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10.4: Fix full-speed fan issue with 10.4.4 on iMac G5s
Authored by: mm2270 on Jan 17, '06 10:12:07AM

I could be wrong, but I really don't think the jet engine fans has anything to do with being caused by single user mode, unless you're saying that the fans were still revving after booting back into normal multi-user OS X mode.

As I understand it, the fan speeds are regulated by a combination of software in the system and hardware monitoring. This is true for G5 towers as well. Since booting into single user mode puts you in a state where the software component is unable to monitor the drive and cpu temps among others, the fans go into high speed to protect the system. It's designed this way on purpose.

I've seen systems that locked up overnight and when coming in the next morning, the fans are going full tilt! Sounds like the space shuttle taking off. It does this because the system is frozen and can't communicate with the hardware (fans) to tell them how fast they should be spinning. So the built-in protection gradually kicks in and the fans rev higher and higher until they are deafening.

At least this is my understanding. If someone knows better, please tell.

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10.4: Fix full-speed fan issue with 10.4.4 on iMac G5s
Authored by: mindsnare1349 on Jan 17, '06 12:33:55PM

single user mode does not necessarily make the fans go wild. well, actually it does since 10.4.2... but it definately didnt before.

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10.4: Fix full-speed fan issue with 10.4.4 on iMac G5s
Authored by: ballardmac on Jan 17, '06 11:24:27PM

Yes, mm2270 is pretty much right on. Recent versions of the PMG5 & IMG5 (ALS) have an advanced SMU instead of a PMU, but it serves a similar purpose.

The SMU controls the fans and regulates the speeds to run each fan. The SMU derives fan speed from sensors in each thermal zone. Some of these sensors monitor thermal only, while others monitor both thermal and power.

If the SMU does not receive an update from the operating system within two minutes, it begins to ramp up the speed of the fans to full speed. This is sort of a self-preservation function.

Follow previous steps (in previous post) to reset SMU. Remember that your date and time will be reset, unless you have enabled "Set date & time automatically" in System Preferences.

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