As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT 500 FT WHEN PACS WERE TURNED ON, CABIN WENT INTO A DIVE APPROXIMATELY 1500 FEET PER MINUTE. CABIN ALTITUDE AT 9000 FEET WAS APPROXIMATELY 3000 FEET BELOW SEA LEVEL. DIFFERENTIAL PRESSURE 8.5 PSI. AIRCRAFT WAS PRESSURIZED WITH MANUAL DEPRESSION. FUEL DUMP WAS PERFORMED TO OBTAIN LANDING WEIGHT. AIRCRAFT LANDED NORMAL. REMOVED AND REPLACED PRESSURE REGULATOR SEQUENCE BOX. PRESSURIZATION CHECK ON GROUND IAW MM 21-31-00. AIRCRAFT RETURNED TO SERVICE. (M)