A B767-400 WAS DISPATCHED IN EXTENDED RANGE OVERWATER OPS WITH THE HYD MOTOR DRIVEN GENERATOR INOP IN CONFLICT WITH THE MEL.

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I AM A MECH FOR ACR X AIRLINES. WE ACCOMPLISH THE ETOPS PREFLT CHK HERE IN ZZZ FOR ACR Y AIRLINES. WHILE ACCOMPLISHING THE ETOPS PREDEP CHK PER ACR Y PAPERWORK; DISCOVERED THE HYD MOTOR GENERATOR WAS SHOWING NO INDICATION OF VOLTS OR FREQS. TALKED WITH THE ACR Y LINE MAINT CTLR. HE ASKED IF WE COULD TROUBLESHOOT THIS PROB FURTHER. WE ADVISED HIM WE COULD SINCE WE HAD ANOTHER 3-4 HRS UNTIL DEP. DISCOVERED THE HYD MOTOR GENERATOR WAS TURNING WHEN WE ACCOMPLISHED THE HYD MOTOR GENERATOR TEST; BUT STILL NO VOLTS AND FREQS. ALL OTHER COCKPIT INDICATIONS WERE NORMAL. AFTER HAVING ANOTHER CONVERSATION WITH THE ACR Y MAINT CTLR; HE ASKED US TO CHK FOR VOLTAGE AT THE 'HYD GENERATOR AC VOLT SENSE' CIRCUIT BREAKER (C846) ON THE P65 PANEL. WE ACCOMPLISHED THIS AND FOUND THAT WE DID HAVE 115 VOLTS WHEN ACCOMPLISHING THE HYD MOTOR GENERATOR TEST. PER THE ACFT SCHEMATIC MANUAL; THIS BREAKER GOES TO THE R DISPLAY CTL UNIT PER 24-28-01 SHEET 1. THIS IN TURN DETERMINED THAT THE R DISPLAY CTL UNIT (M7242) WAS NO GOOD. AFTER AGAIN CONFERRING WITH THE ACR Y MAINT CTLR; HE TOLD ME TO DEFER THE HYD MOTOR GENERATOR AS INOP PER THE MEL SINCE THIS IS WHAT WAS NOT WORKING PER THE ETOPS PREDEP CHK. AFTER TAKING A FEW DAYS OFF AND TESTING A COUPLE MORE ACFT; I FELT THIS WAS NOT PROPER. I THOUGHT THE R DISPLAY CTL UNIT SHOULD HAVE BEEN DEFERRED. AFTER MORE RESEARCH OF THE MEL; I DO NOT BELIEVE THE DISPLAY CTL UNIT CAN BE DEFERRED. THEREFORE; THE ACFT SHOULD HAVE BEEN GNDED UNTIL REPAIRED.