A method comprises the transmission to a regulator (8) of the alternator of width-modulated pulses (PWM1) exhibiting a duty ratio representative of a setpoint value (Vref) continuously included between a first reference duty ratio (DC1) and a second reference duty ratio (DC2), and the detection of these pulses by the regulator so as to generate the setpoint value. The method also comprises the transmission to the regulator of other width-modulated pulses (PWM2) exhibiting a specific duty ratio (EXOF_R) which is less than the first reference duty ratio (DC1) and ties between a low duty ratio (DCL) and a high duty ratio (DCH), and the detection of these other pulses by the regulator, in a first step, and the generation of an opening signal (EXOF) for a circuit for excitation of the alternator, in a second step, following this latter detection.