Field Relay
On vehicles equipped with a charge indicator light, when the ignition switch is turned to the "ON" position, current flows from the battery through the indicator light, through the voltage regulator, and through the alternator field coil to ground. This current flow illuminates the indicator light and energizes the alternator field. When the engine begins to run and the alternator begins to supply voltage and current, voltage is delivered to the field relay coil. This closes the relay contacts and completes a circuit that provides an alternate path for system voltage to the regulator. Voltage is also applied to the opposite terminal of the indicator light. The system voltage is now applied to both sides of the indicator light, and the voltage difference across the light is zero. When the voltage is zero, no current flows through the bulb and it goes out. As long as the alternator is operating the field relay contacts remain closed.
On vehicles with an ammeter and field relay, current flows from the battery, through the ignition switch, to the field relay coil. When the coil is energized, the relay contacts are closed. The battery current then flows through the relay contacts, through the voltage regulator, and to the alternator field. As long as the ignition switch is "ON" the relay contacts remain closed.
On vehicles with an ammeter and no field relay, field current flows from the battery, through the ignition switch directly to the voltage regulator. Current flows through the regulator to the alternator field.