Method for controlling a regulation device of a motor vehicle alternator slaving a DC voltage (B+, B-) of an onboard electrical network powered by said alternator to a predetermined setpoint value (Vref) by controlling an excitation circuit, said method, in a first nominal mode of operation of said alternator, comprising the steps of: - transmitting to said regulation device by means of a communication line first width-modulated pulses exhibiting a predetermined period and a setpoint duty cycle representative of said setpoint value (Vref) continually lying between a first reference duty cycle and a second reference duty cycle; and - detecting said first pulses by said regulation device and generating said setpoint value (Vref); and in a second mode of operation without power generation from said alternator, comprising the steps of: - transmitting to said regulation device second width-modulated pulses exhibiting said predetermined period and a specific duty cycle (EXOF_R) lower than said first reference duty cycle and lying between a low duty cycle (DC L) and a high duty cycle (DC H); - detecting said second pulses by said regulation device in a first step (22, 23, 24) and generating an opening signal (EXOF, for said excitation circuit in a second step; characterized in that in the second mode of operation without power generation, said method also comprises the steps of: - detecting an activity of said communication line prior to said first step (22, 23, 24); - if said communication line is active, calculating averages (DCav k) of current duty cycles (DCn) of a plurality of said first or second pulses by disregarding a first predetermined number of the first of said current duty cycles (DC 0, DC 1, DC 2, DC 3) received, and comparing said averages (DCav k) to said low duty cycle (DC L) and high duty cycle (DC H) during said first step (22, 23, 24); and - generating said opening signal (EXOF) during said second step if a second predetermined number of said averages (DCav k, DCav k-1, DCav k-2) lie between said low duty cycle (DC L) and said high duty cycle (DC H). - transmitting to said regulation device by means of a communication line first width-modulated pulses exhibiting a predetermined period and a setpoint duty cycle representative of said setpoint value (Vref) continually lying between a first reference duty cycle and a second reference duty cycle; and - detecting said first pulses by said regulation device and generating said setpoint value (Vref); and - transmitting to said regulation device second width-modulated pulses exhibiting said predetermined period and a specific duty cycle (EXOF_R) lower than said first reference duty cycle and lying between a low duty cycle (DC L) and a high duty cycle (DC H); - detecting said second pulses by said regulation device in a first step (22, 23, 24) and generating an opening signal (EXOF, for said excitation circuit in a second step; characterized in that - detecting an activity of said communication line prior to said first step (22, 23, 24); - if said communication line is active, calculating averages (DCav k) of current duty cycles (DCn) of a plurality of said first or second pulses by disregarding a first predetermined number of the first of said current duty cycles (DC 0, DC 1, DC 2, DC 3) received, and comparing said averages (DCav k) to said low duty cycle (DC L) and high duty cycle (DC H) during said first step (22, 23, 24); and - generating said opening signal (EXOF) during said second step if a second predetermined number of said averages (DCav k, DCav k-1, DCav k-2) lie between said low duty cycle (DC L) and said high duty cycle (DC H).