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So it would seem that the fan clutch on my '72 Supreme may be bad, but I need further direction. When the engine is cold, I can spin the fan about 1/6 of a turn, and when the engine is hot, it turns about 3 times. When I shut the engine off and it's warm, the fan continues to spin maybe 5 times. There is no play or wobble either at the clutch or the fan.
I also know that it's a bi-metallic coil and it's made in the USA by Thermotrol. If I need a new one, then it would have to be for a U code with HD cooling.
I saw this one on ebay, but dunno if it would work and dunno how to find out. Fan clutch
Also, what is the difference between a "thermal" and a "non-thermal" fan clutch?
So it would seem that the fan clutch on my '72 Supreme may be bad, but I need further direction. When the engine is cold, I can spin the fan about 1/6 of a turn, and when the engine is hot, it turns about 3 times. When I shut the engine off and it's warm, the fan continues to spin maybe 5 times. There is no play or wobble either at the clutch or the fan.
I also know that it's a bi-metallic coil and it's made in the USA by Thermotrol. If I need a new one, then it would have to be for a U code with HD cooling.
I saw this one on ebay, but dunno if it would work and dunno how to find out. Fan clutch
Also, what is the difference between a "thermal" and a "non-thermal" fan clutch?
Any help would be appreciated.
Dave - If it's bi-metal it's a thermal fan clutch & operates based on engine temp. A non-thermal fan clutch operates based on centrifugal force.
EDIT: IOW, based on your image, that's a bi-metal thermal fan clutch.
I posted this is your other thread regarding same fan clutch topic.
It would "appear" the fan clutch is working as you'd expect I believe. You have a thermal fan clutch w/ the bi-metal circular labyrinth (e.g. looks similar to a choke coil). As the engine heats up (or remains hot), the spring/coil expands & silicone fluid flows & engages the fan e.g. the fan should rotate more than if the engine was cool. As the engine cools, the spring/coil contracts/relaxes & disengages the fan - fewer rotations of the fan.
I have absolutely no idea how this equates to number of rotations. Obviously you can expect variances based on diameter, flange length, etc.
Put a heavy blanket over the grill, make sure it can’t get caught in the fan. Start the engine leave it on high idle. If the clutch is good, as the engine gets hot you should eventually hear and feel a noticeable difference in noise and airflow as the clutch engages. Obviously, pay close attention to the temp gauge/light, don’t overheat the engine. Once you hear the fan engage, remove the blanket and let the engine idle. As the engine cools you should hear the roar of the fan stop, and airflow decrease.
Unless this is a 1000 point restoration if it was me I would buy a new AC Delco replacement clutch before taking a chance on a 40 (?) year old used one on E-bay.
Should be AC Delco 18-80245 ?
Put a heavy blanket over the grill, make sure it can’t get caught in the fan. Start the engine leave it on high idle. If the clutch is good, as the engine gets hot you should eventually hear and feel a noticeable difference in noise and airflow as the clutch engages. Obviously, pay close attention to the temp gauge/light, don’t overheat the engine. Once you hear the fan engage, remove the blanket and let the engine idle. As the engine cools you should hear the roar of the fan stop, and airflow decrease.
As soon as you start it, notice the amount of air and noise common the fan. If the clutch is working correctly, there will be no question when it begins to engage. It will feel and sound like your in a wind tunnel.
Unless this is a 1000 point restoration if it was me I would buy a new AC Delco replacement clutch before taking a chance on a 40 (?) year old used one on E-bay.
Should be AC Delco 18-80245 ?
As soon as you start it, notice the amount of air and noise common the fan. If the clutch is working correctly, there will be no question when it begins to engage. It will feel and sound like your in a wind tunnel.
Ok...and just so I understand, the blanket goes in front of the grille, correct?
All of the ones listed for my application are standard duty. Wouldn't I need a heavy duty with the Y72 option?
I would bet money that you would never know the difference. Maybe, just maybe if you do a whole lot of sitting in traffic on 90+ degree days but otherwise the one I showed should work just fine.
Unless this is a 1000 point restoration if it was me I would buy a new AC Delco replacement clutch before taking a chance on a 40 (?) year old used one on E-bay.
Should be AC Delco 18-80245 ?
If you're looking to replace, going new is probably best idea vs swapping another used one in unless you know what it's coming from and it's super cheap.
Otherwise, Hayden 2747 and 2797 (HD version) are good replacements, as is Carquest 215046 (HD). These are all essentially the same units, albeit only difference being HD or non-HD - I currently use the Carquest unit noted above on my '72 and it performs very well.
I'm also using an OEM 7-blade fan from a '76 Corvette, Milodon water pump and a Be Kool aluminum rad. I rarely see more than 180* at idle in the FL heat with this combo but even without the aluminum rad, I don't know if you should routinely see 200+ at idle if all cooling system components working properly
Dave, did you see my response on your part# thread? I won't repeat it here. I think the "better" clutch fan from my story was a Hayden now that I heard the name again...
Thanks so much to everyone for the replies but based on what I've described in my opening post, does it sound like I need a new fan clutch?
I don't think it is possible to make that decision based on the information you have given. Too many variables in temperature to say for certain. I have never seen or heard of an actual specification as to how long the fan should spin when hot or cold. It is very very subjective. Like the others have said you should be able to hear the fan roar when the clutch starts to lock up. I guess a lot of it comes from experience ?
I suppose one test might be to get a hair dryer and aim it at the coil on the clutch and see if it eventually starts to lock up ?
Once it's hot, if it gives you 1/2 revolution or less while trying to spin by hand, with no wobble/clunking end play, and no silicon seepage (dirty/oily front of clutch), your clutch can be considered good. If it's always loose or always "locked up" then that's not good. These are just generalizations. Everything wears, especially if it's an OEM piece. So there's no definitive test for a fan clutch that's 100% accurate.
Agree with listening for the wind tunnel effect. When the thermo unit gets to temperature, it will expand the spring which moves the controlling shaft to line up the inner silicon ports. This allows the silicon to flow into the "clutch" plate and effectively solidify the connection to the main shaft. Most of the time, the fan with clutches will be a 5-blade fan, which is made a bit lopsided to minimize that roaring noise the fan makes when the clutch locks up. Standard/HD/Severe Duty are the flavors. Standards run a bit over half water pump speed, and HD runs about 85% of water pump speed. Severe Duty types are the same as HD, except they're larger in diameter so the guts run a bit cooler than HDs.
Non-thermal fan clutches work backwards from a thermal type. They use centrifugal force to close the valve ports for the silicon. So basically, they're always engaged up to a certain rpm, where the ports close and the fan clutch disengages. Basically it maxes out at a certain rpm and lets go.
The latest part number listed for your GM supersession parts number string from 1972-2009-ish, was 22049774 (either OJ or OL coded). The original part number was 4950788. By the time they got to the last number, it was sort of an all-encompassing part number that had a lot of umbrella coverage. All the older fan clutches are discontinued, obviously. But aftermarkets are still available.
There's oil inside the clutch, so it's thick when cold and thin when hot. I'm mostly certain it's something like a torque converter in there - blades on the hub, blades on the body, fluid coupling transfers the torque. The bi-metalic spring turns as it gets hotter, eventually turning enough to close a switch of some kind that makes it harder for the fluid to move which forces a more direct engagement.
What you're feeling - high drag when cold, little drag when hot- is totally normal.
Listen to the fan - when you start with the engine cold it makes a hell of a roar, right? Then as the engine warms up, the oil thins and the fan slows down.
The switch should flip somewhere in the 200 degree area and it'll suddenly go from the same purr to a load roar again. It's extremely noticeable with the windows down.
One thing you could try is get the clutch warm so it spins easily. Heat the coil with a lighter or hair dryer and see if if gets really hard to turn. If the drag doesn't change then it's pooched.
At 210 it absolutely should be engaged and you should have a small tornado at the front of the car.
The HD versions have a larger body. Is it actually worth anything? I have no idea, but that's what I always bought. I doubt there's much of a difference other than just being larger.
Good stuff! My concern is when it gets hotter here in SE VA that my car will run hotter. Also, what is your opinion as to whether or not I need to replace the fan clutch?
what is your opinion as to whether or not I need to replace the fan clutch?
At the top of this thread, you said this:
When the engine is cold, I can spin the fan about 1/6 of a turn, and when the engine is hot, it turns about 3 times.
What happens after 1/6 of a turn? It locks up? If you can't spin the fan, or if in turning it you feel kind of a rough, grating feeling, replace the clutch. If you have any doubts, replace the clutch. They're not expensive. I've done it twice, and it's not difficult.
You talk about needing one for a car with heavy-duty cooling. As I understand it, a fan clutch that fits your car is a fan clutch that fits your car. Fan clutches were used only on cars with A/C and/or heavy-duty cooling. I don't think there's a fan clutch for cars with "regular" cooling and a different clutch for cars with "heavy-duty" cooling. There are just fan clutches.
My '73 Delta 88, which does not have A/C, does not have a fan clutch.
Rockauto shows fan clutches available from several manufacturers. They have an AC/Delco brand clutch. If it were me, I'd buy that one, install it, and see how it goes. You'll probably have no more concerns.
What happens after 1/6 of a turn? It locks up? If you can't spin the fan, or if in turning it you feel kind of a rough, grating feeling, replace the clutch. If you have any doubts, replace the clutch. They're not expensive. I've done it twice, and it's not difficult.
You talk about needing one for a car with heavy-duty cooling. As I understand it, a fan clutch that fits your car is a fan clutch that fits your car. Fan clutches were used only on cars with A/C and/or heavy-duty cooling. I don't think there's a fan clutch for cars with "regular" cooling and a different clutch for cars with "heavy-duty" cooling. There are just fan clutches.
My '73 Delta 88, which does not have A/C, does not have a fan clutch.
Rockauto shows fan clutches available from several manufacturers. They have an AC/Delco brand clutch. If it were me, I'd buy that one, install it, and see how it goes. You'll probably have no more concerns.
It doesn't lock up after that 1/6 of a turn, it just stops turning. IOW, I have resistance when it's cold, but no lock up, and no roughness or grating...it feels really smooth.
I'm confused. Isn't stopping turning the same thing as locking up?
I'm pretty sure he means that with the engine OFF, he can flick the fan by hand and it will continue rotating for only 1/6 of a turn, while doing the same when the engine is hot it turns ~3 full turns (i.e.: its "looser" hot than it is cold, which seems the opposite of what is required for proper fan operation vs. temperature).
I believe even "thermostatic" fan clutches can appear to be "tight" when they are "cold" due to higher viscosity of the internal fluid coupling. It is not uncommon for them to "roar" on a cold start, only to quiet down after a minute or so. The important thing is that they "roar" again when the engine temp gets above the normal "hot" range.
Last edited by JohnnyBs68S; Mar 24, 2025 at 02:03 PM.
The original complaint is that he can't turn the fan without it stopping on him. From my reading of the factory service manual, this means replace the clutch.
I have the '73 manual, not the '72, but it's probably close enough.
In the fan clutch troubleshooting section, the first topic is noise. It says that some noise is normal, as you say, "When the clutch is engaged for maximum cooling" and "during the first few minutes after start-up until the clutch can redistribute the silicone fluid back to its normal disengaged operating condition after overnight setting."
Then it says, "However, fan noise or an excessive roar will generally occur continuously under all high engine speed conditions (2500 rpm and up) if the clutch assembly is locked up due to an internal failure."
Now here's the key part: "If the fan cannot be rotated by hand or there is a rough grating feel as the fan is turned, the clutch should be replaced."
The fact that, when cold, he can turn it well less than a full revolution and then it stops seems to fit the above sentence, and the clutch should be replaced.
Heck, it might very well be the original clutch. After 53 years, it's done its job and is entitled to be worn out.
I'm confused. Isn't stopping turning the same thing as locking up?
It doesn't lock up. By stopping turning I mean that when cold and I spin the fan, it rotates about 1/6 of a turn. I can continue to do this for as long as I want with no lockup. However, when it's warm and I give the fan a spin, it's will rotate about 3 times before it stops. Again, no lockup.
The original complaint is that he can't turn the fan without it stopping on him. From my reading of the factory service manual, this means replace the clutch.
I have the '73 manual, not the '72, but it's probably close enough.
In the fan clutch troubleshooting section, the first topic is noise. It says that some noise is normal, as you say, "When the clutch is engaged for maximum cooling" and "during the first few minutes after start-up until the clutch can redistribute the silicone fluid back to its normal disengaged operating condition after overnight setting."
Then it says, "However, fan noise or an excessive roar will generally occur continuously under all high engine speed conditions (2500 rpm and up) if the clutch assembly is locked up due to an internal failure."
Now here's the key part: "If the fan cannot be rotated by hand or there is a rough grating feel as the fan is turned, the clutch should be replaced."
The fact that, when cold, he can turn it well less than a full revolution and then it stops seems to fit the above sentence, and the clutch should be replaced.
Heck, it might very well be the original clutch. After 53 years, it's done its job and is entitled to be worn out.
So do I need to listen for a roar if it's good or if it's bad?
So do I need to listen for a roar if it's good or if it's bad?
If it roars briefly on cold start, no problem. If the roar persists, it needs to be replaced.
One of the clutches I replaced was on a '75 Delta 88 with a 455 and a/c. Whenever I would get the car up to any speed above about 20 mph, it would sound like Niagara Falls was under the hood. The clutch was seized and would turn as fast as the engine instead of freewheeling like it was supposed to at higher rpm.
On balance, I would guess your fan clutch is fine.
Based on the information given, I have to disagree about the clutch being fine. It spins 3 times or more when it’s hot. It should be virtually locked when hot. 1/2 turn or less manual spin when hot as a rule of thumb.
Hot should be harder to spin. 2 minutes from cold it should be easy to spin.
It doesn't lock up. By stopping turning I mean that when cold and I spin the fan, it rotates about 1/6 of a turn. I can continue to do this for as long as I want with no lockup. However, when it's warm and I give the fan a spin, it's will rotate about 3 times before it stops. Again, no lockup.
This indicates to me that the clutch is defective.
Another indication is the fan continues to spin when a warm engine is shut down.
