Abstract: A control system (10) for a varible displacement internal combustion engine (12) is disclosed as including a sensor (14) for sensing intake manifold vacuum and a control unit (20) for operating valve deactuators (22a, b, c, and d) of the engine with a hysteretic action in response to the vacuum sensed so as to prevent oscillatory on-off operation of the valve deactuators. In its preferred construction, the control unit includes switches (28a, b, c, and d) for operating the valve deactuators and a switch operator (32, 34) including a lost motion connection that provides the hysteretic operation of the valve deactuators. The switch operator preferably includes rod (32) that is movable in response to the vacuum sensed and a switch operating member (34) slidably mounted on the rod and moved by stops (36, 38) to provide the lost motion connection. Electrical, fluid, and mechanical systems are all capable of operating valve deactuators by the hysteretic operation disclosed.