Just bought a 1995 Kenworth T450 water tanker, ran fine yesterday for 45 miles of highway driving. Today we ran it stationary with the water hose and it over heated. Fan will not engage manually. You have to manually turn on engine fan for it to come on. Cools right down with fan manually on. Been awhile since I messed with over the road truck, isn't fan clutch tied into temp sensor for engine??? engine gets a certain temp and it kicks on fan if I remember correctly. Just leave the fan switch on for now.
Being a 95 I'd say your fan is controlled by a shutterstat, either normally open for spring applied or normally closed for spring released fan clutch. What engine is in your truck?? Follow the airline from the fan clutch, if it goes to a valve plumbed into the cooling system that's the shutterstat, if it just goes to an electric solenoid it may be controlled through the ECM (Depends what engine you have) or have an electric temp switch plumbed into the cooling system. Could be a wiring problem also. Need more info or pics would be the best!