Alternator & Regulator: Operation

The voltage regulator controls the field with a Pulse Width Modulated (PWM) signal, measured in duty cycles. A PWM signal continuously cycles on and off; the on time can vary within each cycle.

When the ignition switch is turned to the RUN position, before engine is started, the BCM applies voltage to the regulator. The regulator is now in field strobe function and applies a small percentage of the duty cycle to the field winding, which produces a magnetic field. As alternator RPM increases, the field strobe function is disabled and normal regulation occurs.

When field current is on, the regulator switches the field current on and off at a fixed frequency of about 400 cycles per second. By varying the overall time field current is on and off, correct average field current for proper system voltage control is achieved. At high speeds, the time on may be 10 percent. At low speeds, with a heavy electrical load, the time on 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 on time, the BCM will sense a fault and indicate a problem, illuminating the ELECTRICAL PROBLEM telltale.