AN MD80 CAPT RPTED THAT HIS ACFT WAS UNABLE TO PRESSURIZE AFTER DEPARTING ORD DUE TO THE AFT CARGO DOOR BEING CLOSED IMPROPERLY.

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UNABLE TO PRESSURIZE ON CLBOUT FROM ORD. UPON ARR AT STL; FOUND PAX LUGGAGE LOCK WEDGED BTWN AFT CARGO DOOR FRAME AND DOOR. FRONT PART OF DOOR WHERE LIGHT SENSOR SWITCH IS WAS FLUSH. AFT PART OF DOOR WAS OPEN 2-4 INCHES. STL PERSONNEL TOOK PICTURES. ON DEP FROM ORD; WE LEVELED OUT AT 5000 FT. FO RAN THE PRESSURIZATION FLOW LIGHT/UNSCHEDULED CABIN CLB CHKLIST. I THEN TOOK MY TURN AT RUNNING THE CHKLIST. WE CLBED UP TO 8000 FT AND STILL HAD PROBS. ACR MAINT VERIFIED THAT IF THE PACKS ARE WORKING; THE OUTFLOW VALVE IS CLOSED AND WE ARE NOT PRESSURIZING; THERE IS A HOLE IN THE AIRFRAME AND WE SHOULD LAND. DISPATCH TOLD US TO GO BACK TO ORD. WE WERE VECTORED FOR ORD; THEN PLACED IN HOLDING. WX MOVED INTO ORD AND WE WERE GIVEN AN INDEFINITE HOLD. WE WERE VECTORED FOR THE APCH TO ORD. 2 ACFT AHEAD OF US ON FINAL WENT AROUND DUE TO WINDSHEAR. WE TOLD APCH WE NEEDED A 240 DEG HDG AWAY FROM THE WX. APCH TOLD US TO CONTINUE AHEAD AND CALL TWR. RATHER THAN FLY THROUGH PURPLE AND RED RETURNS; I HEADED 240 DEGS. TWR SENT US TO DEP. DISPATCH SAID THEY WANTED US IN STL; BUT THEY COULD NOT GENERATE ANY DATA FOR A FLT AT 6000 FT. ORD DEP HAD SAID ALL ACFT GOING AROUND HAD TO GO TO THEIR ALTERNATE. WE GUESSED; BASED ON OUR FUEL FLOW AND DISTANCE TO STL; THAT WE COULD GO THERE. BESIDES WE HAD OTHER ARPTS TO LAND AT ALONG THE WAY.