A system is disclosed for processing vehicle powertrain torsional information resulting from vibration of the vehicle powertrain. A speed sensor produces a speed signal indicative of rotational speed of one of the powertrain components, and a control computer is operable to determine a magnitude of an Nth-order torsional component of vehicle powertrain vibration as a function of the speed signal. The control computer is further operable to execute either of a diagnostic routine relating to the Nth-order torsional component and a control routine controlling operation of the vehicle powertrain away from conditions at which the magnitude of the Nth-order torsional component exceeds a threshold magnitude if the magnitude of the Nth-order torsional component exceeds the threshold magnitude for at least a first predefined duration.