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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 Görseli


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


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Thanks that was a sensible, relatable actual useful answer. Appreciate it.


I second that as you answered for so many machines at once and help us upgrade without degradation of the equipment.


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


Thanks Dan, however as I stated previously the fan is working normally without the cable fitted, they generally run full tilt without the cable I gather.

I don’t know what to say, other than it’s working ok and has been since my first post here.

All the best,



@Wayne Jones - your good! You have the late 2013 model not the early 2013, so the firmware has the fix.

Just dotting the I and crossing the T here as others may get confused.


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I have a mid-2011 i5 27" iMac (12,2). I have an SSD to install.

I'm not clear on whether the SMART updates will be applicable here (you mention 2010 and 2012), but I'm guessing the answer is "no". Is there a software alternative to the adaptor kit or is the adaptor still required?

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You need to OWC sensor as the SSD does not offer access to the onboard thermal sensor via the SATA power connection like the custom Apple HDDs did.


I've read about software, such as "SSD Fan Control", will this not suffice?

Thank you for responding so quickly.


@originalmacbabe - Sorry no, you’re just forcing the system to a fixed fan RPM, not allowing the SMC to manage things.


@danj OK. Thank you. So my choice is either get the kit from OWC or run the computer from an external SSD. Glad I asked.


@originalmacbabe - leave the HDD connected otherwise you’ll have a racing fan issue.


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This is also not entirely correct and thus might be misleading:

I have the 27" iMac 10,1 from late 2009, i. e. the Intel Core 2 Duo version and it DID have a "pinned" thermal sensor for Apple's proprietary HDD.

This MIGHT be different on the Core i5 and Core i7 models from the same year (I believe they're called iMac 11,1), but I don't know.

There are some other noticeable differences between these two lines _from the same year_, which is very confusing, unfortunately.

E. g. the i5 and i7 models can handle 32 GB of RAM, the Core 2 Duo only 16.

Anyway: As per a tip from a private website from someone who modded his iMac with a pinned thermal sensor, I shorted the cable from that HDD and left it pinned to the motherboard. I stuck a bent metal (duh!) paper clip into the two pin-outs that would normally go into the HDD.

Apparently that tricks the iMac into not ramping up the fans to full speed. And that works.

BUT -- and it would be great if s.o. could confirm this -- I have not heard the fans go beyond the minimum speed yet on their own, although I left several intense programs running and used the infamous terminal command to max out the CPU to 100 %.

The smcFanControl (which I installed only to test if the fans still do work) lets me ramp up the fans, but of course I would need the iMac also do that by itself if need be. Otherwise it throttles down or might even fry parts inside. According to the sensors it never got hotter than around 55° Celsius. But I don't know if all this is normal.

This is my 27" iMac from late 2009 (iMac 10,1, Intel Core 2 Duo, 3,06 MHz):

Block Image

This cable (593-1033 A) connects to the thermal sensor on the Apple HDD:

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As advised in other forums, short the cable and leave it attached to the motherboard. Check with a multimeter that there's connection:

Block Image

Heat-shrink to not short out anything after reassembly:

Block Image

I drilled two holes in the provided bracket and mounted the SSD. The taped on (and shorted) cable doesn't measure anything here and neither does it get thermal data from the SSD. The tape is just for it to not dangle around inside.

Block Image

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@kerneltask - The external sensor plugged into the hard drive and routed though the SATA cable. At you can see in Step 10 in this guide iMac Intel 27" EMC 2309 and 2374 Hard Drive Replacement for your system whereas in the 2009 21.5” system has the external sensor plugged directly into the logic board as you can see here in Step 10 I. This guide iMac Intel 21.5" EMC 2389 Hard Drive Replacement.

So yes it can be confusing where the external sensor is plugged in. And yes, the Core2Duo is based on the 21.5” setup. This was a transition year between the two architectures.


I'm referring to your comment "And the 2009 and older use a glued on sensor as the HD vendors didn’t offer internal sensors back then." for the above 27" model.

Mine did not have a glued on sensor that one can simply transfer nor was the sensor routed through the SATA cable.

The Apple HDD in my 27" iMac from 2009 (Intel Core 2 Duo) had a _third_ cable (the thermal sensor) attached. THAT cable I shorted.

I would love to upload the image from my modification here but I can't. However, it looks like the picture from the 21" model you linked.

(Never mind, I updated my comment with pictures.)


@kerneltask - I’m suspecting this was a replacement HDD and the person who installed it could figure out how to plug in the sensor as the newer generation drives did away with the header the cable plugs into. I’ve seen that a few times.

Your image shows the cable connector, not the sensor side so it’s hidden below the drive or just hanging loose. Is that the case here?


I was the first one to open this iMac and it had a Seagate Barracuda 7200.12 with an Apple logo in it: 1.0TB PHARA 655-1565A.

Just wanted to point out that also the/ some (?) 2009 models (perhaps only the 27" ones) did not have a glued on thermal sensor that one could easily transfer, but a cable that connected to the HDD's sensor on the board.


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Good day


I am having a iMac (21.5-inch, Mid 2011) - Model : A1311, do I also need to replace a sensor when I replace with a new SSD drive



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