A DC9-30 IN CRUISE AT FL330 DECLARED AN EMER AND DIVERTED DUE TO LOSS OF CABIN PRESSURIZATION CAUSED BY R AIR CYCLE MACHINE DUCT SEPARATION.

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INBOUND TO ZZZ ARPT DURING DSCNT; CABIN ALT WOULD CLB WITH THRUST LEVERS AT IDLE. AUTOPRESSURIZATION WAS MEL'ED BY MAINT. MANUAL PRESSURIZATION WAS REQUIRED AND WAS PERFORMING THROUGHOUT CLBOUT AND TO CRUISE ALT. ESTABLISHED AT CRUISE; CABIN WAS AT 8000 FT AND WAS STILL CLBING AT 200 FPM. L AND R PACK PRESSURES WERE NORMAL; BUT L PACK WAS GREATER THAN 150 DEGS TEMP; WHILE R PACK TEMP WAS BELOW 30 DEGS. ATTEMPTS TO BAL THE TEMP TO MORE NORMAL TEMPS WERE NOT SUCCESSFUL; IN EITHER THE AUTO OR MANUAL TEMP CTL MODES. EVENTUALLY THE R PACK SHUT DOWN AND WE REQUESTED LOWER ALT. ATC GAVE US FL260; PASSING FL290 THE L PACK SHUT DOWN. WE DONNED OUR OXYGEN MASKS. I FLEW AND HANDLED THE RADIO WITH ATC WHILE THE CAPT PERFORMED THE RED BORDER CHKLIST AND COMPANY PROCS. WE DECLARED AN EMER; REQUESTED A LOWER ALT DUE TO LOSS OF PRESSURIZATION AND RADAR VECTORS TO RETURN TO ZZZ1 ARPT. ZZZ1 CTR CLRED US TO FL240 AND I RESTATED WE NEEDED LOWER. ZZZ1 CTR STATED THAT FL240 WAS ALL WE COULD GET NOW. BEFORE I COULD RESPOND; ANOTHER ACFT TOLD ZZZ1 CTR THAT I HAD DECLARED AN EMER; AT WHICH TIME ZZZ1 CTR CLRED US TO 10000 FT AND A HDG FOR ZZZ1 ARPT. AT 10000 FT WE REMOVED OUR MASKS; OPENED THE RAM AIR TO COOL THE CABIN; RAN ALL CHKLISTS TO PREPARE FOR LNDG. IN HINDSIGHT; WE SHOULD HAVE REFUSED TO TAKE THE ACFT ONCE THE AUTOPRESSURIZATION WAS MEL'ED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED ON THE GND IN ZZZ THAT MAINT FOUND THE R AIR CONDITIONING PACK AIR CYCLE MACHINE INLET AND OUTLET DUCTING NOT CONNECTED. THE RPTR SAID THE AIRPLANE AUTO PRESSURIAZATION SYSTEM WAS DEFERRED PRIOR TO THIS TRIP. THE RPTR STATED MAINT HAD WORKED ON THE R PACK AND APPARENTLY NEVER FINISHED THE JOB.