The Power Management Controller (PMC) failed for the center (hard disk) fan in my iMac Core 2 Duo 3.06 27-Inch (Late 2009). The two outer fans work perfectly, but no center fan gets no power at all.
It took me three tries, but I finally got it right. It's an ugly fix, but it works great.
1. First i got Marcel Bresnik's free Temperature Monitor
to read the disk's temperature. It shows the hard disk temperature in the dock. when it started to show close to 130 F, I would shut down the Mac. This was unacceptable.
2. I got a small, square USB fan and attached it over the rear center hole with velcro. This made the heat buildup at the hard disk worse. I figured the new fan was blowing air out of the original fan's intake, so air was moving in the wrong direction. And I couldn't tilt the screen down.
3. So I watched some how-to videos before cracking the case. I had to pull off the motherboard to access the center fan and place a double layer of clear tape over the fan's intake. Then I re-attached the USB fan on the back, and this seemed to cool the disk much better. However, I still could not tilt the screen down. If you are comfortable with the screen not tilting downward properly, you could stop here, but I got better cooling by continuing...
4. I found a short piece of hose from a vacuum cleaner, which even had a small HEPA filter. Then I made a small wooden box for the fan, complete with the air filter.
Next, I used a hole saw and cut a 2-1/8 inch circle out of the stand, directly behind the iMac's center air hole. I installed my custom-filtered fan on the stand with more velcro, and the hose running thru the stand hole directly into the center hole of the iMac. The original fan's air intake is still taped over.
a. My hard disk's temperature has not exceeded 120 degrees in over three years. I keep Temperature Monitor in my Dock to make sure.
b. The two (working) original fans operate normally, without additional software. I think this is important, as I doubt they were intended to run at full speed forever, and running them at full speed pulls dirt into the machine.
c. The new fan makes a very, very small amount of noise, and it's filtered air.
d. The three-inch section of vacuum hose extending thru the stand hole has enough flex that I can tilt the screen easily, while maintaining proper air flow.
e. I already had a hole saw, so for me the only cost went to a cheap USB fan.
Notes: Opening the iMac case is not simple. The hard part is getting just the right parts. You need a suitable USB fan, a fan housing (filter optional), wide cellophane tape, velcro, some kind of 2-inch hose that is flexible, a drill that isn't too fast, and a hole saw available for $10 at any hardware store.
Cutting the hole is scary to think about, but it's a lot easier than it sounds. It does not harm the integrity of the stand (there is already a larger hole in it). The aluminum is soft enough to cut cleanly and easily. Put a piece of paper underneath to catch the filings, and don't allow the aluminum particles to enter the iMac. Mark the center and use the saw's center bit to start the cut and maintain the center position. Do not try to go too fast, or press hard. It cuts like butter!
Do not use a high-speed drill; it will get hot, and you don't want that. Use a medium- low-speed drill and take your time. You might even pause for a few minutes halfway through the cut to avoid heat build up. It will stay cool enough that the silver paint will be completely unaffected. A nail file will clean up any sharp edges.
In the end the only problem is that the fan uses one USB port, and I guess it looks a little strange when you peer around the back, but i don't care about that.
: Well, I have to admit, I'd never have done something like that, and I find the solution quite creative. It's certainly a lot cheaper than replacing whatever part of the iMac would need to be changed.]