Although checking charging voltage at the battery terminals is the most popular method for testing an alternator, a charging voltage test seldom reveals less common problems like open or shorted alternator diodes, intermittent high or low charging rates, charging system wiring problems, intermittent cranking/no start complaints or intermittent dead battery complaints. In addition, testing charging voltage at the battery terminals does not accurately indicate the condition of a load-sensing or computer-controlled charging system.

ALTERNATOR BASICS
Early in the nineteenth century, Michael Faraday discovered that electricity flows through a copper wire as it passes through a magnetic field. Alternating current is produced as the wire passes through the north and south poles of the magnetic field, and the polarity of the current changes from positive to negative.

Modern alternators use a rotating magnet called a rotor to create a rotating magnetic field that is "cut" by three windings of copper wire. A positive diode is attached to one end of each field wire and a negative diode is attached to the opposite end to "rectify" the alternator's electrical output from alternating to direct current.

The rotor's magnetic field is created by passing a variable electrical current through a carbon brush riding against a copper slip ring that, in turn, is attached to a winding of copper wire in the rotor. Another slip ring and br...