The present invention relates to a method for improving the operation of a grain cleaning system in a combine harvester characterized in that the method comprises the step of adjusting the position of the upper sieve in the grain cleaning system upon registering a deviance from a threshold value for crop material load on the upper sieve such that the material return flow is adjusted. The present invention further relates to a sieve overload control algorithm capable of activating an optimal sieve control action upon registration of a deviance from a threshold value for crop material load on the upper sieve of a grain cleaning system.