Western Digital drives have an 8 pin connection for plugging the temperature sensor cable into. 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. I installed a Western Digital drive exactly like the old drive and my fans are still going nuts! The LCD temperature sensor cable is located here on the motherboard:

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. 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 your fans are each in the 1,000rpm range (give or take a couple hundred) everything is running normally. 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. The LCD sensor controls the optical and CPU fans. The hard drive sensor controls-you guessed it-the hard drive fan. The LCD sensor and the hard drive sensor control different fans inside the iMac.

Luckily you don’t have to rip your iMac back apart to diagnose whether your LCD sensor is the problem. 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. 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. 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. So why are the fans in my iMac spinning wildly after my hard drive upgrade? 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.

After seeing an uptick in reports of out of control fan speeds in 20 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.

New hard drive for imac