I think locked is better than un locked at this point

 

I recently replaced mine. Mine was locked up solid and it sounded like a small Cessna. Once it was up to around 3000 rpm the thing would howl like crazy, it had me thinking my transmission was going. I actually went to a trans. shop. During the test drive the guy said sounds like it's pushing allot of air. Sure enough the thing was locked up solid. After I replaced it the performance change was really surprising.

 

Yeah, that's excessive. Mine is stiffer than my old F-150 5.0L by a good bit, but does cycle.

 

They start out really stiff, but when you turn them a bit and get a little warmth in the fluid they start to freewheel better. Then you shouldn't hear them till the temp gauge goes past the "r" on normal. Of course that's if your temp sender is working right.

 

Yeah, I'd have to agree with that.

Got the info from PLC7.3 last night. Apparently starting off cold, the centrifugal force moves the fluid around inside the clutch, warming it up and decreasing its viscocity. As it warms up and flows better it "un-locks" and starts to slip more. Then when it warms up past a certain point, the bimetallic clock spring heats up, unwinds as it expands, and closes off some of the valving inside the clutch. That's what causes them to "lock up" at the higher temps. Then they stay locked until the fan pulls enough air to cool the clock spring. At which point the internal valving opens, allowing the fluid to flow inside the clutch again and it "un-locks".

I'm pretty sure the only part of that cycle mine is doing is the first - where it is most of the way locked up while cold - and maybe the last part, where it locks up completely.

 

Yup, there's some more free power waiting for ya.

 

....and even if it is only a couple of ponies, let me say again,

I WANT IT!

 

Truer words have never been spoken my friend!

I just swapped the one off my spare motor into my truck, and took 'er for a test drive... HOLY SMOKES! I'm absolutely AMAZED at the improvement in the way it accelerates and pulls!

I ran my favorite "test hill" on the way home from work, and then again after the fan clutch swap. Both runs I hit the bottom of the hill at 62-63 MPH. Before the swap I topped it at 45 MPH, as usual, but after the swap I topped the hill at 50 MPH!

It held a full 5 MPH more speed up that hill than what it was able to with the old fan clutch. Right before I hit the hill the second time I filled both tanks. 22.12 gallons to go 300.7 miles - 13.6 MPG in mixed city/highway driving. Not bad, but I can't wait to see how much better it gets without the old locked up fan clutch dragging it down. Plus that 22 gallons of fuel added another 150 pounds for it to pull up the hill.

I swear, I must have regained somewhere between 5 and 10 HP......

Oh, and BTW, the temp increase was hardly even noticable - just enough to move the needle on the stock gauge from between the N and O of NORMAL to between the O and the R.

 

OH yeah, one other thing I forgot to mention. I can hear my ENGINE now. Not just the howling of the fan. In fact, I can hear a LOT of things I couldn't hear over the fan noise before

Like the tire treads howling on the highway, things rolling around in the bed, squeaks in the suspension, rattles in the dash,......

 

I just fixed mine. It was locked up when I bought the truck so it took a while to realize it was not supposted to be on all the time. I hated the fan noise so much I was either going to stop it or sell the truck.
I had to buy the special wrenches from KD tools to get it removed. The first one I got from NAPA worked for two days then it locked up so I had to do the job all over agin.
Now I have one that works properly so I can tell you how they work. At 2000 rpm running down the highway my fan is silent. After being stopped in traffic espesially when towing the fan engages for a mile or two until the radiator is cooled down by the air.

There is a thremostatic coil on the front that turns a bypass valve. When the air at this coil is hot it truns a shaft that closes the oil bypass valve, this engages the clutch. When the truck starts moving agin cool air comes thru the radiator and hits the thermostatic coil causing it to unwind and turn the shaft that opens the oil bypass valve disengaging the clutch. When you start out with a cold engine the bypass valve is open but it takes time for the thick oil to be pumped into the bypass chamber, this is why the fan seems to be engaged for the first mile you drive.
My quess is that if the truck sits for a few months without running the shaft that the thermostatic coil turns gets bound up causing full engagment.

 

I think you pretty much have it down - except that I believe you have this one part of it backwards.

I believe that as the coil heats it unwinds. They are a bimetallic clock spring - two strips of diffrent metals bonded together then rolled into a spiral. As the spring heats up it uncoils - due to the different expansion rates of the two metals (usually the one that expands the most as it is heated faces the inside of the spiral).

I'm pretty sure that's how MOST bimetallic springs are designed to work. But I could be wrong in this particular case....