Abstract: An improved control method for a solenoid-operated hydraulic actuator for deactivating an engine valve mechanism characterizes the dynamic response of the mechanism based on a lumped parameter model of the solenoid, the hydraulic sub-system, and a locking pin mechanism actuated by a control pressure developed by the hydraulic sub-system. In response to a mode change request, constituent delay times associated with the solenoid, the hydraulic sub-system, and the locking pin mechanism are determined and summed to form an estimate of the overall delay time required to complete the requested cylinder deactivation. The solenoid activation is then scheduled based on the estimated delay time and a window of opportunity in the engine cycle for cylinder deactivation.