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Restore normal fan operation on an iMac G5 System
I attempted to boot my iMac G5 with Bluetooth keyboard into single user mode. Sadly, it didn't work and caused my fans to subsequently run at full speed, despite CPU usage around the 4% mark. I almost went nuts trying to find how to turn the fans down, until I came across something.

Now the reason this is a hint, is because it almost makes no sense and when I posted my own answer on the Apple discussion boards, I got a lot of reponses from people saying they hadn't ever come across it.

It seems the rules for reseting the power management unit for the iMac G5s is the same as that of the PowerMac G5 machines. In my case, the late 2004 G5 processor, that simply involves removing the main power cord for a few minutes and plugging it back in. Once I did that, my fans returned to normal speed.

But a hint of advice ... if you are having a problem, search for an answer to your processor and not your machine! It may save you lots of time.
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Restore normal fan operation on an iMac G5
Authored by: jeffulri on Mar 08, '06 11:10:07AM

If the unplugging hint doesn't work for you, try opening up your machine and resetting the two hardware switches on the motherboard. That was the only thing that worked for me. Leaving the machine unplugged between 5 and 60 minutes did not fix the ramped up fan problem for me, but pressing the two hardware switches on the motherboard for 10 seconds each did.

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Restore normal fan operation on an iMac G5
Authored by: webjakob on Mar 08, '06 12:07:03PM

For my model of iMac G5, resetting was done by inserting the powercord while pressing the powerbutton.

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Restore normal fan operation on an iMac G5
Authored by: dsjkvf on Mar 08, '06 09:48:18PM

where those two hardware switches on the motherboard an be found? is there any photo on the net?

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Restore normal fan operation on an iMac G5
Authored by: pi314 on Mar 08, '06 03:39:56PM

I also have iMac G5 of 2004. I think it is the feature of 10.4.5 to keep maximum fans after single user mode because of the power management unit failure.
When you try Apple Hardware Test, it reports fan and hard-drive error. But after resetting power management unit, the error is never reported.
I think Apple should announce this problem, and fix it on 10.4.6. We should feed-back to Apple.

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Restore normal fan operation on an iMac G5
Authored by: Raven on Mar 09, '06 06:22:04AM

I actually did check with Apple while I was traning there a few weeks back. I had to enter Single User Mode on a G5 dual processor tower to fix and issue and obviouslt every one in the class "kind of" noticed the full blast fan noise in the quiet room.
Since it was the first time I had to boot a G5 in Single User mode I was surprised myself, so I asked the trainer. He said that by default G5 towers and G5 iMacs will runf ull blast in Single User mode as power management/temp monitoring don't load if booted this way, so, to prevent issues, they added info on the ahrdware to run at full capacity to make sure that the machine would not overheat even though its not being monitored. So it IS a feature... Just that it look slike there is a way to get around it... but wouldn't run processor intensive tasks in SUM with this diosabled as even if the processor starts heating up, the fans probably won't pick up speed.

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also in target disk mode
Authored by: mbaudis on Mar 10, '06 06:09:40AM

i just did a (5h) user transfer from a 2GHz 17in G5 iMac to a new Mini, using target disk mode on the iMac. basically, one could not stay in the same room ... should default to some intermediate setting, i think; i wonder how well the fan can handle that if unobserved for a longer time.

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clean your machine
Authored by: kd4ttc on Mar 30, '06 06:57:06PM

I finally fixed my problem. I have had the fan noise problem on my iMac flat panel since the 10.4 install. By cleaning the vents the thing has gotten back to normal. I think that the new system had a different power curve vs fan setting and that my machine must have been marginal in heat clearance. I was afraid I would need to open the machine, but just vacuuming the top and bottom holes worked. I think I need to still open it up. I burned out a 10 year old power supply on a 6116CD a year and a half ago. Inspection showed dust on the air grates. Invisible until disassembled.

Steve Holland

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