A noninvasive method for detecting failure of brush-less exciter rotating diodes is presented in this paper. Diodes can fail in one of two ways: either open circuit or short circuit. It is shown by actual test, on a 31.5-kVA alternator, that the exciter field current waveform changes distinctly when a diode fails. Harmonic analysis of the exciter field current waveform is performed. The results of this analysis form the basis of a method for real-time detection of diode failure on a microprocessor-based platform.