What was found
AT FL 370 PRESSURIZATION AUTO FAIL LIGHT CAME ON. SWITCHED TO ALTERNATE CONTROLLER. THE CABIN PRESSURE BECAME ERRATIC WITH ALT LT ON AND FLT ALT & LAND ALT WINDOW BLANK. THE FLIGHT DECLARED AN EMERGENCY AND DIVERTED TO SJU WITHOUT FURTHER INCIDENT. MAINTENANCE REMOVED AND REPLACED NO 2 CABIN PRESS CONTROLLER IAW AMM 21-31-01. ACCOMPLISHED PRESSURIZATION SYSTEM GROUND TEST IAW AMM 31-31-00-700-802 AND CKD SATIS. THE AIRCRAFT WAS NOT GROUNDED.