Alternator & Regulator: Operation

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

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

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 proper system voltage control is achieved. At high speeds, on time may be 10 percent. At low speeds, with a heavy electrical load, on time may be as much as 90 percent.

BCM monitors regulator from alternator field terminal. If PWM duty cycle falls below 7 percent, BCM will sense a fault and indicate a problem, illuminating ELECTRICAL PROBLEM telltale.