What was found
AFTER DEPARTING PBI PILOT SELECTED LANDING GEAR UP. LANDING GEAR RETRACTED AND HANDLE IN-TRANSIT LIGHT EXTINGUISHED. A COUPLE OF SECONDS LATER, LANDING GEAR HANDLE IN TRANSIT LIGHT ILLUMINATED FOLLOWED BY IN TRANSIT LIGHT EXTINGUISHING AND LEFT MAIN DOWN AND LOCKED LIGHT ILLUMINATING. PILOTS SELECTED GEAR DOWN AND RETURNED TO PBI. LANDING GEAR WAS UNEVENTFUL. AIRCRAFT WAS PLACED ON JACKS AND A RETRACTION TEST WAS PERFORMED. IT WAS DETERMINED THE UPLOCK SWITCH HAD FAILED WHICH PREVENTED SOLENOID VALVE FROM OPENING AND APPLYING HYDRAULIC PRESSURE TO UPLOCK ACTUATOR. UPLOCK SWITCH REPLACED. OPERATIONS CHECKED GOOD.