A B747 IN CARGO CONFIGN DIVERTED DUE TO INSUFFICIENT FUEL IN #3 MAIN TANK.

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UPON LEVEL OFF AT FL310; FE DISCOVERED #3 MAIN TANK FUEL QUANTITY GAUGE STUCK AT 61000 LBS. SINCE #3 AND #2 ARE IDENTICAL; HE DID SOME TROUBLESHOOTING AND SWAPPED QUANTITY GAUGES. UPON DISCOVERING THAT THE PROB WAS NEITHER GAUGE; HE SWAPPED THEM BACK TO ORIGINAL POS. #3 GAUGE THEN STUCK AT 46000 LBS. ABOUT 4 1/2 HRS INTO PLANNED 6 HR FLT; #3 FORWARD BOOST PUMP LOW PRESSURE LIGHT CAME ON. ACFT ATTITUDE WAS APPROX 2 1/2 DEGS NOSE UP. IN ORDER TO CHK FUEL QUANTITY IN #3 TANK; #3 TANK JETTISON/TRANSFER PUMPS WERE TURNED ON. LOW PRESSURE LIGHTS CAME ON FOR THOSE PUMPS ALSO; INDICATING FUEL QUANTITY IN THAT TANK TO BE BELOW STANDPIPE LEVEL. QUANTITY GAUGING SYS FOR #3 TANK HAD BEEN WORKED ON AT HOME STATION ON FEB/XX/96; CHANGING SENSING HARNESS AND OTHER COMPONENTS. THERE WERE NO LOGBOOK ENTRIES ABOUT THIS SYS SINCE THAT DATE. DEST WAS JFK; WHICH WAS RPTING INST WX CONDITIONS. UPON DISCUSSION AMONG THE CREW; AND PHONE PATCH WITH COMPANY DISPATCH AND MAINT CTL; FLT DIVERTED TO ORD. WE LANDED WITH 31600 LBS; 2200 LBS IN #3 MAIN TANK.