iMac Temperature Sensor Dummy Load

iMac Temperature Sensor Dummy Load

This is an update to my other posting about the upgrade of my 2010 iMac to the Samsung 840 SSD. In that post I had an issue with controlling the HDD fan since the sensor was unable to be connected to the SSD. With the sensor disconnected the fan would run at full RPM continuously. I was able to control the speed with software on just about every event the computer would go through, such as startup, shutdown and wake from sleep. The only event I couldn’t control was wake for network access. That event occurs when I access the iTunes library from the Apple TV at which point the fan would run at full RPM again. The only way I got around that was to never put the iMac to sleep, leaving it on all the time and only putting the display to sleep.

Samsung Electronics Samsung 840 Series Solid State Drive (SSD) 500 GB SATAIII 2.5-Inch MZ-7TD500BW

I got to thinking the other day that Apple probably didn’t read the HDD temperature by sending data from the HDD itself, but rather measuring the temperature of the jumper pins on the HDD, where the sensor plugged in.


I had an old HDD which didn’t work anymore and I removed the controller board from it because it had the IDE jumper pins on it. I took that and used some 3M 2-sided tape and stuck the controller board to the SSD and connected the temperature sensor cable to the jumper pins. I didn’t connect any power or anything else.

I put the iMac back together and turned it on, removed all my software tricks and scripts and restarted again. I went though all the events of startup, shutdown, restart, wake from sleep and wake on network access and never once did the HDD fan spin uncontrollably. It worked!

The fan stays at a speed of around 1100 to 1200 RPM’s and I figure the ambient temperature inside the iMac affects the pins a little and that’s what gives the various speed readings of the HDD fan. The dummy load HDD controller board from a dead HDD did the trick. It’s too bad for me that I didn’t think of this sooner, like 2 or 3 months ago.

6 responses

  1. Pingback: Controlling the fan speed in Mac models after an SSD upgrade. » SandboxGeneral

  2. Thinking about upgrading to an SSD on my 2010 iMac and was excited to stumble across this article. I have an old, extra HD, so am thinking about using the controller board as you’ve proposed. At this point, have you had any issues with fan speed or heat since installing the SSD+controller board as you’ve described? Do you use fan-speed software in addition to the controller board tweak, or does that modification eliminate entirely the need for the fan software?

    Thanks in advance for your help!


    • Hi Chris,

      I’m glad you found my article! To answer your questions, I can happily say that I have had no problems with anything since I posted that “fix” earlier this year. I removed all software that controls fan speeds and by using the controller board on the temperature sensor, the fan stays around 1100 RPMS’s or thereabouts. Not once has it spun up to full speed no matter what I am doing with it, or how it’s turned on, put to sleep or woken up again.

      I hope that helps and good luck on your upgrade.



      • Thank you, Steve! That is great news. Glad to hear it has worked out so well. I’ll let you know if my upgrade ends up differently. — Chris


  3. I’m thinking of upgrading my 2009 iMac. First I was just thinking of putting an SSD in the optical bay, and either leaving my HDD or getting a bigger one for either a Fusion or internal Time Machine setup.
    My understanding based on the settings for smcFanControl is that I have three fans, one for the HDD, one for the optical bay, and one for the CPU. From what I’ve read, SSDs use about 1/3rd or less the energy of and HDD, so they generate far less heat from electricity and none from moving parts. They shouldn’t need any cooling at all, so why not just remove the HDD and optical drive fans altogether?


    • Hi Dave, removing the fans altogether would be an option I suppose and would obviously solve the max RPM issue. I haven’t tried it, but I don’t see why it wouldn’t work. My only concern would be about overall case cooling. I bet Apple designed the inside cooling to work with all fans. If, by removing these two other fans and leaving only the CPU fan, there could be a build up of nominally generated heat that has nowhere to go since active cooling methods were removed. That could potentially be a problem – or maybe not, I’m not sure as I haven’t tried it myself or read of anyone else doing it.

      If it’s something you end up doing, please write back with how it turns out.



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