It's my understanding that a fan clutch can be of two types. Type 1 is a thermal, fan will be come functional at a preset temp. Type 2 fan is functional up to a preset rpm and then cuts off. Guess a 3rd type could be a combo of 1 & 2. Thanks for any info. Ed

and then you have the flex fan... :D
it does not have a clutch but it twists to a not working mode at higher speeds.