As the title says my 6420 will not get up to operating temperature (180 degrees). Ive used my ir digital thermometer (with new batteries) to verify my heat gauge is accurate and the temp tops out at 120 degrees on the top rad hose, and 105 on the bottom after running hard for a half hour, so my sensor and gauge are fine. I realize the first thing most people will think is faulty thermostats, so Ive already replaced both the engine and low temp side thermostats, and have the same temperatures. This is a new to me tractor but my other 6320 and 7320 both get up 170-180 degrees so its definitely not a normal characteristic of this machine. Not sure where else to go other than replacing the viscous fan clutch, but wow they are a lot of $ just to test it out. Also it doesn't seem to be kicking out too much air compared to what they are capable of. Any other ideas? Also replaced coolant with new and bled the system. heater works fine but but not real hot, obviously. I repair most of my own stuff but this is an odd one to me, still scratching my head.