ACR MLG UNABLE TO MAINTAIN CABIN ALT ON ONE PACK MADE AN EMERGENCY DESCENT AND DIVERTED TO ALTERNATE.

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THE INITIAL PROB WAS A TAIL COMPARTMENT HIGH TEMP LIGHT. FOLLOWING EMER AND ABNORMAL PROC RESULTED IN SHUTTING DOWN ONE PACK. THIS SOLVED THE PROB; BUT LEFT US WITH A 1-PACK OPERATION. THE 1 PACK COULD NOT MAINTAIN CABIN ALT RESULTING IN CABIN ALT GOING ABOVE 10000'. THIS RESULTED IN AN EMER DSCNT DOWN TO A HABITUAL ALT; AND A DIVERSION TO BWI. OXYGEN MASKS WERE DEPLOYED AND PAX ADVISED TO USE THEM; WHICH THEY DID. FLT LANDED AT BWI W/O FURTHER INCIDENT. WHEN THE EMER DSCNT WAS INITIATED WE DID DECLARE AN EMER AND SQUAWKED 7700'. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: A LOOSE DUCT CONNECTION WAS THE SOURCE OF HEAT THAT TRIGGERED THE HIGH TEMP LIGHT IN THE TAIL COMPARTMENT. RPTR SAYS IT IS NOT AN UNCOMMON OCCURRENCE ON THIS ACFT WHEN OPERATING AT HIGH AMBIENT TEMP AND THAT TURNING OFF THE HIGH PRESSURE BLEEDS USUALLY CORRECTS THE PROB. ON THIS OCCASION THE FLT CREW HAD TO TURN OFF THE PACKS ALTERNATELY TO ISOLATE THE PROB AND THEY DID NOT NOTICE THE CABIN PRESSURIZATION LOSS UNTIL THE WARNING HORN SOUNDED AND THE MASKS DROPPED. IT WAS LATER DETERMINED THAT THE REMAINING PACK WAS NOT FUNCTIONING AT FULL OUTPUT AS EXPECTED AND THIS ALLOWED THE CABIN ALT IN INCREASE.