AN EMB WITH AUTO PRESSURE SYS INOP AND DEFERRED PER THE MEL DIVERTED DUE TO UNABLE TO PRESSURIZE WITH MANUAL SYS. FOUND MEL SPECIAL PROC INOP'ING THE OUTFLOW NOT ACCOMPLISHED CORRECTLY.

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ACFT WAS ENRTE. THE CREW EXPERIENCED A FAILURE OF THE AUTOMATIC PRESSURIZATION SYS. MANUAL PRESSURIZATION WAS FULLY FUNCTIONAL SO THE FLT CONTINUED TO ZZZ. MAINT CTL WAS NOTIFIED AND I CONTACTED CONTRACT MAINT VENDOR. A&P MECH; AN APPROVED MECH ON THE VENDOR LIST; CAME TO THE ARPT AND WAS INSTRUCTED TO FOLLOW THE PROCS TO APPLY MEL 21-16 TO DISABLE THE AUTOMATIC PRESSURIZATION SYS PER THE MEL PROCS. THE MEL CONTAINS INSTRUCTIONS AND A DIAGRAM FOR THE PROCS AND AMM REFS WERE SENT TO PROVIDE DOCUMENTATION AND PROC FOR ENTERING THE EQUIP SPACE. UPON THE MECH'S RPT OF SUCCESSFUL COMPLETION OF THE PROC; I ISSUED MEL 21-16. ACFT DEPARTED ZZZ AND WAS FORCED TO DIVERT TO ZZZ1 BECAUSE THE CREW FOUND MANUAL PRESSURIZATION INOP DURING CLB AND THEY WERE UNABLE TO CTL CABIN ALT. CONTRACT MAINT AT ZZZ1 INVESTIGATED AND FOUND THAT THE MEL PROC HAD NOT BEEN CORRECTLY APPLIED. THE MEL CALLS FOR ONE LINE ON THE ELECTROMECHANICAL OUTFLOW VALVE TO BE DISCONNECTED. INSTEAD; THE CONTRACT MECH HAD APPARENTLY DISCONNECTED 2 LINES; THUS DISABLING PRESSURIZATION. NOTE: THE MEL CALLS FOR 1 LINE TO BE DISCONNECTED AND 2 CAPS TO BE INSTALLED. I SUSPECT THE MECH MISUNDERSTOOD THE WRITTEN PROC. TO CONTINUE; ZZZ1 MAINT CORRECTED THE MEL APPLICATION; PERFORMED AN OPS CHK OF THE MANUAL PRESSURIZATION AND THE ACFT WAS RETURNED TO SVC. I LATER CALLED AND SPOKE WITH MECH AND MADE HIM AWARE OF THE DIVERSION AND REVIEWED THE MEL PROC AND EXPLAINED THE ERROR AS WELL AS THE CORRECT PROC. HE CONCEDED THE ERROR AND NOW UNDERSTANDS THE CORRECT PROC.