with an internal combustion engine 7 itself provided with a toothed wheel sensor 8 used for controlling the engine after shaping in a circuit 9 the output signal from the sensor 8 referred to as a u201c first u201d signal is applied to one input terminal 10 e1 of a switch 10 the toothed wheel sensor 8 is known to be reliable for speeds of rotation greater than the minimum measurement speed v m furthermore angular position estimator means 11 receive electrical signals representative of the voltage and current electrical characteristics of the alternator starter 4 these angular position estimator means 11 respond to said input signal voltage current to generate an output signal referred to as the u201c second u201d signal that is representative of the angular position of the rotor of the alternator starter the second signal is applied to another input 10 e2 of the switch 10 the position estimator means 11 are of a simplified type that is known to be reliable only at speeds below the maximum estimation speed v a the position estimator means may be constituted in the form of a calculator of simple structure possibly being incorporated in the engine controlling software the switch 10 has an output terminal 10 s which is connected to an input of the control means 5 of the alternator starter 4 the output terminal 10 s is also connected to means for generating a signal representative of an estimated speed value with its output being connected to a member 13 for actuating the moving contact.