Suspect Electric Engine Fan

I had a issue to day and suspect the electric engine fan to be at fault just getting second opinions. Its a a 2008 670vn with d12 and the engine fan is the variable speed electronic fan. It is a municiple snow plow and today there was a overheat complaint. I took for a drive the temp gauge will start into the red throw a overheat code and shut down. Restart let it idle it will start to cool back down until you start driving again and it will overheat again. Coolant level good, had new rad last year, also fan hub brg assy was replaced. At idle the fan is moving a large amount of air however going down the road can't hear it fan is engaged or not? My question is with this type of fan should you hear it engage at road speed? Also I understand the basic principle of this style fan its a viscouse fan that reacts with engine temp like automotive fans but also has the ability to lock up harder as needed by the electronics. My understanding is it is a fuel saving system. What controls the electric side of fan is it ecm controlled or simply controlled by a temp switch. Monday I will unplug the fan if memory serves me correct this locks fan on full at least this should allow me to get it from the dispatch depot to the repair depot were I work. Any info or diagnostic advice is appreciated.