Chosen Solution
I have checked other threads regarding this issue and they suggest replacing the resistor with the “47K 1/16W 5% in a 0402 package” on digikey. However, the issue described in other threads are about the fan constantly at full speed . My issue is random and only happens about once a day, for about 30 seconds. This is what happens: For no apparent reason (ie I’m not using more processing power than usual), the fan starts to rev up, a bit like car racers rev up their engines just before a race. It’s goes a bit like this: VR…. VRRRRRR…. VRR.. VRR.. VRR.. VRRRRRRRRRRRRRRRRRRRRRR The pattern is not the same every time but it’s a bit like that. (smcFanControl indicates 0 rpm during that time) The thing is, it only happens once a day, so the resistor must still be in place (since the fan functions normally 99% of the time). How come the issue happens only sometimes? What could be wrong?
Sounds like the fan unit its self is failing here. What is happening is SMC is going into failsafe mode as it is trying to keep the system cool but since the fans tachometer (glitchy) is not working its pushes the fan to the max. I’m not a lover of smcFanControl software as it has messed me up a few times. While I understand why people use it I prefer this app instead: TG Pro between its better information panel it also allows you to turbo the fan when the system gets to a set threshold which is better than what smcFanControl offers. I would also strongly recommend you do a good dusting out of the system and look at re-pasting the CPU & GPU’s as the thermal paste in this series was not very good.
Was the hard drive was changed recently? Try installing this fan utility, and use the hard drive’s internal SMART thermal monitoring instead of the one inside the iMac. Doesn’t matter if its a regular hard drive or SSD, it works for both.
Hi Alex, I had a similar problem with my late 2009 27" iMac. I tried resetting the SMC controller and followed all the advice on Apple support forums etc. without joy. When I had PCs, taking the side off the tower and giving the whole thing a good clean with compressed air was a breeze (forgive the pun) but I was worried about doing this with my all in one iMac. In the end, I bit the bullet and disassembled it for cleaning. It was full of dust and the cleaning alone took around 30mins. You’ll have to remove the coverglass which is attached with strong magnets (little suction cups from eBay were the route I took) as well as the display panel itself to get to the fans and other components. The panel removal is the most delicate part as there are a couple of fiddly ribbon cables and a temperature probe that must be disconnected/reconnected very carefully. All in all I’d give yourself an hour to do this. There are some good guides on youtube that helped me. If you don’t have an air compressor you can buy compressed air in cans from a photography shop or similar, if you’ve got a compressor just make sure you go easy on the pressure and don’t squirt the fans up to serious speeds as you’ll damage the bearings. So long story short, I agree with Dan. Give her a good clean. I would be very surprised if the fault was caused by the resistor you mentioned.
