B737-300 FLT CREW OVER-PRESSURIZES THE ACFT DURING A FERRY FLT.

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I WAS ASSIGNED TO FERRY ACFT X FROM MDW FOR REPOSITION TO FIX SKIN CRACKS. I RECEIVED INFORMATION WITH MY RELEASE PAPERWORK AND WAS BRIEFED BY DISPATCH THAT THEY HAD PLANNED THIS FLT AT FL200 BECAUSE OF PRESSURIZATION LIMITATIONS. DISPATCH INFORMED ME IN THE PHONE BRIEF THAT THE ACFT COULD BE PRESSURIZED UP TO 4.0 CABIN PRESSURE; AND THAT FL260 WAS THE APPROX MAX ALT WITH A 3.9 CABIN PRESSURE. I WAS INCLUDED IN A 3-WAY CALL WITH THE CHIEF PLT ON CALL. AFTER INSPECTION OF THE FUSELAGE AND INFORMING THE DISPATCHER AND THE CHIEF PLT ON CALL THAT I WAS COMFORTABLE TAKING THE JET AT THE BRIEFED ALT; I REFED THE PRESSURIZATION PLACARD ON THE PRESSURIZATION PANEL ON THE JET. IT CONFIRMED THAT THE CABIN PRESSURE AT FL260 WOULD BE 3.9. AS WE CLBED OUT TO FL200; WE NOTICED THE PRESSURIZATION DIFFERENTIAL ABOVE 7.0; BUT THE CABIN WAS STILL AT 1.9. AFTER LEVELOFF; I CONTACTED DISPATCH AND MAINT TO CONFIRM THOSE READINGS AS WHAT WE WANTED. BOTH CONFIRMED WITH US THE READINGS AS ACCEPTABLE. I ALSO HAD A MECH ONBOARD WHO CONFIRMED FOR US THAT WE WANTED NO MORE THAN 4.0 ON THE CABIN PRESSURE READING. LATER ON IN THE FLT; WE WERE CONTACTED VIA VOICE BY DISPATCH TO RPT OUR PRESSURE READINGS. AT THIS POINT THERE WAS CONFUSION AS TO WHICH READINGS WE WERE TO FOLLOW. I RPTED AGAIN THE INNER POINTER READING OF 1.9; AND DISPATCH SAID WE NEEDED TO DECREASE THE CABIN PRESSURE DIFFERENTIAL BY SETTING A HIGHER READING IN THE CRUISE ALT WINDOW. WE THEN LOST VOICE COM WITH DISPATCH. LATER WE WERE CONTACTED BY DISPATCH VIA ACARS AND TOLD; PER MAINT AND THE CHIEF PLT; TO DECREASE THE PRESSURE DIFFERENTIAL. I ASKED HOW THEY WANTED ME TO DO THIS AND I WAS TOLD TO 'CTL THE OUTFLOW VALVE.' I WAS CONFUSED AS TO WHAT THEY WANTED SINCE THE OUTFLOW VALVE WAS WORKING PERFECTLY AND HOLDING THE DESIRED CABIN PRESSURE. AT THIS POINT WE ENCOUNTERED TURBULENT AIR AND ICING; AND HAD TO DSND TO 16000 FT. THE PRESSURIZATION DECREASED PER SCHEDULE TO BELOW 4.0 DIFFERENTIAL ON THE DSCNT. I ASKED 3 MECHS HOW I COULD HAVE DONE THIS BETTER; AND ALL 3 TOLD ME I HAD DONE THE CORRECT PROCS.