What was found
PDX FLIGHT NR 2093, AFTER TAKEOFF AIRCRAFT BEGAN PRESSURIZATION AT HIGH RATE, CABIN DIFF OF 5.75 REACHED BY 4000 FEET MSL. SWITCHED TO MAIN MODE, NO CONTROL. ATTEMPTED TO DUMP CABIN WOULDN'T DUMP. FINALLY DUMPED CABIN PRESSURE FOR LANDING BY OPENING FORWARD OUTFLOW VALVE. RE-SECURED CONNECTOR ON CABIN PRESSURE CONTROLLER. PRESSURIZED AIRCRAFT ON GROUND IN DUMP, AUTO AND MANUAL IAW DHC-8-200 AMM 21-30-00. OPERATIONAL CHECK GOOD. NO PARTS REPLACED. AIRCRAFT RETURNED TO SERVICE.