What do you mean by "keeps coming on"? How often and for how long. Any testing must be done while the light is on, otherwise everything is working and there's no defect to be found. A real common cause of intermittent problems with the Nippendenso generators is worn brushes. When used on Chrysler products they can be replaced for about ten dollars. As they continue to wear the warning light will turn on more often and for longer periods of time. That will make it easier to diagnose.