The invention relates to a device and a method for controlling a shiftable planetary gear mechanism in a belt drive of an internal combustion engine in a motor vehicle having a sun gear which is fixedly connected to a belt pulley, and a ring gear which can be connected, fixedly with respect to the housing by means of a brake which is activated by an actuator which is electrically operated and is effective along a distance which is monitored by a relative travel sensor, as well as a web which is connected to a crankshaft of the internal combustion engine and rotatably accommodates planetary gears, wherein arranged between the sun gear and the web is a freewheel which can be shifted and can be rolled over in both directions and is shifted by the same actuator along the distance.; In order to be able to assign the relative sensor signals of the travel sensor to an unambiguous travel position of the actuator, the distance from the actuator is determined by means of relative distance signals of the travel sensor, and the relative travel sensor is referenced to an absolute value by means of at least one reference movement of the actuator, wherein electrical behaviour of the actuator is evaluated at least at a predefined travel point on the distance, and the at least one predefined travel point is predefined as an absolute value as a function of the electrical behaviour of the actuator.