B737 CREW HAD THE CABIN ALT WARNING HORN SOUND ON DEP.

Narrative

WHILE CLBING ON DEP AT 11500 FT; THE ALT WARNING HORN ACTIVATED. SILENCED HORN AND REALIZED THE CABIN WAS AT 10500 FT AND CLBING APPROX 800-1000 FPM. LEVELED OFF AT 12500 FT AND REQUESTED A 180 DEG TURN BECAUSE OF 14000 FT MEA ON AIRWAY. ATC APPROVED TURN; AND CLRED US DIRECT VOR. WE RAN ALL CHKLISTS AND PREVENTED THE CABIN FROM CLBING ABOVE 12000 FT. DSNDED TO 10000 FT PER ATC INSTRUCTIONS. THE L PACK WAS INOP PER MEL; AND R PACK WAS CONFIGURED PER MEL WITH ISOLATION VALVE CLOSED. DUCT PRESSURE WAS APPROX 45 PSI; AND THE OUTFLOW VALVE WAS FULLY CLOSED. LANDED ZZZZ WITHOUT INCIDENT. THE L PACK HAD MULTIPLE WRITE-UPS DURING PREVIOUS WK. I HAD REQUESTED AN ACFT SWAP 45 MINS PRIOR TO DEP. THIS REQUEST WAS DENIED. I RECOMMEND THAT WE NOT DISPATCH A PACK INOP ACFT TO A HIGH ALT ARPT; NOR S OF THE BORDER. ZZZZ HAS ONLY CONTRACT MAINT WITH MECHS WHO ONLY SPEAK SPANISH. SUPPLEMENTAL INFO FROM ACN 622389: OUTFLOW VALVE FULL CLOSED. RETURNED TO LAND ZZZZ. THE L PACK HAD MULTIPLE WRITE-UPS AND THIS ACFT; IN MY OPINION; SHOULD NOT HAVE BEEN DISPATCHED TO A HIGH ALT ARPT IN MOUNTAINOUS TERRAIN WITH ITS PERSISTENT AIR CONDITIONING PROBS. TROUBLESHOOTING WAS A JOKE WITH SPANISH SPEAKING CONTRACT MAINT THAT CLRLY DIDN'T KNOW THE SYS.