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Apple Hardware Test = Apple Incineration Utility

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Original poster
Feb 18, 2009
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I'm running AHT on my new 21" iMac because it looks like the additional Kingston memory I have might be bad. (I get random lockups with the additional RAM installed; smooth operation when not installed).

I have the extended tests running with looping enabled, and I can cook an egg on the back of this thing. The fans don't run at all during the memory tests. The aluminum is absolutely scorching in some spots.

What's the point of a hardware test that cooks the machine's innards? Or should I be investigating a possible fan issue now too? This just doesn't seem right & the machine has been here for less than a week.
 
The back of the iMac is made of aluminium (metal), thus it dissipates heat better than the former plastic back, thus you will feel the heat on the entire surface. The iMac is designed to use its fans only when the heat is too much, and it seems it isn't yet. If you get the iMac running again, transcode some big video you have to another format via Handbrake and see, if the fans speed up or not.
 
When considering that the aluminum shell is made as a heatsink (the heatsinks from the CPU and GPU are "fed into" the casing) I wouldn't worry.

If the fans run as they should when in normal operation, I wouldn't worry.
 
My iMac gets really hot in the back too. But it's normal. When you own an Apple laptop, you'll see what's an egg frying computer hahaha
 
Might be worthwhile to created a pinned topic saying "the top/back of the aluminum iMac is supposed to get real hot"
 
I understand that the aluminum assists with heat dissipation; I didn't realize heat dissipation was among the primary functions of the case, so much so that it is literally too hot to handle. Just caught me off guard.

And I've had a MacBook Pro the past 1.5 years. It gets warm but not scorching. ;-)

Edit: Again, this is during the Apple Hardware Test specifically, which never seems to fire up the fans except during logic board testing. Under normal operations the iMac is much cooler, with all three fans running at 1k RPM minimally.
 
f00f said:
And I've had a MacBook Pro the past 1.5 years. It gets warm but not scorching. ;-)
Click to expand...

The CPU used in your iMac has double the TDP compared to your MBP, thus double the heat. The 3.5" HDD also radiates more heat than a 2.5" HDD and the GPU in the iMac is hotter than the MBP's.

http://ark.intel.com/Product.aspx?id=48505
http://ark.intel.com/Product.aspx?id=35563

Edit: Again, this is during the Apple Hardware Test specifically, which never seems to fire up the fans except during logic board testing. Under normal operations the iMac is much cooler, with all three fans running at 1k RPM minimally
Click to expand...

That is the base speed (CPU - 1200RPM, ODD - 1000RPM, HDD - 1200RPM), audible, but very silent in my experience. Even if I stress my older C2D CPU (2GHz) with Handbrake or other nifty tools, the fans don't speed up, and I have a plastic back on my iMac, though my iMac has a notebook CPU with a TDP of 35W.

http://ark.intel.com/Product.aspx?id=29760
 
So 120F surface temp on the iMac's case is normal? Guess I don't need a portable heater in the office this winter! ;-)

That temp is during AHT, a diagnostic that is designed to run potentially for many hours such as overnight. I was concerned that the temp was abnormally high; it's very hot to the touch & I can't imagine how hot inside. Running any computer at high temps for extended periods of time is, in my experience, unwise. But if what we're saying is "it's normal", so be it -- so long as the innards don't melt and ooze all over! :p
 
The cpu and gpu can handle temps in excess of 200f, so no need to worry. :)

The hard drive, however, will start to lessen its lifespan when its temp reaches above 120f. Thankfully apple put a dedicated vent for where it is, and it's rather far away from each of the major heatsinks, with empty space for extra air insulation aroun it, so it tends to stay cool.
 
I shouldn't worry too much about the temps. At this moment my GPU is running at 119F and its heat sink at 117F.

Electronic components are designed to run at much higher temps. For example the average surface mount capacitor will tolerate up to 257F.
 
If your iMac did not power off during the AHT loops, then it has not reached the thermal threshold that could be considered overheating.
 
The iMac heatsink has limited thermal coefficients and will not protect components from overheating entirely. The amount of heat generated DOES decrease the life of the computer. Please understand that this is the price you pay for a minimalist AIO design.
 
You can always do what I did with my 1986 Mac Plus. Get a small desk fan and aim it at the hottest part of the machine. Not only will this cool the computer, but it will also scare away any cats from chewing cables and pissing on the keyboard.

Hey, it worked for me.
 
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