Monday, December 31, 2007

iMac G5 CPU Fan

My father has an iMac G5, and these machines are notorious for emitting horrible high pitched noises caused by the CPU fan, and his was no different. Since I was there for XMas I decided to work with him to modify it to use a different fan. I followed the basic premise found here. The major difference was there was NOT a CPU fan hookup on the motherboard in his G5(Trust me I looked and unplugged everything ;) So instead we ripped out the CPU fan, cut the wires, found the 2 with the biggest voltage difference, and used that for the new fan. Also we were able to completely get the heatsink cover off, although I suspect this was because we got rid of the entire fan assembly.

The only issue was that the lines for cutting were drawn too large and we ended up with a slightly larger hole than it should have been(About a millimeter off). Also I tried one of those new-fangled liquid bearing fans. I used a 1200 RPM one, which isn't completely inaudible, but definitely less noisy than similar ones I have.

Anywho, pics or it didn't happen:

Also a side rant against Apple. First it was impossible(Yes I said impossible!) to figure out fan speed or system heat in software. Nothing was for his version of OSX(10.39 AKA: Hello Kitty) and as far I can tell there is no basic BIOS to look at. Also I couldn't find it in /proc. On the hardware side, the original CPU fan design was utterly retarded. It pushed air through a sealed up copper heatsink. This wouldn't be bad, except that the area the fan blows out is incredibly small(About 1/3 the size of a normal CPU fan). This obviously means the fan has insanely high RPMs and thus an insane amount of high pitched noise. I realize they wanted a slim form factor to look 'stylish' but did they fail to test the thing? They could easily have used multiple fans, or bigger fans on the back, but it wouldn't be Apple if it didn't suck.

Edit: To Rachel, your processor definitely should not slow down when the fan comes on. There is such a thing as variable speed processors, but those are almost exclusively used for mobile computers. I'd think it would error out if there was somehow a problem with your PSU that was draining off power from the CPU. My guess(Without knowing more) is that the fan noise is so irritating that everything just seems to be taking longer :)

Edit: A little more detail on the actual steps taken:

Modification Steps

Pretty simple, but definitely not for the faint of heart:
  1. Take off the back cover of the iMac case
  2. Remove the CPU heatsink cover(It has a big G5 emblazoned on the front). You should be greated with a big set of copper fins. There are some small tricky tabs that hold it in place. Also mine came completely off, but apparently some versions have to be cut off
  3. Find the CPU fan connector on the motherboard and unplug it, plug in your new fan and test it. But my version didn't have a CPU fan plugin(Trust me I checked and unplugged everything several times), so I forcefully removed the CPU fan(it's towards the bottom) turned on the computer really quickly, used a multimeter to check which leads were positive and negative(It had 3 wires, one is for speed sensing I think), and then cut the wires on my fan and spliced them in.
  4. Next I marked on the back where to cut for the fan(Directly over the heatsink) and cut it out with a Dremel
  5. Finally mounted the fan to the back over the hole and put everything together again


The computer has been running over 3 years now with no problems. It is cooler than it was with the stock CPU fan, so the modification should prolong the life. It is going at about 60C(140F) after being in use for a while. The maximum temperature a PowerPC G5 processor can run looks to be about 75-80C(~170F).


Anonymous said...

Wow! I don't know if I would have the confidence to go in do what you did. I found your blog while looking for the reason why my iMac G5 fan runs like a plane taking off! At the start of my day it is nice and quiet then by the afternoon it is full RPM mode! So annoying! I found one mention of resetting PMU inside.

It seems that when the fan takes off my processor slows down. Have you heard of that?

Anonymous said...

Hello Gavin,
I recently bought an Imac g5 with temperature issues like yours. After no luck talking to Apple, I would like to do a fan mod like yours. I would really appreciate some more details on how you pulled things apart, and put them back together again. Also, is your imac g5 still working , as I notice you modded it in 2007. I can send you my email address if that's ok. Thank you!