Does the fan clutch engage because of engine temp? I originally though it was based on engine speed, however with my truck I notice increased fan noise after I have been pulling my trailor for a few miles and it is good and warm. I don't get this fan noise when I am running the truck empty, so it appears that clutch engagement is based on engine temp. If it is, how does it sense it? Just curious. Thanks

Bimetalic spring senses ambient (basically engine compartment) temperature.

Fan clutch operates on air passing through the radiator core, over the clutch and bi-metal spring. The hotter the air temp, the more fan lockup you get.

Thanks for the replies. Just noticed that the fan clutch is a $300 item. I guess that explains why.

FYI You can find some diesel vendors that modify the OEM fan clutch to lock up at a lower temp to start moving more air sooner.