How do fan clutches work?
Thread Starter
Racer
Joined: Jan 2020
Posts: 388
Likes: 117
From: Tacoma, Wa
How do fan clutches work?
So here’s what I know. When the engine gets warm, the fan clutch “grabs” causing it to spin faster to create more airflow.
First question, why does my fan clutch grab when the engine is cold? It has always done this, freezing morning or 90 degree evening. I can hear the fan until it ‘releases’ after a half mile to a mile of driving. Seemingly when the engine reaches operating temp.
Second, recently it is ‘grabbing’ at strangely high rpms. I was driving on the freeway during a warm day, and I heard this screeching sound. I turned my music off and I’m 99% sure it was the fan clutch grabbing at too high of rpm. It would continue to to do this on and off. Engine was barely at 200. Is this a sign of a fan clutch starting to fail? A few time it screeched on city streets as well, like when I would stay in a low gear going up a hill, the fan clutch would screech once i got to a higher rpm.
First question, why does my fan clutch grab when the engine is cold? It has always done this, freezing morning or 90 degree evening. I can hear the fan until it ‘releases’ after a half mile to a mile of driving. Seemingly when the engine reaches operating temp.
Second, recently it is ‘grabbing’ at strangely high rpms. I was driving on the freeway during a warm day, and I heard this screeching sound. I turned my music off and I’m 99% sure it was the fan clutch grabbing at too high of rpm. It would continue to to do this on and off. Engine was barely at 200. Is this a sign of a fan clutch starting to fail? A few time it screeched on city streets as well, like when I would stay in a low gear going up a hill, the fan clutch would screech once i got to a higher rpm.
Drifting
Joined: Dec 2009
Posts: 1,349
Likes: 34
From: Porterville Ca
Factory clutch will be thermostatic and speed controlled. It'll lock up when it gets warm, and freewheel at higher RPM's so that it's not a HP drain. Cheaper aftermarket fan clutches are only speed "controlled. A good rule of thumb is that it doesn't spin more than about 1.5 revolutions after you turn the engine off. If it just spins it's bad. It shouldn't spin more than about one rev if you spin it by hand. Usually they fail by being too loose and not cooling enough. That, or they seize, but it's somewhat rare.
Safety Car
Joined: Jul 2005
Posts: 3,694
Likes: 833
From: WI
There's a viscious fluid in them that allows slippage or not. It can leak out (or partially leak out) if ya take off an older fan and lay it horizontally on the ground. Maybe yours has partially leaked out, or just failing. There's a little more to it than that:
Thread Starter
Racer
Joined: Jan 2020
Posts: 388
Likes: 117
From: Tacoma, Wa
Factory clutch will be thermostatic and speed controlled. It'll lock up when it gets warm, and freewheel at higher RPM's so that it's not a HP drain. Cheaper aftermarket fan clutches are only speed "controlled. A good rule of thumb is that it doesn't spin more than about 1.5 revolutions after you turn the engine off. If it just spins it's bad. It shouldn't spin more than about one rev if you spin it by hand. Usually they fail by being too loose and not cooling enough. That, or they seize, but it's somewhat rare.
This is my 2nd fan clutch, the original seized not too long after I got the car. I payed extra for an AC Delco clutch. Still doesnt make sense that it locks during intitial driving while cold, as it always has. The original on the car also did it.
Thread Starter
Racer
Joined: Jan 2020
Posts: 388
Likes: 117
From: Tacoma, Wa
There's a viscious fluid in them that allows slippage or not. It can leak out (or partially leak out) if ya take off an older fan and lay it horizontally on the ground. Maybe yours has partially leaked out, or just failing. There's a little more to it than that:
https://www.youtube.com/watch?v=UwM4OqsLek4
https://www.youtube.com/watch?v=UwM4OqsLek4
well now I know how they work. And Im even more confused about why mine is operating the way it does
Advanced
Joined: Apr 2020
Posts: 80
Likes: 14
From: Ont, Canada
As derekderek wrote it sounds like fan belt needs replacement
If you bought a thermal clutch it will have a spring on the front face of the clutch.
A thermal fan clutch is engaged ( temporarily ) on a cold startup because the fluid because the fluid drains into the working area when the engine is shut off.
If you bought a thermal clutch it will have a spring on the front face of the clutch.
A thermal fan clutch is engaged ( temporarily ) on a cold startup because the fluid because the fluid drains into the working area when the engine is shut off.
Last edited by relivingthepast; Jul 17, 2022 at 05:26 AM.
Race Director
Joined: Nov 2006
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From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
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Cruise-In VIII Veteran
How do fan clutches work?
Well,
Not really well.
Do they stop spinning completely when not needed thus freeing up horse power?
No.
Are they better than a flex fan?
Yes.
in general do they keep air moving through your radiator to keep engine temps in check?
Yes.
When moving down the highway do you need a fan running at all?
Rarely.
I'll keep my electric fans.
Well,
Not really well.
Do they stop spinning completely when not needed thus freeing up horse power?
No.
Are they better than a flex fan?
Yes.
in general do they keep air moving through your radiator to keep engine temps in check?
Yes.
When moving down the highway do you need a fan running at all?
Rarely.
I'll keep my electric fans.
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Advanced
Joined: Apr 2020
Posts: 80
Likes: 14
From: Ont, Canada
How do fan clutches work?
Well,
Not really well.
Do they stop spinning completely when not needed thus freeing up horse power?
No.
Are they better than a flex fan?
Yes.
in general do they keep air moving through your radiator to keep engine temps in check?
Yes.
When moving down the highway do you need a fan running at all?
Rarely.
I'll keep my electric fans.
Well,
Not really well.
Do they stop spinning completely when not needed thus freeing up horse power?
No.
Are they better than a flex fan?
Yes.
in general do they keep air moving through your radiator to keep engine temps in check?
Yes.
When moving down the highway do you need a fan running at all?
Rarely.
I'll keep my electric fans.
Le Mans Master
Joined: Jun 2019
Posts: 7,650
Likes: 2,466
From: Indiana
Melting Slicks
Joined: May 2021
Posts: 2,265
Likes: 1,310
From: Fleming Island, FL
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,074
Likes: 4,208
From: US-PNW
First off, no matter the engine's temp, the fan is going to rotate. First start or totally warm, just the pulley rotating is going to turn the fan. The difference is that the internal fluid drive gets "locked" similar to an automatic transmission's torque converter so that the fluid couples the internal "plates" together. When the clutch reaches the proper temp and locks, the fan can make a roar sound. When first started, the fan should easily slip against the rotation of the pulley. If the engine is shut down before the fan's internal fluid "locks" up, then the fan will/can free wheel a number of times before coming to a stop. Once fully warmed and "locked", the fan should come to a stop quite quickly with engine shut down.
Le Mans Master
Joined: May 2007
Posts: 5,021
Likes: 1,120
From: North tonawanda NY
Pun intended? 
Just my 2 cents as someone who struggled with trying to get my stock setup to work with my 180 degree stat only to find the temps climbing well over 200 when sitting in traffic. I had replaced the rad, hoses, seals, and clutch. In my case, swapping to the dual electric fans not only freed up more HP than going from an old cast iron stock intake manifold to an edelbrock RPM, it just works better to keep my engine cooler. I have 2 fans on 2 separate controlled relays so I have no worries of being stranded which can be a real concern for electric fans as I had it happen with the stock fans on my 84 c4. As others mentioned the clutch allows for slippage if its working correctly and the fluid hasn't all leaked out but even then it adds unnecessary drag on the engine. electric fans draw 0 HP while the car is actually under speed and the fans are turned off. This is just not the case with a clutch fan.
Just my 2 cents as someone who struggled with trying to get my stock setup to work with my 180 degree stat only to find the temps climbing well over 200 when sitting in traffic. I had replaced the rad, hoses, seals, and clutch. In my case, swapping to the dual electric fans not only freed up more HP than going from an old cast iron stock intake manifold to an edelbrock RPM, it just works better to keep my engine cooler. I have 2 fans on 2 separate controlled relays so I have no worries of being stranded which can be a real concern for electric fans as I had it happen with the stock fans on my 84 c4. As others mentioned the clutch allows for slippage if its working correctly and the fluid hasn't all leaked out but even then it adds unnecessary drag on the engine. electric fans draw 0 HP while the car is actually under speed and the fans are turned off. This is just not the case with a clutch fan.
Le Mans Master
Joined: May 2007
Posts: 5,021
Likes: 1,120
From: North tonawanda NY
Very true... mechanical fans without the clutch cause are typically found to have 3 times the HP losses as a clutch fan.
I have seen multiple tests on multiple dynos done and the lowest HP loss with a clutch fan that Ive seen was 7hp... not a whole lot. if it all works well Its really a tossup in my mind to upgrade to a modern fan setup.
In my case I spend about $175 for my electric setup... Its pretty fair to say a lot of folks have spent more $ here for other bolt ons that only netted them 7 hp or so and one additional plus is I had no issues with clearance with the spreader bar I just installed today. (which was $130 and netted me 0HP gains)
I have seen multiple tests on multiple dynos done and the lowest HP loss with a clutch fan that Ive seen was 7hp... not a whole lot. if it all works well Its really a tossup in my mind to upgrade to a modern fan setup.
In my case I spend about $175 for my electric setup... Its pretty fair to say a lot of folks have spent more $ here for other bolt ons that only netted them 7 hp or so and one additional plus is I had no issues with clearance with the spreader bar I just installed today. (which was $130 and netted me 0HP gains)