Modifying Thermal Fan Clutches

Since no one reads my build threads....

I spent $130 on an OEM thermal fan clutch for my V8 Grand Cherokee because the cheap replacement I bought was wonky. Now that it's installed, it isn't locking up when hot - like I ran it to 229*F and it didn't lock.  The truck slowly overheats if running out of OD on the interstate. Needless to say, I'm not a happy camper. I bought it online through a Jeep dealership, and I doubt I've got a good return process. (Hindsight says I should have spent the extra $70 to buy it from the local dealer)

I've read on other forums that you can adjust the lock-up temp by rotating the spring and/or center post. The spring expands when hot, so if you artificially advance the center screw you effectively decrease the engagement temp.

Has anyone hear ever adjusted or modified a thermal fan clutch? Can you confirm or deny?

Wow, I'm sure you're frustrated with that.

I don't know about modifications to the fan spring, but if it doesn't work I would consider changing over to an aftermarket Flex fan and adding an electric pusher fan.

Have you tried a junkyard one?

How's the radiator? I thought fan's were really for low vehicle speed air movement since when you're on the highway you really don't need the fan.

That's not a fan problem. The fan does nothing over about 10-15mph. Sounds like a radiator or possibly water pump issue.

Yeah, all of this has been covered in the build thread. Here's the whole issue:  The fan does not lock at the right temp. Can I adjust it?

Longer explanation. I was running a HD fan clutch and the truck ran cool. I switched to a standard fan, but it doesn't lock up at all, and the truck slowly gets overly warm.

Even longer explanation that was covered in the build thread: The entire cooling system is new. Aluminum radiator, water pump, all hoses, HD fan clutch was new, belt's new, idler pulley is new, even the heater core is new. Vent holes cut in the hood. Also, the entire AC system including the condenser is new, so it's not blocked. The factory trans cooler was upgraded to one almost 2x larger, so it's not blocked. In 95* ambient temps the truck ran fine when moving (even towing in the interstate) but would slowly overheat if sitting still. Plus the HD fan clutch never UNlocked, so it was loud as hell and killing my mileage and acceleration. I put this new OEM standard fan clutch on and suddenly the truck accelerates much better and is SO much quieter, but the fan never locks up.  Now it overheats running with OD off on the interstate AND still overheats at the same rate sitting still. ZJs have such poor air management through the front end that they tend to loose cooling efficiency over 55mph, unlike most vehicles.

Pretty sure I have 2 competing problems - 1, the fan clutch isn't working right (and that's the point of THIS thread.)  2, I have some other lingering cooling problem that is slight enough to be mostly masked if the fan clutch is working correctly. The water pump is is leaking ever so slightly at the block, but I'm pretty sure that's not enough to cause the problem. I'm only topping it up about 8oz every 2 weeks. I didn't flush the system before the rebuild, so it's entirely possible I have a flow restriction somewhere, but the coolant has always looked good, not muddy or rusty. I think there's also an outside chance that the new trans cooler is so large that it's hurting airflow at parked idle enough to be problematic. It does cover almost 1/2 the radiator. I also thought it was possible that the HD clutch was partially locked - not UNlocked enough when cold, and not LOCKED enough when hot, so you got the slow overheat as a result. Not completely ruling that out.

At any rate, I'm hoping to eliminate the fan issue by FIXING the OEM fan so that it spins (fairly) free at low temps and actually locks up a temp that does some good. That would allow me to chase the other issue.

It should lock at any speed if the air coming through the radiator is hot enough. Is there a chance you have a missing air dam under the front end. It could be as simple as the air pressure behind the radiator is high enough to stop flow through the radiator. No flow through the radiator means not enough heat and no fan lock up. 

In reply to Toyman01 :

Forgot to add to the above rant...All factory ducts, air dams, and shrouds are in place. I even made a panel to seal the bumper to the frame where I cut off the front bumper for clearance. I've also run a compression check and it was good, so probably not a head gasket issue

And you're right. But at 225* I pulled off and shut down the truck and the fan spun by hand with the same resistance it did this morning dead cold at 60*. And incidentally, it doesn't free spin, so it's not broken (in THAT way.)  When you spin the fan it turns two blade widths and stops. 

In reply to ultraclyde :

So it's just un-clutched. 

Is it engaged on start up in the mornings? Usually the goo settles to the bottom of the clutch and it takes a few seconds for it to clear on a cold start so the fan is engaged. If not, it may not have any goo in it. At that point the only real option is to send it back. 

Are you using the factory temp gauge? Could your gauge be wrong? I'd be tempted to install a mechanical gauge long enough to verify the accuracy of the the one you are using. 

Can you put heat to the fan with a heat gun or something and confirm that the spring isn't flexing? 

I have adjusted these before, but it has not fixed the problem.

Can you replace the clutch with a fixed fan drive and confirm that the problem is gone?

i have never heard of adjusting the spring on the fan but the fj80 landcruiser guys talk a lot about changing out the viscous fluid inside the clutch fan to get it to engage at different temps. It's been a while since i read about it but probably pretty easy to find the info, if i remember right you can get the viscous fluid at some hobby shops.

not sure if that's even possible on your Jeep fan but might be worth looking into

Toyman01 said:

In reply to ultraclyde :

So it's just un-clutched. 

Is it engaged on start up in the mornings? Usually the goo settles to the bottom of the clutch and it takes a few seconds for it to clear on a cold start so the fan is engaged. If not, it may not have any goo in it. At that point the only real option is to send it back. 

Are you using the factory temp gauge? Could your gauge be wrong? I'd be tempted to install a mechanical gauge long enough to verify the accuracy of the the one you are using. 

The first time I started the truck after installation, it started locked and after about a minute of revving the motor by hand and thinking"What the hell, it's still locked up!?" it unlocked and went quiet. Now it doesn't lock even if the truck has been parked overnight. I admit, I had the same thought, but you do get some viscous resistance when turning it by hand, plus it locked the first time. My impression is that the gate inside that stops/allows goo movement isn't working right - which is the same issue I had with the HD aftermarket fan, just in the opposite direction. (incidentally, the two fans are IDENTICAL except for a different part number. I suspect my high dollar OEM fan is probably made on the same production line, which pisses me off as I spent the exorbitant fee specifically to get a better quality item.)

I'm reading the temp off a bluetooth app connection to the ECU.  The factory gauge seems like it matches up until you get over 210 (center of gauge) and then it lags dangerously behind the reading from the ECU. Both are reading off the factory sensor in the manifold. I'm taking an IR thermometer home from work today to triple check accuracy.

Pushrod - Yeah, but I'll have to pull the unit to do it. That's my plan for Saturday. With the the HD fan (almost fully locked all the time) it ran fine on the interstate. As I said, I think a functional fan is covering another issue, but it covers it well enough to tow a boat 3 hours, which allows me to keep troubleshooting without being parked.

edizzle - Yeah, I've read some of that too. These are sealed units, so a fluid swap isn't a good option. At least not one I'm comfortable with.

If you have a slight leak at the water pump, what kind of pressure are getting in the system?

In reply to RossD :

Not sure. That's one of the issues I need to chase. I would bet from the nature of the leak that it's building some but not a full 16psi.

I'm sure you want to spend more money on parts, so, is there a flex fan for this application?  Something that doesn't use the thermo-clutch at all?  

Pressure is pretty important to the system. 

I chased a overheat problem on my Polaris Ranger that ended up being a brand new radiator cap that was defective. It wouldn't keep enough pressure in the system and caused overheat problems. 

Though it doesn't sound like you have much of a leak. 

Maybe the fan clutch is bad?  New parts do fail right out of the box.  See if the vendor will warranty it and send you another one.

Well, according to Toyman's chart I must be building some kind of pressure since it's 50/50 mix and I've had it to 230 and it hasn't boiled.  I know the pressure directly affects BP, but does it effect cooling effectiveness? Will an identical system run cooler at 16psi vs 10psi?

I'm doubtful that I can get them to take it back, but maybe. It took them over a week to put the part in a box last time, not counting delivery time. I'm not real hopeful about their return process....

02Pilot said:

That's not a fan problem. The fan does nothing over about 10-15mph. Sounds like a radiator or possibly water pump issue.

This is a common belief, but owners of 2004-2005 LLY Duramax trucks will all tell you otherwise...

There is a lot more involved, including aerodynamics of the front end and +/- pressure zones.  I've got one of said trucks.  When towing in warm weather, the fan clutch will cycle on and off, and you can see the results by watching the coolant temp gauge, and in my case the oil temp as well, as it is a water/oil cooler setup.  This is considered normal for these trucks.

Sorry to hijack, just wanted to put this out there as there are definitely other factors involved than just ground speed.

doc_speeder said:

02Pilot said:

That's not a fan problem. The fan does nothing over about 10-15mph. Sounds like a radiator or possibly water pump issue.

This is a common belief, but owners of 2004-2005 LLY Duramax trucks will all tell you otherwise...

There is a lot more involved, including aerodynamics of the front end and +/- pressure zones.  I've got one of said trucks.  When towing in warm weather, the fan clutch will cycle on and off, and you can see the results by watching the coolant temp gauge, and in my case the oil temp as well, as it is a water/oil cooler setup.  This is considered normal for these trucks.

Sorry to hijack, just wanted to put this out there as there are definitely other factors involved than just ground speed.

That's a new one on me. I've dealt with more than a few viscous fan clutches and never encountered one that was working properly that would lock up at speed. I don't understand how a forward-mounted radiator with a grill feeding it outside air at speed would generate any more airflow with a fan locking up (given that the velocity of the air moved by the fan is well below highway speeds). Unless there's some aero/venting trickery involved, I suppose.

If the engine temp gets high enough, a thermal fan clutch will engage even at highway speeds. The 79 GMC Dually I had would do that while towing, and since it didn't have overdrive, you could definitely tell (and hear) when it was engaging.

To the OP, it does sound like your clutch is not working right, and it needs to be replaced.

Well I've been digging around the interwebz about fan clutches. I've found exactly one clutch that is listed to fit my application that looks like a different design. Interestingly enough, the company that "makes" it is only about 8 miles from here, and I used to do tradeshow work for them when I was in the sign biz. I've got an email out to them to see if it's possible to buy locally or if I really have to buy from Rock auto and then have it shipped all over to get to me 8 miles away. Depends on how they run their supply chain, but I bet I will. Either way, it's about $50 and I'm willing to give that a shot since it seems it's the only option besides the two types/brands I've tried.

I've got to see if I can find the box I got the factory clutch in. If I can't get it to lock up with some outside heating I'll return it. $130 ain't no joke. That buys a new aftermarket fan clutch AND a junkyard push bar off a police car!

I've also got an email out to Amazon customer service about warranty claims on the SD one I bought that stays locked up. I call that defective, and it's got a 2 year warranty. Unfortunately almost everyone I read about says the SD from Hayden/FourSeasons just stays locked all the time. I'm not sure if this qualifies as defective, or just a crappy design.

 I don't understand how a forward-mounted radiator with a grill feeding it outside air at speed would generate any more airflow with a fan locking up (given that the velocity of the air moved by the fan is well below highway speeds). Unless there's some aero/venting trickery involved, I suppose.

If the fan is ducted to the back of the radiator and it's moving less air than ambient air speed, the fan is actually impeding air flow. If you make it impede air flow less, you should get more flow. I'm still of the opinion that most vehicles shouldn't need their fans to work at highway speed to not overheat. Some cars are designed better than others, some cars operate with a much higher average load during their careers, but a whole lot of cars just don't work right and aren't yet broken enough for their owners to be aware of it.  That's one thing that diesel truck owners watching their coolant and oil temp gauges are going through. They're often assessing a state of partial brokenness. The distinction between bad design and lack of reliability is kind of a line in the sand. Suffice to say you just don't see a lot of owners of other vehicles accumulating huge data sets of what coolant and oil temps are 'good' or 'bad' in their forums because....maybe they just don't need to. 

In reply to ultraclyde :

The trans cooler will definitely make the rad work harder. I did the same thing, large trans cooler, ~1/3 rad sized, in front of rad. My coolant temps went up 2-4 degrees depending on trans temps. But I dont think that is all of your issue...

In reply to Vigo :

The diesel trucks also dump a lot of heat in front of the rad. Mine has oil, trans and intercooler stacked right in front of the rad. It gets pretty hot air by the time it gets there when being worked hard. And add bad aero to the package and the cooling system gets a workout.

Vigo said:

They're often assessing a state of partial brokenness. 

Perfect description of Jeep ownership.

Can you lock up you new fan with wire or something to confirm that it is indeed the lack of clutch lockup that is the issue?