An open-loop control system that provides control signals to active engine mounts to reduce or eliminate the transfer of engine vibration to a vehicle body structure, where the system uses only a crank signal from the engine as an input. The control system includes an instant crank speed variation sensing processor that receives the crank signal from the engine, where the crank signal is a pulsed signal including missing pulses as a result of teeth missing on the vehicle crank wheel. The crank speed variation sensing processor provides a measurement of the instant crank speed variation of the crank signal and minimizes an error in the measurement as a result of the missing pulses. The crank speed variation sensing processor outputs a crank speed sensing variation signal as a sine wave that identifies order content in the crank pulse signal.