Mac Classic

iMac G3 Cooling Modifications: Part II

In my previous iMac cooling article I detailed the cooling modifications made to my iMac G3 with the intention of prolonging it's life. After running this setup for a while I came across some issues, and found improvements that could be made.

When first performing these modifications, I knew the cooling gear I was using was cheap/low quality, and expected to replace it soon after. The Winsinn fans bought from AliExpress failed within ~1 week. Despite the low quality gear, they did a sufficient job and brought down the ambient temperature immensely.


The cheap and noisy fans were no longer suitable so, I purchased 4x Noctua fans:

I also wanted to reduce the added strain on the iMac's power supply and purchased an external power supply for the fans. This is the only "cheap" part used in the project due to unavailability of a better quality alternative in New Zealand. For this I purchased a 4-pin molex power supply from AliExpress:


This time I wanted to cool the iMac properly and get more airflow through the case, so I decided to mount the two 20mm fans on the underside of the upper vent. This required dismantling the case and removing the shell, so I could safely install the fans without going near the CRT.

I secured these by inserting the included rubber mounts into the fans, and then pulled them through the vent. When they were set in place I trimmed the outside part of the rubber mounts.
I also used the fan cable extensions that came with these fans so they could reach the underside of the machine.

Note: if the plastics have become brittle in your iMac (very common), be prepared for some small breakages. Be extremely careful not to be too forceful and keep the iMac on a soft surface while working on it.


Extreme caution must be taken when working near CRTs. CRT power supply units can store enormous amounts of energy even after being unplugged and unused for a long period of time. An electric shock from one of these can kill you due to the extremely high buildup of current stored in the machine. Do not touch any of the power supply internals unless you know what you are doing and have correctly discharged the machine!

The only other fans I installed this time were the CPU/GPU fan, and one facing the flyback transformer (both 10mm). As mentioned in the previous post, the CPU/GPU fan draws air in from the lower vents, which then makes it's way through the case. The flyback facing fan doesn't have a lot of room to draw air from, but it's enough and keeping this part as cool as possible was essential to me (see iMac issues and care for more information).

I used the same rubber mount method as before for securing the CPU/GPU fan into the pre-drilled shield holes (which I made in the previous post). For the flyback facing fan there isn't a lot of room to do anything fancy. Luckily the 10mm fan with it's rubber corners fits snugly inside the iMac's case.


While I had the iMac open I also took the opportunity to add some heat sinks to the GPU, cache and other chipsets.

    After securing the fans I placed folded electrical tape over all fan cables that the shielding would sit over to prevent any damage to the cables.

    Before putting the case back together I connected each fan cable to a Molex to fan splitter cable, which I had coming through the case where the external video door is. This allowed me to bring the external power supply's cable through and connect it to the fans.


    These modifications work extremely well. I have a digital thermometer within the iMac, and with these fans installed the ambient temperature doesn't rise above ~38 degrees Celsius while running a game like SimCity 3000 for 2 hours.

    The fans are very quiet and I would compare the noise to running a G4 Tower (Not a MDD wind tunnel). When holding a hand over the top air vent there is a huge amount of air being pushed through which is fantastic.

    To top it off, the finished product looks great, the only thing to show that any modifications have been made are the small rubber mounts at the top of the vent. The only potential downside is that you must remember to turn on/off your external power supply when you turn on/off the computer. Although I believe this compromise is fine considering it is taking a decent load off the iMac's power supply/PAV board. I also like to leave the fans running for a short while after turning off the iMac to properly cool it down and ensure there is no trapped heat to cook it while it's off.

    The Finished Product


    Christopher Bartleson

    Christopher Bartleson

    31 August 2023

    It looks like you can fit some larger fans on the metal mesh part to get more airflow in the case.


    Greystash - Admin

    31 August 2023

    There is actually a lot of room in there to play with. A bigger fan would be a good idea, there's plenty of vent space at the bottom to draw from.


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