A B737-400 WITH A CTR TANK FUEL QUANTITY INDICATOR INOP PER THE MEL WAS FUELED IMPROPERLY. NEITHER FLT CREW NOR GND CREW KNEW THE ACTUAL FUEL IN THE CTR TANK.

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I WAS SENT TO ACFT X TO FUEL ACFT BECAUSE OF THE MEL CTR WING TANK INOP. WHEN I ARRIVED TO THE ACFT; THE FUELER WAS UNHOOKED AND I ASKED HIM WHAT HE WAS DOING AND HE SAID THE MECH SAID TO GO AHEAD AND FUEL; SO I WENT UPSTAIRS AND THE AVIONICS TECHNICIAN WAS MEL THE CTR TANK. I ASKED HIM HOW MUCH FUEL HE TOLD THE FUELER TO PUT IN THE TANK AND HE SAID 500 GALLONS; SO I TOLD MR X WE NEED 530 GALLONS. HE WENT DOWN TO TELL THE FUELER WE NEED 30 MORE GALLONS AND THE FUELER SAID HE PUT 560 GALLONS AND SHOWED HIM THE GAUGE ON THE PUMP CART. I TALKED TO THE PLT AND COPLT AND TOLD HIM WHAT HAPPENED AND SAID I CAN ONLY TAKE THE WORD OF THE AVIONICS TECHNICIAN THAT WAS WORKING THE ACFT THAT THE TANK WAS EMPTY. AFTER TALKING TO HIM ABOUT THE MEL THAT WAS BEING RE-ACCOMPLISHED AND THE DROP DEAD DATE THAT HE WAS WORRIED ABOUT; I CALLED MY LEAD AND SUPVR AND TOLD HIM THE PLT'S CONCERNS. AFTER THAT; THE PLT AGREED TO TAKE THE ACFT. MY CONCERN IS; I SHOULD NOT HAVE TAKEN THE WORD OF THE AVIONICS TECHNICIAN AND DE-FUELED ACFT AND STARTED OVER. MY OTHER CONCERN IS WHY THE FUELER DIDN'T TELL US HE HAD PUT FUEL IN THE TANK BEFORE WHEN HE DISCOVERED THE PROB.