What are you doing with the tractor when it overheats? What implement are you using? Have you done the same job in the past without overheating? The 6420 uses engine coolant circulated through the cooler which is located behind the engine and in is in a circle around the driveshaft. Be sure too check the lines to and from the cooler to see if there are any kinks or restrictions. Also be sure to check the radiator and screens for unobstructed airflow. Check the upper and lower tanks on the radiator. some of the early 6020 series had plastic upper and lower tanks that had problems. the aluminum upper and lower tanks were more relaible - The cooling system is a split system. the engine portion of radiator runs at 180 degrees. The trans oil cooler, engine oil cooler, and heater portion of radiator runs at 140 degrees. The earlyu radiators tended to allow the "hot" and "cool" sides mix together resulting in transmission overheating.