Abstract: A detector monitoring the function of the stator and rectifying diodes in a multiphase alternator battery charging system is disclosed. A circuit combines individual phase signals from the alternator to form an artificial neutral at which a normally symmetric wave signal is present. A first comparison means detects deviations in the normally symmetric wave signal greater in value than a first threshold voltage level. A second comparison means detects deviations in the normally symmetric wave signal lesser than a second threshold voltage level. Means coupled to the first and second comparators produce an output signal whenever the normally symmetric wave signal deviates above the first threshold voltage level or below the second threshold voltage level. The detection of assymmetry in the normally symmetric wave signal indicates a fault in the stator or rectifying diodes of the alternator.