I've seen the fiber washer between the field terminal nut and the generator housing get broken or misplaced and cause this problem. The nut grounds the terminal to the housing and puts the generator into uncontrolled output.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!