Breaking Fan Belts

[Phil Waterman]

On the way to a parade this weekend I shredded two fan belts on my C60 driving on the highway at speedometer speed of 50 mph I was 70 miles into a 80 mile trip when suddenly I noticed that amp meter was showing discharge followed quickly by smell of antifreeze. Pulled over into a weigh station and discovered that the fan belt was shredded. See photo. No big problem carried a spare old fan belt. After letting it cool off replaced the belt and check radiator which was down a little. ( The expansion tank had expanded as its vent was to small to take the rapid increase in flow coming out of the radiator.) Started engine everything seemed fine 3 miles down the road same thing happened. Yes I had checked to see that the fan/water pump and generator were turning freely. Pull down off ramp and stopped good Samaritan took me to the nearest auto parts store and luck again they had the belt. Back to the truck, replace belt again decided to head home on the back roads every thing seemed fine up to about 40 mph then could hear the fan changing tone like it was slowing down. When down shifting it squeals.

Any thoughts on how to check for cause?