New member here and to the Dodge with Cummins 5.9 Diesel. (Std Output) I bought the truck used (300K) and found a small leak in Radiator. Installed a NEW Radiator, cleaned all the other coolers in front of the Radiator and here's what's going on. Drive the truck at Hwy speed (70-75mph) temp gets up to 215 and if I slow down or get in stop and go traffic the truck will cool down to 190 or just below. Sitting still the fan won't come on and the temp has been up to 215. If I continue to set still the temp will go down and still no fan coming on? The fan will come on if I turn on the A/C and if I leave the A/C on the temp will go down. This seems backward to me from my experence from gas burners. I also noticed that if I start the truck cold and pull away the fan isn't enguaged like others says their's is. I am spooked to TOW with this truck with the temps acting like this. Thanks in advance for any help you can give a confushed new 5.9 Cummins owner. Kid...