Voltage Regulator: Full Field Test
Test the alternator regulator with a full field test when the alternator has low output. This test bypasses the regulator, energizing the rotor with unregulated voltage.
A Type-A circuit has the voltage regulator on the ground side of the field coil. A Type-B circuit has a voltage regulator on the field coil's power side. In many late-model vehicles, the regulator is controlled by the PCM. They're controlled by a duty cycle and best diagnosed with a scan tool. Always check with the manufacturer for the steps required to field test a circuit.
Some alternators have a tab on the back that, when grounded to the case, full-fields the alternator for maximum voltage. The regulator is at fault if the alternator produces low voltage and, when full-fielded, suddenly produces maximum voltage. The alternator itself is bad if it still produces a low voltage when full-fielded.