In car wash machine manufacturing, the assembly line is characterized by product diversity and workforce flexibility. The main challenge of operational control is to schedule assembly tasks to workstations by taking workforce availability and qualification into consideration. In order to help planners to better analyse, plan, and evaluate the assembly line, in this paper, we develop a framework in which simulation and optimization are applied. Particularly, a simulation-based optimization approach is used to reduce excessive overload peaks for workstations. At last, an example of a solution for our industrial partner is given to show the feasibility and the applicability of our framework.