What was found
ON THE GROUND WITH THE APU RUNNING AND GENERATOR ONLINE, THE COCKPIT LIGHT STARTED GETTING BRIGHT AND THE AVIONICS FAN STARTED SPEEDING UP. I SAW THE VOLTS RAPIDLY CLIMB, AND THE GCU DID NOT KICK THE GENERATOR OFFLINE UNTIL APPROX 33 VOLTS. AN UNUSUAL SMELL WAS ALSO NOTED FROM UNKNOWN SOURCE. REMOVED AND REPLACED APU GCU IAW MM 24-31-02. OPERATIONAL CHECK GOOD. PERFORMED GENERAL OPERATION ON SYSTEMS IN COCKPIT WITH REF TO A/C FLIGHT MANUAL CHECKLIST AND OPERATIONAL CHECK GOOD. NO UNUSUAL SMELLS NOTED.