AN EMER DSCNT WAS MADE BY A B737-300 FLT CREW WHEN LOSING CABIN PRESSURE AT FL290; WITH A RETURN LAND AT PHX; AZ.

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CLBING OUT OF 26000 FT TO 27000 FT MSL; THE CABIN ALT WARNING HORN SOUNDED AND I NOTICED THE ACFT CABIN AT 10500 FT (ROUGHLY) AND IMMEDIATELY INITIATED A RAPID DSCNT THAT WAS COORDINATED WITH ATC. THERE WERE NO OTHER WARNINGS OF ANY TYPE (IE; AUTO FAIL LIGHT; PRESSURIZATION BUMP; NOTHING). WE WERE CLRED TO 12000 FT AND NOTICED THE CABIN WAS BASICALLY COMING DOWN SLOWLY FROM ABOUT 10500 FT TO 9000 FT. WE NOTICED THE OUTFLOW VALVE WAS FULL OPEN AFTER WE HAD ALREADY GONE TO MANUAL AC. WE MOVED THE SWITCH TO THE CLOSED POS AND VERIFIED THAT BOTH BLEED SWITCHES AND BOTH PACK SWITCHES WERE IN THE ON POS. WE BECAME CONCERNED THAT WE COULD NOT CTL THE OUTFLOW VALVE AT THIS TIME. CAPT IMMEDIATELY STARTED THE APU AND PUT IT ON THE L PACK WITH THE ISOLATION VALVE CLOSED. AT THIS TIME; THE ACFT BEGAN TO START PRESSURIZING VERY QUICKLY WITH A NOTICEABLE PRESSURE BUMP FELT IN OUR EARS. WE SELECTED MANUAL DC AND WORKED THE OUTFLOW VALVE TO HELP CORRECT THE RUSH OF AIR. WE NOW KNEW WE HAD CTL OF THE PRESSURIZATION AND SELECTED STANDBY MODE WITH THE PIP SET AT 300 PSI AND THE PRESSURE SPIKE BECAME A LOT MORE COMFORTABLE. CONTINUED DOWN TO 12000 FT MSL BY ATC AND UNEVENTFULLY DSNDED TO 10000 FT MSL UNTIL THE CABIN RETURNED TO NORMAL. THEN; WE CLBED BACK UP TO 12000 FT MSL AND PROCEEDED BACK TO PHX AT THIS ALT DUE TO THE VERY ROUGH AIR AT 10000 FT MSL. I MADE AN UNEVENTFUL LNDG AT PHX. AT THE GATE; WE WERE NOTIFIED THAT A FEW PEOPLE HAD EXPERIENCED AIR SICKNESS DUE TO THE TURB. WE HAD HEARD THAT ONE FEMALE HAD MENTIONED SHE MAY HAVE FAINTED AT SOME TIME; BUT DID NOT INFORM ANYONE ABOUT THIS UNTIL REACHING THE GATE. WE BOTH HAD NO WARNING OF ANY TYPE OF PROB WITH THE PRESSURIZATION; EITHER FELT OR SEEN. WE BOTH HAD CONSIDERED USING OUR OXYGEN MASKS; BUT THIS WAS NOT A RAPID DECOMPRESSION AND WE HAD VERIFIED TO EACH OTHER THAT; INDEED; BOTH BLEED AND PACK SWITCHES WERE ON THE ENTIRE FLT. WE WERE VERY STUMPED AS TO WHY OR WHAT WAS HAPPENING. I DO NOT REMEMBER IF THIS WAS; OR WAS NOT; A NO BLEEDS TKOF. IT WAS THEN THE AFTER TKOF CHK WAS COMPLETED CORRECTLY; BY THE FACT THAT WE BOTH DISCUSSED THAT SWITCH POS WERE CORRECT FOR NORMAL FLT. UPON ARR AT THE GATE; MAINT CAME ON BOARD VERY QUICKLY AND WE TOLD THEM WHAT HAPPENED. THE MAINT PERSON INFORMED US THAT A FEW DAYS PRIOR THAT THIS ACFT HAD A HISTORY; THAT IT CAME OUT OF SAN TO PHX A FEW DAYS PRIOR AND THE CREW STAYED AT 19000 FT MSL BECAUSE THEY COULD NOT HOLD PRESSURIZATION. HE ALSO MENTIONED TO HIS CO-WORKER; IN FRONT OF US; THAT HIS CO-WORKER SHOULD GO CHK SOMETHING THAT WAS SENDING A BAD SIGNAL TO THE OUTFLOW VALVE; WHATEVER THAT MIGHT BE. THE MAINT PERSON SEEMED VERY CONFIDENT OF WHAT OUR PROB WAS AND OF HOW TO FIX IT OR FIND WHAT MIGHT HAVE CAUSED OUR PROB.