What was found
APU FAULT LIGHT ILLUMINATED. ACFT GROUNDED. FOUND THE FOLLOWING FAULTS ON APU MAINTENANCE FAULT HISTORY: 49-11019, 49-41012. AND 49-41-1. APU WILL START FINE WITH EXTERNAL POWER BUT WILL SHUTDOWN ALONG WITH AC LOSING ELECTRICAL POWER AS SOON AS THE APU BUSSES ARE TURNED ON, EITHER. TRYING TO START THE APU ON BATTERY POWER ALONE, THE K8 RELAY ON THE SPCU RAPIDLY CLICKS. THE APU NEVER SPOOLS UP, "ELEC" FAULT COMES ON FOR THE STATIC INVERTER, THEN THE APU STOPS THE START CYCLE. WAITING FOR SPCU TO BE DELIVERED. REPLACED SPCU, REF MM 24-34-11. NO HELP, REINSTALLED ORIGINAL UNIT. REPLACED BOTH MAIN BATTERIES, REF MM 24-31-11. "ELEC" FAULT CLEARED. ALL OPS NORMAL IAW SAME. APU OPS TEST NORMAL, REF MM 49-11-00.