not necessarily... depends what kind of clutch fan it is. there is a clutch that is rpm based and then the one with a spring that reads the temp coming from the radiator.
to test a fan clutch u put a mark on the clutch and water pump snout. then use a timing light to see if and when there is a differencial in speed.
I agree...A heavy duty thermal clutch fan will still spin at highway speed if the temp is still comming through the radiator hot enough to keep it spinning..if it is bad it will not...