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Posted July 10, 2007

ScienceMan's iMac G5 Fan Mod Forum - Special Feature

Welcome to a special mac modification edition of the iMac G5 Mac Mod forum! Contributor "John" has provided us a special treat - his whining iMac G5 20" prompted him to attempt a radical fan modification. What follows are the steps involved in the procedure.

As usual, be aware that when you dig into your own machine, you void any warranties and do so completely at your own risk. That being said, John's work looks incredible, and I'm guessing at lot of G5 iMac owners with noise headaches will be attempting to copy his success!

A big hearty thanks to John for his work in preparing this page - well done Sir!

(ScienceMan note - John has a spectacular photoblog - it's time well spent!


Dear ScienceMan,
I stumbled across your iMac G5 Fan mod page a few months back whilst trying to find a way to quieten my 20" G5. It wasn't the whirring that was irritating, but more the "whine" noise that I had coming from the unit.
I had seen another modification where someone had cut a hole through the back of the unit and attached an external 12v Fan, giving better cooling - Rather than noise reduction. I have to admit, I stole his idea; but used it differently!
As my father is better than I with DIY, I called upon him to help me out with the electronic side of things. We cut a large, circular hole, big enough for a 120mm fan and attached it to the outside of the case. Internally, we made some *major* changes. Firstly, we pulled off the G5 "badge" from the processor heatsink and then removed a second metal covering, which covered the heatsink and allowed air to travel in a vertical motion past the heatsink. Secondly, we removed the CPU fan that was positioned above. This was the main culprit of the irritating whining noise. We did however cut the wire that connected the fan to the LogicBoard instead of unplugging it. The reason for this was because the 120mm fan we were fitting was powered by a traditional 3pin PC connection.
After finding the correct pin connections to solder to, we connected the 120mm fan to the already existing connection on the logic board. Placing the other case back onto the iMac finished the job!

iMac G5 20" Fan Modification - Step-by-Step

First off, if you attempt any of this modification, you do so at your own risk. This is just a documentation of how I did MY modification.
I accept no responsibility for any damage this may cause to your own iMac.

Before you can even attempt any of this modification, your Mac internals *must* look like this. My modification has only been performed with this setup and as such, may or may not work on any other Mac revision.

You have to remove the top right fan and the cover that connects that to the G5 CPU heatsink. Do this with a Philips head (+) screwdriver. That's the easy part! In regards to the fan, leave it plugged in for the time being!

Firstly, you need to position your fan in the appropriate position. Because my iMac happens to be wallmounted, it was a little more difficult positioning the fan as I needed to maintain maximum support for the bracket. For reference, the fan is a standard 120mm.

If you haven't already noticed, the fan is position here as it is directly above the CPU heatsink.

Once you've figured where you fan is going, mark the screw holes with a permanent marker and draw around the INSIDE of the fan to create the circular hole you see here. Once you've done that, use a small dremel to cut the hole. You may find it easier to drill the holes out first too. Please, be aware you need the correct sized drill piece, otherwise the fan screws will not fit! Measure up before doing any drilling or cutting.

Note: Cut slowly and along the inside of the markings, not directly on. If you cut within the markings you have room for mistakes. Also, do not put too much pressure down as you may crack the plastic. It would be wise to give some support to the areas you are drilling and cutting.

Check the inside of the backplate. Sand off any shards of metal that were created by the cutting and drilling. This minimises the risk of any flecks of metal causing a short circuit on the inside of the Mac.
Once completed, place the fan on the outside of the case with air movement positioned so it's blowing inwards, towards the processor. Also, screw from the inside-out. This is because the screws are self-tapping and will cut into the plastic on the fan, giving more support. Once you have done this, use some electric tape to cover the screws, just in case the worst happens. This is how the completed fan position should look from the inside. Ideally you would want the cable in a downward position for ease of access later on.
Now to the scary part. The G5 processor is covered with 2 layers. The first layer is a simple, G5 badge. It's easy enough to remove, just get yourself a flathead screwdriver and gently pry away at the edges to lift it up. It's only held down by what appeared to be double-sided tape, so don't worry if it sounds like it's cracking. The second later is more awkward. This layer is "clamped" down by some folded metal along the sides. Before this part though, remove any "tin foil" you find around the edges and please, make sure you remove the flakey deposit's that tin foil always leaves.
Once you've done that you can attempt to remove the heatsink shield. Click the above image and you'll see I highlighted where the folds are. You'll find that there are 3 clips on the right side (if you're looking down at the heatsink) and 2 clips on the left. Use the flathead screwdriver to get underneath the clips and pry them up, allowing you to fold back the shield. I had to get my father to do this part as I didn't have the guts. You literally have to bend it back - It doesn't fold back on it's own. Please, whatever you do, make sure there is enough support around the area so as not to bend the LogicBoard or any other component. You have been warned! You have two options here. We decided to cut the metal towards the base of the hinge. We found that the actual shield was held in at the base of the unit, folded around what looked like a joint between the power supply and the speaker. The only way we could find that we could get it off, would be to remove the LogicBoard from the chassis. We also decided to cut it in this position for a far more logical reason too. Whilst we were making the decision of what to do, we fired up the G5 and placed the fan over the heatsink to make sure it could do a decent enough job. Sure enough it did, but we noticed that from the bottom, there was some dust flicking around. We couldn't actually see anything there, but we came to the conclusion that there is a very small fan drawing air through the heatsink. Kind of giving it a gentle push through and leaving the rest to the large generic Apple fan.
Once the heatsink had been modified, we located the CPU fan control. A voltmeter was used to find which pins we had to use to connect our fan too as the fan we purchased was a general PC fan, with the tradional PC motherboard fan connection on. Remember I said to leave the fan plugged in at the start? This was for a good reason. Move on... Do not attempt to plug the PC fan straight into the LogicBoard. We tried, and it worked, but we do not suggest it! Cut the Apple CPU fan cable and solder the Brown cable to the Red PC fan cable, and the Black cable to the Black PC fan cable as shown in the picture above. The Yellow PC fan cable (the pulse control) was not used in my setup. We also used electrical tape again and wrapped it around the solder, just in case it came undone over time. Plug the fan back in, remove anything side which is now loose and doesn't need to be in there, place the backplate back on the unit and you're finished!

The fan does maintain a constant speed, which is it's maximum RPM. Because of the size of the fan and the slow rotation speed, the noise is minimum and there is no longer an annoying buzzing and whining sound, but a simple whirring sound. It also doesn't speed up and slow down as you open different applications, which would drive me mad also.

So far, the tempetures I have experiance have been exactly the same. When we benchtested the unit in our living room, we actually experienced much lower tempetures, as low as 60celcius under maximum load. As this unit is now back into my bedroom where there isn't as much air movement as downstairs, the tempetures are back to normal but no longer with the irritating noise.

Oddly enough, the Mac still maintains that the main CPU fan spins at around 2000rpm when idle and well up to 4200rpm when under load, despite the fact that the new fan fitted will only do a maximum of 1200rpm.

Contact John

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