A self-propelled vehicle alternator used both as a generator and as an electric motor for starting the engine of the vehicle. A coil-carrying rotor and a multiphase stator connected to a bridge of diodes and switches for rectifying and controlling the phases cooperate for selective operation as a generator or starting motor. A control unit for the switches recognizes a code signal transmitted by a code transmitter and controls the phases of the stator to cause the alternator to operate as an electric motor and start the engine only if it receives a code signal from the transmitter authorizing starting.