The process and device for controlling at least one operating condition parameter (54) of working parts (45) of a combine harvester (1) has at least one sensing device (36, 40) for monitoring a crop tailings stream (31) and for generating a grain tailings signal (X) and a tailings flow rate signal (Y) for control of the operating condition parameters (54), such as blower speed, upper sieve mesh, lower sieve mesh, so that changes in the crop harvesting conditions can be reacted to rapidly and efficiently. The device includes an evaluating and display unit (39), which includes a controller (53) for automatically controlling operating condition parameters or for displaying control information produced by control algorithms (56 to 58), so that the operating condition parameters can be optimized according to the grain tailings signal (X) and tailings flow rate signal (Y).