This is a common car/truck/bus type alternator system, any books on those should help you unerestand how this system works. Local lib?
http://www.ibmwr.org/r-tech/airheads...ing.info.shtml
You could have a blown globe, alternator rotor open, brushes worn out or a broken/dissconnected wire. In that order..
Still want a book ?
http://www.motoelekt.com/goodstuff.htm