Just wanted to post a tip for everyone with hot weather upon us. We are having problems with some of our 2005-6 VT365's giving us an overheat light. The diagnosis has been complicated because the fan clutches are not coming on when the bus gets over 210 degrees as they are supposed to. The cause has been the upper tubes in the radiator are clogging and not allowing coolant flow. This is troubling because only the top rows of tubes are in front of the fan clutch. If they become clogged, the air flow across the front of the fan clutch is not hot enough to turn on the clutch. Upon removal, some of the upper parts of the radiators have been so bad that even after removing the tanks, my radiator shop still can not get the tubes to open up. The deisgn of the radiator (both hoses are located at the bottom of their tanks) seems to contribute to the problem. The best way to diagnose it is to test drive the bus and pull several hills, then get out of the bus and reach your fingers through the grill and feel the tubes. If the the top 6 inches of tubes are not hot....bingo...you have your culprit.
We used to have similar troubles with our T444Es with the vertical flow "half" radiator/half intercooler. On those all our viscous fan clutches got switched to the Horton electromagnetic EC-450 clutches that operated off a coolant temp sensor in the engine water jacket.
I came across just this problem today. It was a low mileage vt365 that never got the uptime coolant filter till last year. Sure enough the top of the core is ambient temp while the lower core is 185 degrees. good tip.