a dishwasher is provided with a valve for alternating flow to upper and lower spray arms during washing . the valve is controlled by periodic interruption of the pump for certain lengths of time . an electromechanical timer has a main cam wheel and an auxiliary cam wheel . the main cam wheel controls the pump and other parts of the dishwasher . the auxiliary cam wheel overrides the main cam wheel control of the dishwasher to provide the periodic lobes .