Voltage Regulator: Full Field Test
Test the alternators regulator with a full field test when the alternator has low output. This test will rule out the regulator by bypassing it and 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 the voltage regulator located on the power side of the field coil. 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. If the alternator produces low voltage, and when full-fielded suddenly produces maximum voltage, the regulator is at fault. If it still produces low voltage when full-fielded, the alternator itself is at fault.