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Model A1312 / Mid 2010 / 3.2 GHz Core i3 or 2.8 & 3.6 GHz Core i5 or 2.93 GHz Core i7, ID iMac11,3

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Do I need an OWC thermal sensor if I want to swap SSD?


I noticed there was a lot of fuss about OWC (a scam/controversy about the thermal sensor in an iMac 2011, which could have been resolved using a Y cable splitter).

My system is a iMac mid-2010 27” I want to place an SSD instead of the original HDD (so I want to maintain the disc drive bay).

Thanks to these (1) (2) (3), I came to the conclusion I may not need at all the OWC cable, maybe I can resolve thanks to a thermal sensor from the disc drive. As suggested in the first link previously posted, the only difference would be my iMac is 27”, the one in the link is a 21”.

Thanks in advance!


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Sadly there’s a lot of miss-information still out there!

So let’s see if we can clarify things. The 2011 systems use a custom drive from the HD vendors that supply Apple. These drives have the internal drive thermal sensor wired to the spare wire within the SATA power connector.

The older 2010 systems leverage a header pinout to access the internal thermal sensor of the HDD. And the 2009 and older use a glued on sensor as the HD vendors didn’t offer internal sensors back then.

So what is this thermal sensor? What is it for?

This sensor was used during the manufacturing of the drives so a hot running drive would be pulled as not meeting the specs the maker was contracted to meet. It’s a simple NPN transistor monitoring the power going through How do you get a temperature sensor from a transistor? The biggest issue is the range of the sensor so compensation and an amplifier is often added to properly scale the sensor so it is accurate.

So while you can use the Apple optical drive NPN transistor you may not get the accuracy you want. At the time we where winging it as we hadn’t gotten access to the schematics to review the circuit, today we know better! So I don’t recommend using the Apple optical drive sensor as its not calibrated where the OWC sensor is!

OK, whats the story with the Y connector? All you are doing is shorting out the sensor line so yes SMC is happy as its getting a signal, the only problem its a useless signal! I love people who are not skilled in electronics discover a simple solution without understanding the ramifications! You really don’t want to mislead SMC as you’ll cook your system. Placing a gun at your head is a bad idea too! Don’t do this nonsense!

What also confuses things is if you have a SSD only system Apple shorts the logic board header for the HDD connection so the SSD’s SMART services is accessed. If you have a dual drive setup you need to enable it as the polling of the HDD SATA port is more frequent! The SMART polling is every 5 or so minutes unlike the SATA connection which is constant.

What about newer systems how do they work? The 2012 & 2013 systems still require the OWC sensor as Apple did not update the firmware with the newer version of SMART Apple peddled to the ANSI/EIA groups SMART consortium. It took some effort on Apples part to get the needed change so the SMART thermal sensor IRQ method could be also polled directly. So what is IRQ Interrupt request which causes the computer to go into a wait state for the request loosing processing time for the interruption. If you remember I explained the Apple SSD uses SMART in the 2011 systems and its poll is overly long which is not as big a deal with an SSD (at the time) but would have been an issue with HDD’s.. Todays denser SSD’s get warm and the 2TB and bigger drives can get quite toasty!

So let’s move on the 2014 systems and newer: If you’ve maintained the OS with macOS Mojave or newer your systems firmware would have been updated with the newer SMART services. But there is still the drive! It needs to also be running the newer version too! You then don’t need the OWC sensor.

So! For your system follow this guide iMac Intel 27" EMC 2390 Hard Drive Replacement and getting the kit iMac Intel 21.5" and 27" (Late 2009-Mid 2010) SSD Upgrade Kit will get you everything you need. And sadly you need the sensor!

iMac Intel 21.5" and 27" (Late 2009-Mid 2010) SSD Upgrade Kit Image


iMac Intel 21.5" and 27" (Late 2009-Mid 2010) SSD Upgrade Kit


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Great explanation.

I have the iMac late 2013 27” if I replace HDD with SSD and upgrade the OS to Catalina, would I still have the fan issue?

Update (03/26/2022)

I have installed a Crucial MX500 SSD 1TB in my system without the OWC cable and installed a later macOS Catalina and the internal fan is behaving normally.

Well, I’m happy, in short it seems to be working OK

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1 Comment:

@Wayne Jones - The 2012 & 2013 systems still require the OWC sensor as Apple did not update the firmware with the newer version of SMART


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Fabio will be eternally grateful.
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