What was found
ENROUTE, THE APU EXPERIENCED AN OVERSPEED CONDITION RESULTING IN AN AUTO SHUTDOWN OF THE APU. THE NR 1 ENGINE GENERATOR/CSD WAS ALREADY ON MEL (DUE TO LEAKING), SO THE FLIGHT DIVERTED. FLIGHT LANDED WITHOUT FURTHER INCIDENT. NO EMERGENCY WAS DECLARED. MAINTENANCE REMOVED AND REPLACED THE APU ELECTRONIC 3 SPEED SWITCH. OPERATIONAL CHECK GOOD. ACCOMPLISHED SUCCESSFUL INFLIGHT START OF APU. CLEARED AND REMOVED MEL ON NR 1 ENGINE GENERATOR/ CSD BY REPLACING THE NR 1 ENGINE CSD (PN 735511A). OPERATIONAL CHECK AND LEAK CHECK GOOD.