Drive assembly for driving a rotary member, in particular a combustion engine water pump shaft

In a drive assembly (8; 52) for driving a pulley (10) of a water pump (7) of a combustion engine (1), a movable supporting arm (15) is fitted idly with a drive wheel (16), and is loaded by a torsion spring (30) to bring the drive wheel (16) into angular engagement with the pulley (10) fitted to the shaft of the pump (7), and with a drive belt (5) of the engine (1); an actuating device (58, 81; 22, 47) being activatable to exert a force in opposition to that exerted by the torsion spring (30), and detach the drive wheel (16) from the pulley (10).