Anything over 15V (ideally 14.5V) is too much. Your regulator is toast OR you may have a grounding issue. Make sure the VR is grounded VERY WELL to the body with ground strap and the alternator is as well. If there is a disconnect between the VR and alternator, the VR will not be able to 'sense' the alternator's output and will be rendered useless giving you high voltage.