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Old 06-28-2008, 11:52 AM
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How to Test a Fan Clutch

Is there a reliable method to test the functionality of a fan clutch?
I have a 69' Vette with a 409 SBC and bought a new AC/Delco fan clutch for it.
It is the thermostatic variety (has the spiral spring in front) but it appears to be engaged 100% of the time.

If it's 50 degrees or 90 degrees out, the fan is full-on all the time- Regardless if the car has sat all night or not.
It's very frustrating to be driving around on a nice cool day and having a jet engine roaring under hood at all RPMs.

Any tips, suggestions or advice would be greatly appreciated.
Thanks!
Elm
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Old 06-28-2008, 12:05 PM
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Based on your info, the clutch is defective.
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Old 06-28-2008, 01:25 PM
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How to Test a Fan Clutch?

seems to be ok , it's supposed to freewheel in cold temps of 32 or less
supposed to draw air when warm and hot.
put in freezer and check when cold
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Old 06-28-2008, 01:32 PM
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Take it back and get a replacement one. I had this same complaint problem with a Hayden fan clutch. That problem was on a Ford truck, but the replacement fixed the problem. I'm guessing manufacturing defect.
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Old 06-29-2008, 10:15 AM
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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.
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Old 06-29-2008, 10:34 AM
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Hmm, Sounds like you may have been given a clutch for the wrong application. That one may be designed for a heavier fan or ??
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Old 06-29-2008, 11:15 AM
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Yeah, the fan clutch I have is the HD cooling w/AC option and has a much thicker housing than the old clutch.
I didn't realize the HD units just stay on virtually all the time-

I have ordered another AC/Delco unit that is just the 'standard' use w/o AC (I don't have AC on the car). Hopefully this will give me the desired operation.
Thanks for the advice!
Elm
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