A method has been developed for obtaining engine speed and diagnostic information for a diesel engine without mechanical or electrical connections to the engine. This non contact technique has the potential for making diesel engine power measurement and fault detection extremely simple and rapid. The method employs a single transducer held near the engine and special purpose circuitry which extracts the engine speed information from the signal and performs spectral analysis for diagnostic purposes. Circuit outputs are engine speed, a digital pulse train with repetition rate proportional to engine speed, and a harmonic ratio which sensitively indicates the degree of non-uniformity among engine cylinders.By this method diesel engines with minor compression losses or fuel injection faults can readily be identified without vehicle contact in less than one minute of testing. With additional signal processing, the power producing capability of the engine can also rapidly be evaluated.