18 June 2022
The iMac G3 is an easy favorite for many Apple computer owners. It was originally released in 1998 and had several variations until it was discontinued in 2003.
One of the biggest issues with the iMac G3 is that it was designed to be convection cooled, meaning no internal fans were used and the computer relied on heat rising out through it's top vent. Since there was no active cooling some components within the iMac are prone to damage and failure over time. The PAV board (Power, analog, video) and flyback transformer are the biggest points of failure due to heat damage, the GPU is also known to suffer from some of these issues.
When the PAV board or flyback transformer start to fail you may notice the CRT arcing - a thin line appears quickly across the screen accompanied with a static popping sound. Your iMac may also shut off unexpectedly, or refuse to power on. These symptoms are serious, arcing issues can become fire hazards and should be taken seriously.
I recently came into posession of an almost new condition iMac G3 "Flower Power" model and wanted to take care of it since it had been so well kept. I decided to install some internal fans to help the machine keep cool and avoid any serious component failures in the future.
The case design of the iMac makes it difficult to install standard sized computer fans. I didn't want to open the top of the case because since I'm not comfortable working near the CRT. I also didn't want to risk breaking any of the plastic case tabs which become brittle from years of heat and UV damage.
I purchased a variety of fans and a Molex fan adapter from AliExpress and local trading websites. I also installed a solid state hard drive (SSD) which made more free space where the HDD would sit which I made use of with the fan cables etc.
Several of the fans I purchased wouldn't fit anywhere useful since I didn't want to open the top case, so I began with the smallest fans I received. I placed one in front of the flyback transformer inside the case. Be very careful not to adjust the CRT dials on the transformer. I also placed packing foam inside the fan's screw holes and folded it through to reduce vibration when placing it inside the case. I used a hot glue gun to secure the fans because it's transparent and easy to remove if needed. It also meant that I didn't have to make any alterations to the case. This fan pulls air from the side of the case into the flyback transformer and across the PAV board. There's not much of a gap for airflow inwards but it does seem to pull enough air through. This fan runs at 12v.
The second fan I installed was under the processor, and secured to the EMI shield. I drilled three holes into the shield because I have a spare if I ever wanted to replace it. This fan pulls in air from both of the bottom vents and runs at 12v. The air is directed at the CPU + GPU and spreads across the logic board and up the sides of the case.
After running the iMac for a while with the two fans installed I noticed that it still became hot at the bottom left side of the case (flyback transformer, PAV heat sink area) after running for +1hr. I opened the machine again and mounted another of the smaller fans to the case running at 12v blowing air directly onto the larger PAV heat sink. This pulls air from the side of the case and directs it straight onto the heat sink, which is then deflected upwards towards the top vent. After installing this fan there was a much more noticeable breeze coming from the top vent of the machine as it was running. The left side of the case is still warm with general use, but it's cooler than it was without the fans.
This fan is the noisiest in terms of vibration. I couldn't dampen the vibration with foam because I didn't want to risk it melting onto the heat sink. Instead I held the fan in place while it wasn't touching anything and hot glued it in place.
I didn't plan on using LED fans here, I'll probably replace them in the future.
I placed a fourth fan above the top end of the logic board PSU near the heat sinks. This also pulls air from the vents at the bottom of the case and directs it onto the PSU. This fan runs at 5v.
Ideally the machine should have a large fan above the PAV board, near the top vent increasing airflow out of the case. I wanted to do this but I couldn't find a way of doing this without removing the top case.
There is of course increased noise after installing the fans. This is why Apple opted for convection cooling rather than fan cooling. The noise is comparable to the sound of running an external mechanical HDD on top of a wooden desk. This of course could be improved with better quality fans.
There is only one temperature sensor present within the iMac G3 which is on the CPU. The maximum temperature I have noticed after vigorous testing was 48°C (which is very good) reported by G4 Strip CSM, and 46°C reported by Gauge Pro. Testing included running the games SimCity 3000 and Tropico for two hours, then general use of the machine for another 2 hours.
There is a good breeze coming from the top vent of the machine now. The does case still gets warm inside, but feels a lot cooler than what it did without any cooling.
When this article was originally posted I mentioned that I noticed the CPU and Cache were running at faster clock speeds for ~5mins after booting before returning to normal values. After testing this on several other machines I found that this is either a quirk of Mac OS 9 or the Newer Technology Gauge application. I have noticed this happening on an iMac G3, PowerBook Pismo and PowerBook 1400 regardless of any modifications.
To me the cooling additions were worth it and I believe they make a big difference with the internal temperatures. The added noise isn't that bad, I felt it was more annoying when I had a mechanical HDD installed (without cooling) and the constant seeking noises it would make. I plan on getting a temperature sensor to check the internal case temps and will update this page when I get the results.