A B747-400 ACFT REJECTED BY PILOT DUE TO HYDRAULIC OVERTEMP WARNING SYSTEM INOP. PILOT BELIEVES MEL PROCEDURE IS INADEQUATE AND UNSAFE.

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NO HYD OVERTEMP WARNING. AN UNCORRECTED ISSUE FOR 17 DAYS. PLANE REJECTED PREVIOUS DAY FOR HYD #4 OVERTEMP PRIOR TO TKOF; RETURN TO GATE; PLANE REJECTED BY CAPT FOR FURTHER FLT. NO REPAIRS OVERNIGHT; 18 HRS AVAILABLE. ONLY WORK DONE WAS TO MOVE HYD TEMP XMITTER FROM HYD #4 TO HYD #3. PROB FOLLOWED; WITH NO HYD TEMP ON EICAS; EICAS HYD OVERHEAT SYS ON; AND SYS FAULT ON. XMITTER SUPPLIES ALL 3 INDICATIONS OF HYD TEMP AND OVERTEMP WARNING FROM A SINGLE CARD PER WIRING PRINTS I HAVE. BLIND REDUNDANT; POOR DESIGN. MEL ALLOWED RELEASE WITH ZERO INDICATION OF HYD TEMP OR OVERTEMP FOR PLT. MEL NEEDS CHANGE SUBMITTED TO COMPANY MAINT OFFICE. PLANE SAT OVERNIGHT ABOUT 18 HRS; NO REPAIR. XMITTERS AVAILABLE AT OTHER LCL COMPANIES. NO PART ACQUISITION ATTEMPTED. NO REPAIRS PLANNED IN ZZZ IF PLANE LEFT ZZZ1 NOT PLANNING TO SEND PART ZZZ2 TO ZZZ. REFUSED PLANE FOR POLAR OPS; PLAN FLT OVER ARPTS. REFUSED PLANE ALSO UNLESS PART SHIPPED TO ZZZ FOR INSTALL PRIOR TO RETURN FLT. CHIEF PLT X AND DUTY MGR Y DID GREAT JOB OF ARRANGING FOR PART SHIPMENT TO ZZZ AND NOTIFIED OUTBOUND CAPT OF REPAIRS TO BE MADE. NO IRREGULAR PROCS FOR HANDLING ENG OVERHEAT CAUSED BY HYD SYS OVERHEATING AND HYD TEMP SYS OTS. MEL DOES NOT DISTINGUISH ALL 3 TEMP/OVERTEMP INDICATIONS INOP VERSUS ONLY 1 OR 2 AND PRINT A PROC FOR MONITORING REMAINING HYD TEMP DATA. MEL SHOULD NOT ALLOW ALL 3 HYD TEMP INDICATIONS INOP.