So, I have a 15" MacBook Pro (MacBookPro5,3) the fan behavior of which I've been struggling to understand. A few weeks back, I'd noticed that sometimes, the fans will run at 6200 RPM (as shown by the iStat pro widget or smcFanControl) even when the system isn't doing anything (CPU utilization under 5%, no significant disk or network activity) and a temperature as low as 100ºF. In so far as this goes, there are dozens of posts that are similar and lots of suggestions with few encouraging results. Still, I followed the most useful advice, and had no success... but I did run into something that I have not seen in and of the other posts, so I'm going to give th details of my story in hopes that someone can share some useful insight.
I tried the basics and even the extremes:
I applied all updates and rebooted.
I blew out the fans and vents along the hinge with canned air. (it wasn't all that dusty, but I did it anyway, just in case.)
I reset the SMC and rebooted.
I reset the PRAM on a subsequent reboot.
I reset the SMC and then reset the PRAM on the same reboot.
I reinstalled the OS on top of the existing installation.
I reinstalled the OS on a clean partition.
In all cases, the behavior would (mostly) consistently recurr. That is to say that none of the above fixed the problem, however I would sometimes reboot the machine and not have the problem immediately.
That's when things started to get a bit weird.
I noticed that a number of things seem to get the fans to behave normally (i.e. fan speeds of 2000 RPM when the temperature is low and the system mostly idle, with escallating speeds as the CPU is more loaded and/or the the temperature increases with 6200 RPM being difficult to reach without taking steps to intentionally load the CPU and disk activity for a long time.)
Here's a list of things that seem to change the behavior to normal:
Partially closing the lid. If the lid is about 75º rather than 90º or more open, the fan speed seems to behave as expected. Returning the lid to 90º causes the fan to instantly spin back up to 6200 RPM.
Tilting the laptop so that the rear is elevated ~20º (even if still on a flat surface such as a lap desk) seems to cause it to revert to normal behavior fairly consistently. This doesn't always work... sometimes it will cause a temporary lowering of fan speed with a return to full speed after the latop stops moving.
Applying flexing pressure to the lid while open 90º or more will on rare occasion get the fan speed to return to normal. This isn't very consistent, but it does occasionally happen.
Is there a sensor in the hinge that could contribute to the SMCs decisions about fan speed? Is there a tilt sensor that somehow contributes to it as well? I've no idea, but I've not been able to find any discussions that bring up these particular behaviors. Any help?