Alternator & Regulator: Operation

The voltage regulator controls field current with a Pulse Width Modulated (PWM) signal, measured in percent of duty cycle. The PWM signal continuously cycles on and off. Pulse width can vary within each cycle.

When the ignition switch is turned to the RUN position, the BCM applies voltage to the regulator for initial excitation, setting regulator in field strobe function, before the engine starts. As alternator RPM increases, the field strobe function is disabled, and normal regulation occurs.

The regulator switches field current on and off at a fixed frequency of about 400 cycles per second. By varying pulse width, correct average field current for system voltage control is achieved. At high speeds, field-on time may be as low as 10 percent. At low speeds with a heavy electrical load, field-on time may be as much as 90 percent.

The BCM monitors the regulator from the alternator field terminal. If the PWM duty cycle falls below 7 percent, the BCM senses a fault, and indicates a problem by lighting the warning lamp on the instrument cluster.