HD fans and temperature sensor

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I have a 2009 iMac that runs well. I did eventually get a S.M.A.R.T error, and it was time to replace the internal HD. I replaced the Seagate disk with another Seagate disk that OWC said would be temp-sensor compatible such that the fan would run properly automatically. Well, it doesn't. The fan wants to go at a high speed (though interestingly not the highest speed.) No biggie. I just use Macs Fan Control to set the HD fan to a reasonable speed, and everything is dandy.

BUT, Macs Fan Control reports temperatures, and the temperature reported for the HD all seem to be right. 40C or so. That's pretty much what my second iMac HD temp shows. It has the original disk with the fan automatically running slowly. So I'm confused. If my Mac can read the HD thermal sensor properly, why is the fan not being controlled properly by the Mac? I can understand that if the temperature being read was all bolluxed up, the fan control would be as well. But that seems not to be the case. Explanation appreciated.
 

Cory Cooper

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Hello,

Since the iMac's HD isn't a user-serviceable part, it may just be that the new HD temp cable connector is different (I have seen this even with OEM Apple replacement HDs), is not connected, or another temp sensor/fan cable wasn't reconnected properly when you did the HD swap.

It may be wise to take it to an Apple Store and have them run hardware diagnostics on the iMac, which should show what the issue is.

C
 
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Well, again, this isn't a "problem" I'm trying to fix. The incompatability of temperature sensors on hard disks used internally on Macs is well documented. See

http://blog.macsales.com/27918-owc-...or-solves-imac-hard-drive-compatibility-issue

The point being, if you don't replace your hard disk with the "right one", the temperature sensor won't work properly. I thought my replacement disk was a "right one". OWC advertised it as being that. It evidently isn't. But there are easy workarounds.

My question was specific. I now get HD temperature readings that look correct. But the automatic fan control isn't working properly. If the Mac knows the correct HD temperatures, why isn't it automatically setting the fan speed correctly?
 

Cory Cooper

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-It is the HD fan that Macs Fan Control is reporting as running higher than normal?
-Are the other fans running normally?
-Have you tried an SMC/NVRAM reset?

C
 
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Yes, Macs Fan Control is reporting the HD fan running at about half speed when on "automatic" control. Noticeably noisy. It should be running at 1/5 speed or so, and I can manually make it do that. Interestingly, some people who replace their HD with a sensor-uncompliant one evidently find their fans running FULL SPEED. That isn't happening here.

Yes, other fans seems to be running normally and slowly.

Yes, the Mac was powered off while the HD was switched out, so it automatically did a SMC/NVRAM reset.

So the issue is, if Macs Fan Control knows correctly what the HD temperature is, how come the OS doesn't?
 

Cory Cooper

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Well, if everything else is running normally, I am not sure why OS X doesn't detect the same temps that Macs Fan Control does.

You could use Activity Monitor to see if there is some process using a lot of system resources.

-Is this an iMac (20-inch, Mid 2009)?

Also, I assume you attached the thermal sensor to the case of the HD with the foam tape and black plastic bracket and the EMI foam square?

C
 
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Yes, iMac mid-2009.

No I did not buy the thermal sensor with the hard disk, thinking that the internal sensor would work fine. As I said, OWC said that the new HD was a thermal-sensor compliant replacement, and it would work. As I said, no big problem, because the workaround is easy.

But it just seems odd that Macs Fan Control can read the temperature accurately, but the OS doesn't seem to be able to. So yes, the thermal sensor seems to work, but the OS seems unaware of that!
 

Cory Cooper

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The thermal sensor I mentioned above is in Step 18 of the iFixit Guide here: iMac Intel 20" EMC 2266 Hard Drive Replacement

If it isn't attached to the bottom side of the HD, then OS X may not report the temps correctly. There is also a temp sensor cable attached to the logic board on the HD itself, which is what monitors the internal temperature. It is shown in Step 13. I have seen that sensor cable not have the same connection for replacement drives, in which Apple would supply a new cable with their OEM replacement HDs.

C
 
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Yes, that is for a generic hard drive replacement. The iFixit guide is for a Hitachi disk, which allegedly isn't thermal-sensor compatible with the Mac. But allegedly you can buy hard drive replacements that are compliant, such that the temp sensor cable attaches to the hard drive directly (just like with the original disk) and everything works like with the original disk. My thermal sensor is attached to the hard drive, just like it was to the original disk, but the OS is evidently not paying attention to it.

But this is OT. The question is why Macs Fan Control seems to be reading the correct temperature, and the OS doesn't seem to be paying any attention to that reading, and telling the fans to go fast.
 
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OK, I think we're missing the boat here. No, I didn't buy that OWC Thermal Sensor, because documentation by OWC said that if I purchased a compatible disk, I wouldn't need it. I bought that allegedly compatible disk, and evidently I did need it. To be fair, when I ordered that disk from OWC, the online order person suggested that it might be smart to get that thermal sensor anyway (basically contradicting their documentation). I declined.

When I rate my purchase, I'll make it clear that this disk didn't do what they said that it would do. But it really isn't a big deal. The problem isn't that I have to control the fan manually. That's trivial to do. In fact, there is no "problem" at all. It's just a little bit disappointing. A very little bit. I could always retrofit the thermal sensor.

My QUESTION (it's a question, not a problem) is why, if Macs Fan Control can read the temperature properly off the disk, the Mac OS seems not to know it, and insists on running the fan fast.
 

Cory Cooper

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I simply can't give any explanation of why Macs Fan Control seemingly reads the HD temps correctly and OS X does not. I understand that OWC listed that drive as compatible, but the only thing I can narrow it down to from our discussion is that the Apple OEM thermal sensor cable is not working with the drive you bought. The OWC cable may correct the issue, which is why I suggested you call OWC to discuss that option. Your other option is to take the iMac into an Apple Store and have them run a full hardware diagnostic, which could identify if the thermal sensor cable is the culprit.

;)

C
 
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Thanks. Yes, that's the mystery. Why Macs Fan Control is able to figure out the HD temps, but the operating system is not (I guess). Seems to me that if the correct temperature number is available to Macs Fan Control, it sure ought to be available to the OS. When I saw the fan not being properly controlled by the OS, I expected to manually control the fan with Macs Fan Control AND for Macs Fan Control to display a bonkers temperature. But the temperature it displays looks fine.
 

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