Most of these are reprints of posts I have made in various on-line forums over the years. These questions come up time and time again, so I thought I would write them up here and save myself some serious typing. As I come across some of the other posts I have written, I'll add them to this page.
Your fan clutch is a thermal viscous drive unit. This means it uses a fluid-filled coupling between the fan blades and the engine. There is a thermostatic spring in the fan drive that is sensitive to the air temperature flowing out of the radiator. As the temp increases, the fan speed quickly increases to provide additional engine cooling. You can hear the engagement point where the fan speed picks up...kind of a roaring noise (much louder than when the engine was first started and the radiator was cool). When the air temp drops, the fan will disengage and the speed will again drop (roaring subsides)..
If the fan assembly free-wheels without drag (the fan blades will revolve more than five turns when spun by hand), replace the fan drive. This spin test must be performed when the engine is cool (this test is from the FSM). There is a more details method of checking the fan drive. Obtain the FSM and consult the detailed instructions for the test.
A full write-up covering radiator removal, thermostat replacement, etc. is available here.
Here is one Jeeper's solution to that problem.
A full write-up covering a water pump swap is available here.
4x4 Off-Road Homestead Firearms RC Models