with hers i cranked the truck up with the the hood up and you could tell the fan wasnt spinning as fast as it should.... i did find this on another forum though.
Start it up and stick your hand in it If you just cut a finger but its still hanging on by a tendon or flap of skin and the fan stops, its correct for low temp operation. If you lose all five fingers and part of your carpal ligament, its correct for hot operation.
Just kidding. The way I diagnose a fan clutch is pretty simple. Start with the car off. Jiggle the fan forward and back. If there is any play at the clutch shaft, its bad. Give the fan a spin. It should move freely, but stop almost instantly, as if the clutch were filled with mashed potatoes. Start the engine. The fan should turn pretty briskly; almost as fast as the crank. Now use a rolled up newspaper to gently try and stop the fan. You should be able to when its cool. Throw away the ruined newspaper Rev the engine, you should notice that the fan increases speed, but not proportionally to the crank speed. The clutch is full of viscous fluid, so the air friction on the fan starts overcoming the friction of the fluid in the clutch and it stops accelerating with the engine. The fan won't roar. Then let it get hot. You might have to drive it around some stop and go streets to get it to lock up the clutch. Now when you acclerate, you should hear the fan roaring a little louder. If you stop the car, shut off the engine, and try to spin the fan, it shouldn't move.
If any of these tests fail, get a new clutch. Do yourself a favor and don't get a fancy HD or RV clutch. They lock up too soon, the viscous fluid is too thick and makes the fan blow when it shouldn't, and they're typically bigger and you might have fitment issues. Just get a good factory replacement thermal clutch.
