What was found
ALMOST IMMEDIATELY AFTER TAKEOFF, THE CABIN PRESSURIZATION CONTROLLER COMMANDED A SEVERAL-THOUSAND FEET PER MINUTE DESCENT. AFTER SELECTING MANUAL CONTROL, BEGAN TO RAISE THE CABIN ALTITUDE FROM ABOUT 6500 FEET BELOW SEA LEVEL. IN MANUAL, IT WAS VERY DIFFICULT TO CONTROL CABIN PRESSURE. WE DECIDED THE AIRPLANE CONDITION WASN'T SUITABLE TO CONTINUE TO TJSJ, AND SELECTED KAGS FOR OUR DIVERT. SELECTED EMERGENCY DEPRESS TO REMOVE CABIN PRESSURE ONCE WE WERE IN THE PATTERN. LANDING AND TAXI WERE UNEVENTFUL. TROUBLESHOOTING IS STILL IN PROGRESS TO DETERMINE THE CAUSE.