A louver position sensing system for a sieve and chaffer of a combine harvester. One system provides that at least one sensor is in actual, physical contact with one or more louvers of the sieve and chaffer. Another system provides that a one or more magnet holders are mounted on louvers and, spaced away, sensors sense magnets in the magnet holders to determine the rotational position of the louvers. Either system allows for accurate, on-the-fly adjustment of the louvers in order to maximize the efficiency of operation of the sieve and chaffer. Preferably, the sensing systems are configured such that sensed position of the louvers is broadcast on the CAN bus of the combine harvester. As a result, the position information can be used to dynamically adjust the openings between the louvers of the sieve and chaffer to achieve more efficient grain cleaning as the machine and field variables change.