 40 amperes of electrical current, which was enough to power the ignition, fuel and lighting systems. Today, that threshold is rapidly advancing toward 100 amperes of current, which, among other things, must power innovations like heated seats, heated glass, sound systems, electronic power steering, air injection pumps, steering, braking and valvetrain controls.

Even more critical, the advent of electronic automatic overdrive transmissions is keeping engine speeds well below 2,000 rpm at highway speeds. If the alternator can't keep the battery fully charged during periods of high accessory use at low engine speeds, the battery will become partially discharged. When battery voltage drops below 9.6 volts during cranking, the result may be that the electric fuel pump and electronic fuel injection fails to supply enough fuel and the electronic ignition enough spark for a quick start.

Precise voltage control is also critical because charging rates in excess of 17 volts may damage delicate electronics and on-board computer systems. More recently,...