After seeing an uptick in reports of out of control fan speeds in 2009 and 2010 iMacs, we decided to revisit our suggestion of the same brand for brand swapping of the main hard drive in those models to verify that our information was still current. We’re proud to announce that after some extensive testing with every current 3.5″ HDD we carry, the swapping of brand for brand is still 100% completely valid in those models.
So why are the fans in my iMac spinning wildly after my hard drive upgrade?
The most popular reason for the spinning fans is that one of the temperature sensors was not seated fully (or at all) during re-assembly. During the process of removing the iMac’s LCD to get at the hard drive, the LCD temperature sensor is removed from the logic board. If that sensor does not get plugged back in during the re-assembly process, the iMac’s fans will run at high speeds even if the hard drive’s sensor cable is plugged in correctly.
Luckily you don’t have to rip your iMac back apart to diagnose whether your LCD sensor is the problem. The LCD sensor and the hard drive sensor control different fans inside the iMac. The hard drive sensor controls—you guessed it—the hard drive fan. The LCD sensor controls the optical and CPU fans. If you install the iStat widget you can determine easily which fans are running faster than normal and in turn identify which sensor is giving you problems.
If your fans are each in the 1,000rpm range (give or take a couple hundred) everything is running normal.
If the hard drive fan is spinning over 2,000rpm, chances are the cable isn’t fully seated at one end or the other (or you’ve installed a Western Digital drive – see the next section of the article).
If the Optical Drive and/or CPU fan shows the fans spinning over 2,000rpm – then it is the LCD sensor cable that most likely isn’t fully seated.
The LCD temperature sensor cable is located here on the motherboard:
I installed a Western Digital drive exactly like the old drive and my fans are still going nuts!
The second most common reason that the fans on the iMac go haywire is when replacing a Western Digital drive with another Western Digital drive.
Western Digital drives have an 8 pin connection for plugging the temperature sensor cable into. However, that temperature sensor cable has but two wires (one light and one dark) and only 6 pin sockets. This results in four different ways you can plug that cable into the drive, but only one way that sends the right signal down the right wires.
We found something interesting while testing our current Western Digital drives. In the 2009 iMac they did not work correctly in the configuration that they were officially supposed to. This is the way Apple officially states that the WD temperature sensor should plug into drives in the late 2009 iMacs (cable plugged in closest to the SATA connector with the light cable closest to the PCB board on the drive):
And for the late 2010 iMacs, Apple says that the sensor’s position should be reversed. (Cable plugged in closest to the SATA connector with the dark cable closest to the PCB board on the drive):
Now, we’ve found that when replacing that original drive in the 2009 models, you need to plug in the drive with that dark cable closest to the PCB board like on the 2010 models. We suspect this was a running change on Western Digital’s part in regard to which pin sends temperature readings as opposed to Apple changing the cabling around again. So the change doesn’t affect the 2010 model instructions at all but if your hard drive fan is spinning like a jet engine after your Western Digital drive installation, chances are you just need to reverse the cable to correct it.
If you’ve tried the above steps and are still running into issues, please feel free to contact our Technical Support Team at 1-800-275-4576 (or 1-815-338-8685 for international callers), via live chat, or email for individual troubleshooting assistance.