Interesting! Seems to work for many who get roaring fans: Turn off, unplug, push power button to drain (capacitor issue?!) then plug and restart.
Macintouch - Mac OS X 10.4.3Anything that involves capacitors in the iMac worries me, and the pushing power button to drain sounds like a capacitor to me (I'm no EE though.).
10.4.3 Fan Problems: A big thank you to Peter Trondsen for his tip of disconnecting the power cord and pushing the power button. My rip roaring iMac G5 is quite again.
An Apple kb article talks about the same sort of thing -- another bizarre embedded smart device management unit is causing trouble, and has its own exotic reset:
The SMU (System Management Unit) is a microcontroller chip on the logic board that controls all power functions for your computer. If your computer is experiencing any power issue, resetting the SMU may resolve it. The SMU controls several functions, including:Not to be confused with the Power Management Unit! Arggghhh. Apple's embedded OS devices drive me crazy. I wonder if the 10.4.3 update tries to update the SMU logic and that causes the fans to roar. The SMU reset article suggests that owners of 1st generation iMacs won't benefit from the unplug trick -- they'll have to open the iMac and push the internal reset button.
* Telling the computer when to turn on, turn off, sleep, wake, idle, and so on.
* Handling system resets from various commands.
* Controlling the fans.
Note that resetting the SMU does not reset the PRAM. Resetting the SMU will not resolve issues in which your computer is unresponsive—in these situations, restarting your computer will generally suffice. If your computer isn't responding, perform these steps one at a time, in this order, until the issue has been resolved:
1. Force Quit (Option-Command-Escape).
2. Restart (Control-Command-Power).
3. Force Shut Down (press the power button for 10 seconds).
Resetting the SMU can resolve some computer issues such as not starting up, not displaying video, sleep issues, fan noise issues, and so on. If your computer still exhibits these types of issues even after you've restarted the computer, try resetting the SMU. To reset the SMU on an iMac G5 (Ambient Light Sensor) computer, simply do the following:
1. Turn off the computer by choosing Shut Down from the Apple menu, or by holding the power button until the computer turns off.
2. Unplug all cables from the computer, including the power cord.
3. Wait 10 seconds.
4. Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer.
5. Let go of the power button.
6. Press the power button once more to start up your iMac.
Unlike earlier iMac G5 computer models, the iMac G5 (Ambient Light Sensor) and iMac G5 (iSight) computers do not have a button on the main logic board for resetting the SMU. This eliminates the need to remove the computer's back cover to reset the SMU.
I'm getting similar problems though with 10.4.2 and my iMac; I dropped the CPU to "low" to reduce heat and the system quieted. That happened after a memory update; I wonder if installing the memory somehow confused the SMU, but it's true I'm also running iTunes continuously now. I'll try this and report what happens.