What is the proper way to test a clutch fan?

It runs strong when the engine is cold then runs lightly when the engine is warm.

I figured, it should run strong when the engine is warmer, right? And it should run lightly when the engine is cold, right? Theoretically speaking.

One way you can tell if your clutch fan is working properly is to drive the vehicle slowly until operating temperature is reached. This is the normal operating conditions for the clutch fan. After you turn the engine off, immediately lift the hood and try rotating the clutch fan by hand. There should be some noticeable resistance due to the fact the viscous fluid inside the clutch fan is pumped by the clutch into the position where it locks up the clutch. There is a temperature-sensitive valve in the clutch that permits the clutch to pump the fluid to the lock-up position when the clutch gets hot and prevents the fluid being pumped when the clutch is cold.

When the engine has been shut off for a while and cooled down, and the fan has not moved, the oil in the clutch pools in the bottom. This is not where it is supposed to be when the clutch is cold and running. Once you start the engine, it takes 20-30 seconds of turning the clutch for all the oil to be returned to its cold position. During this time, the clutch will behave like it is hot and rotate the fan. After this, the fan will free-wheel as it is supposed to do when the clutch is cold.

I do a similar trick except I pop the hood before I turn off the engine. After you turn it off watch the fan and see if it keeps spinning. It should stop with the engine or within a few revolutions.

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