How does a clutch fan operate?

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70orangeswinger

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Can anyone tell me how a clutch fan on a 70 340 works or more importantly how do I tell if mine is not working properly? It is original to the engine. Thanks.
 
If it's on the engine, try and spin it. It should have some resistance, if it spins freely and keeps spinning when you let go of it, it's no good.
 
Basically, it works similar to a torque converter in that it uses a viscous medium to drive the fan
 
Most clutch fans have bimetal spring located on the radiator side of the clutch. You can feel it with your fingers, its right in the center of the hub. If and when the spring heats up via the radiator air flow or water pump shaft, which in hand causes the solution in the hub to expand and swell causing the hub to spin at the high rpm (motor rpm) When the temp cools down, solutions cools down hub will start to free wheel again. Most of the hubs I have replace where locked up causing a "roar" under the hood. If you wish you can test this by placing a piece of cardboard in front of the radiator and then run the engine at a high idle. You should hear the clutch slowly engage in and out of lockup. You will hear the fan roar when it does. Please be careful! I hope this helps.
 
No spring on 1970 clutch fans, they have a smooth face.
 
I notice a little drag on the fan when cold but I don't notice any change when the engine is hot. I have cooling problems and have to be careful on hot days. How much drag should there be when the engine is hot?
 
70orangeswinger said:
I notice a little drag on the fan when cold but I don't notice any change when the engine is hot. I have cooling problems and have to be careful on hot days. How much drag should there be when the engine is hot?
Click to expand...

according to the interwebs, its 27 degrees in philly...id imagine you should be OK :D
 
I had to reread this thread, I thought I was on the weather channel. Orangeswinger you can go with a flex fan an aftermarket fan blades that flex under high engine rpm. The fan flex blades are made to have full blade pitch when in the idle mode thus having full air flow, when driving the blades of the fan flatten out (high engine rpm) you don't need the fan working if you are going 60 down the road. If you go this route you may have to buy spacers and use longer bolts to go between the pump shaft and fan blade. You have to keep the blade inside the fan housing to have proper cooling. They sell the flex blade and spacers on the net.
 
70orangeswinger said:
Can I replace the non thermal clutch with a thermal clutch unit? I am still running the original fan.
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Yes You can.
Circa '78 replaced the 5 blade fan with a 7 blade & a ' thermal clutch' unit on Tinfish
(My '67 Barracuda forever project).
As to the 'no thermal spring flatface' Maybe--I stand to be corrected,--- as the first
j yard unit I bought (from a Diplomat Police small block unit) was dead & I tore it apart,
Low & behold, viscous fluid & thermal spring.

Apologies 70orangeswinger,
Back on topic :),
The new 'thermal clutch' I purchased moved the fan asy 3/8" closer to the radiator.
inclusive of the 'less aggressive' blade pitch on the 7 blade fan.
Bolt patterns were a non issue (fan to clutch, clutch to water pump).

If Your current setup has the same 'resistance' to rotation hot & cold,
The fan clutch is a door stop.

P.s, flex fans !!! bad ju-ju , unless You are going to examine same & replace
at regular maintenance intervals .
 
the way i test mine is to get the engine up to operating temp then have some one raise the rpm slightly then shut the key off if it spins more than 2-3 revolutions it's toast.
 
On Wheeler Dealers, Ed China showed how to test one. With the engine idling, he grabbed the fan with a thick leather glove and stopped it. He said he wouldn't have been able to do that if the fan clutch was good, I am guessing he then might have lost a finger as a memorable diagnostic. Ed is British so at least sounds smart.
 
BillGrissom said:
On Wheeler Dealers, Ed China showed how to test one. With the engine idling, he grabbed the fan with a thick leather glove and stopped it. He said he wouldn't have been able to do that if the fan clutch was good,QUOTE]

Most retarded method of "testing" something I've ever heard of!
Click to expand...
 
I understand wanting to know how to check one, so good for you looking to learn.
I also think if you are trying to not spend $30 replacing a 46 year old fan clutch, you probably don't deserve to own a 340.
 
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