What was found
AIR DUCT OVERHEAT COCKPIT LIGHT ILLUMINATED 30 SECONDS AFTER TAKEOFF, PASSENGERS SAW SMOKE IN THE CABIN. SELECTED MANUAL TEMPERATURE CONTROL TO COLD FOR 30 SECONDS WITHOUT A CHANGE, THE SOURCE TEMP REACHED 325 DEGREES, EMERGENCY DECLARED, AIRCRAFT DIVERTED TO CHS AND LANDED SAFELY. COMPLIED WITH GROUND RUN CHECKS OF THE TEMP CONTROL SYSTEM, NO DISCREPANCIES NOTED, CHECKED ELECTRICAL CONNECTORS ON CONTROLLER & COCKPIT TEMP CONTROL VALVE. REMOVED & REPLACED VALVE DUE TO CESSNA RECOMMENDATION. SYSTEM OPS CHECK & LEAK CHECK GOOD ON GROUND RUN. ALL WORK DONE IAW CESSNA 560XL MM CHAPTER 21-61-00.