WHEN THE COCKPIT PERFORMANCE COMPUTER FAILS TO PROVIDE TKOF DATA; FLT CREW OF B737-500 USES DATA SUPPLIED BY DISPATCHER. LATER DISCOVER THAT DATA WAS INCORRECT AND THAT TKOF WAS MADE APPROX 8000 LBS OVER ALLOWABLE RWY LIMIT.

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AT PUSHBACK; FO WAS UNABLE TO GET PERFORMANCE COMPUTER TO COMPUTE TKOF DATA. HE PASSED IT TO OUR JUMPSEATER. HE WAS ALSO UNABLE TO GET TO THE TKOF PAGE. I CALLED DISPATCH AFTER A QUICK GLANCE AT THE PERFORMANCE COMPUTER. DISPATCH GAVE US THE NUMBERS FOR RWY 25R. DATA WAS BLEEDS OFF. WE MADE MORE UNSUCCESSFUL ATTEMPTS TO GET THE PERFORMANCE COMPUTER TO CALCULATE THE NUMBERS ON THE TAXI OUT. WE MADE A NORMAL TKOF. AFTER LEVELOFF; I TRIED AGAIN TO GET THE DATA AND NOTICED THE ACFT WAS OVERWT FOR THE RWY (EVEN BLEEDS OFF). WE HAD FAILED TO NOTICE THAT OUR WT WAS BRACKETED. OUR ACTUAL WT WAS 119.2 AND THE RWY LIMIT WAS 111.8! I AM UNSURE HOW DISPATCH WAS ABLE TO CALCULATE SPDS V1 152; VR 155; V2 159 AND 3030 FT STOPPING MARGIN. MODIFY THE PERFORMANCE COMPUTER TO GIVE AN ERROR MESSAGE WHEN TRYING TO GET TKOF DATA WHEN OVERWT FOR THE RWY SELECTED. CURRENTLY; THE PERFORMANCE COMPUTER JUST SITS QUIETLY AND WILL NOT ADVANCE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER ADVISED THAT THE COCKPIT COMPUTER WAS ACTUALLY WORKING; BUT THAT WHEN THE DATA INPUT EXCEEDED LEGAL VALUES; THE PROGRAM SIMPLY REFUSED TO CONTINUE THE COMPUTATION PROCESS. BECAUSE IT DISPLAYED NO ERROR CODE; THE FLT CREW ASSUMED IT WAS A HARDWARE MALFUNCTION. THIS LED THEM TO EMPLOY THE ERRONEOUS DATA PROVIDED BY THEIR DISPATCHER.