B737-400 PLT RPTS THAT THIS ACFT'S REPEATED PRESSURE FAILURE APPEARS TO BE A WEAK AC OUTFLOW VALVE MOTOR BECAUSE THE STANDBY DC MOTOR CTLED PRESSURIZATION.

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DURING PAX BOARDING AT THE GATE THE FO MENTIONED IF I HAD HEARD THE HISTORY OF THE ACFT. I HAD NOT AND HE WENT ON TO TELL ME OF THE PRESSURIZATION PROBS THAT A CAPT HE HAD FLOWN WITH EARLIER IN THE WEEK HAD TOLD HIM. THE PROB RESULTED IN AN 'EMER DSCNT' DUE TO A PRESSURIZATION PROB. I REFERRED TO THE MAINT LOG TO TAKE A CLOSER LOOK AND SAW 3 WRITE UPS OVER THE LAST COUPLE OF WEEKS ABOUT PRESSURIZATION AND ALL OF THEM HAD BEEN CLRED WITH REPAIRS DONE TO SEALS; AND A REPLACEMENT OF A PRESSURIZATION CTLR. I ACCEPTED THE ACFT AND WE PUSHED BACK AND TOOK OFF. THE FO WAS THE PF AND I WAS PNF. THROUGH 30500 FT ON OUR WAY TO FL330; THE FO BROUGHT MY ATTENTION TO THE CABIN ALT INDICATOR. IT WAS INDICATING A CABIN ALT OF 9500 FT AND A DIFFERENTIAL PRESSURE OF 5.5 PSID. THE FO SLOWED THE ACFT CLB AND I VERIFIED THAT THE ACFT WAS CONFIGURED PROPERLY. I CHKED THE OVERHEAD TO VERIFY THAT PAX OXYGEN WAS NOT DEPLOYED. ENG BLEEDS WERE ON AND BOTH PACKS WERE NORMAL AND THE FLT/GRN SWITCH WAS IN FLT. THE PNEUMATIC DUCT PRESSURES WERE INDICATING ABOVE 30 PSI FOR BOTH L AND R DUCTS SO ALL APPEARED NORMAL EXCEPT FOR THE PRESSURIZATION INDICATIONS. THE AUTO FAIL LIGHT WAS NOT ILLUMINATED SO NO INDICATION OF THE AUTO SYSTEM HAD FAILED BUT IT WAS OBVIOUS THAT IT WAS NOT MAINTAINING THE PRESSURIZATION SCHEDULE. JUST AS I WAS SWITCHING THE MODE SELECTOR TO STANDBY MODE; TO ATTEMPT BETTER CTL OF THE PRESSURIZATION; THE CABIN ALT WARNING HORN SOUNDED AND WE PERFORMED THE MEMORY ITEMS OF OXYGEN MASKS ON AND 100 PERCENT AND ESTABLISHED CREW COMMUNICATIONS. I CANCELLED THE CABIN ALT HORN AND VERIFIED THAT THE PAX OXYGEN LIGHT WAS STILL OFF. MY ATTENTION WENT BACK TO THE CTL PANEL AND THE CABIN HAD BEGUN A 400-500 FPM DSCNT IN THE STANDBY MODE. AS PER PREFLT PROC THE CABIN BEGAN A DSCNT TO THE LNDG FIELD ELEVATION MINUS 200 AND THIS INDICATED THAT THE STANDBY MODE WAS WORKING EXACTLY AS IT SHOULD AND I PROCEEDED TO SET STANDBY CABIN ALT OF 6400 FT; WHICH REFERENCING THE PLACARD WAS THE CABIN ALT FOR AN ACFT ALT OF 33000 FT. WE COMPLETED THE QRC/QRH FOR RAPID DECOMPRESSION AND DID NOT INITIATE AN EMER DSCNT SINCE THE CABIN WAS UNDER CTL AND OPERATING AS SCHEDULED IN THE STANDBY MODE. AFTER COMPLETING THE RAPID DECOMPRESSION CHK LIST; THE CABIN ALT HAD COME DOWN TO 8000 FT AND WAS CONTINUING DOWN TO 6400 AT 400 FPM. WE SECURED THE FLT DECK OXYGEN AND PROCEEDED TO ACCOMPLISH THE AUTO FAIL/UNSCHEDULED PRESSURIZATION CHANGE ABNORMAL CHKLIST AND THIS RESULTED IN NO OTHER CONFIGURATION. I THEN CHKED THE MEL FOR ANY ADDITIONAL INFO ABOUT OUR CONDITION ABOUT OPERATING UNDER STANDBY MODE AND THEN PROCEEDED TO CALL MAINT CTL ON FREQ THROUGH THE REMOTE. I EXPLAINED THE CONDITION AND WHAT WE HAD DONE UP TO THIS POINT AND CONSULTED WITH THEM ABOUT CONTINUING ON TO ZZZ2 OR GOING BACK TO ZZZ1. THEY MENTIONED THAT THE ACFT HAD SOME HISTORY AND I TOLD THEM THAT IN LIGHT OF THAT HISTORY IF THEY DIDN'T HAVE ANY CONCERNS WITH PROCEEDING ON UNDER THE STANDBY MODE THAT I WAS COMFORTABLE WITH PROCEEDING ON TO ZZZ2. I THEN CONTACTED DISPATCH VIA THE REMOTE AND WE DISCUSSED ZZZ2 WX (IF ANY OTHER DIVERSION WAS NECESSARY); ZZZ1 AND ZZZ3 WX; AND OPERATING UNDER STANDBY MODE ON TO ZZZ3. WE ALL CONCURRED AND THE FLT PROCEEDED NORMALLY TO ZZZ2. UPON REACHING ZZZ2; I MADE AN ENTRY IN THE DISCREPANCY LOG AND EXPLAINED TO MAINT THE PROB. FOR MY OWN CURIOSITY OF SYSTEMS KNOWLEDGE I VERIFIED THAT THE AUTO CONTROL OF THE OUTFLOW VALVE IS BY THE ACFT MOTOR AND IN PASSING MENTIONED THAT I FELT LIKE THE ACFT MOTOR WAS HAVING TROUBLE CTLING THE VALVE BUT WHEN SWITCHED TO THE STANDBY MODE (DC MOTOR) THAT THE SYSTEM WORKED FLAWLESSLY. FLT TESTING THE PRESSURIZATION SYSTEM INSTEAD OF GND TESTS TO DUPLICATE THE PROBLEM. THIS PROB HAPPENED AT 5.5 PSID AND I THINK GND TEST ONLY ALLOWS GND PRESSURIZATION TO 5.0 PSID SO MAINT COULD NOT DUPLICATE THE PROB ON THE GND.