What OS are you using? Fan control under Linux isn't that bad but under Windows, I've never figured out how to adjust it at all. No idea in regards to Mac...
10.6.6. I've found out that 10.6.5 broke default fan control for a LOT of users, while 10.6.6 made it so the best third party fan control program stop working for many, including me. The program called "Fan Control" was designed for MacBooks, and it looks like there's some serious feedback issues that make it not work at all on desktops. But in the process of mucking about trying to remove Fan Control and otherwise fix permissions in preparation for possibly reverting to 10.5.8, I seem to have made it so that the default fan control sort of works again. The CPU and GPU still run way hotter than they did under 10.5.8, but the fan starts to rev up when things get too hot and I know to kill a process or two.
(Edit: There is no built-in ability to control fan speed manually in OS10.6, because Intel chips aren't supposed to run hot and therefore you don't ever need to adjust fan speed, Thus Sprech Stevethustra.)
Sorry, I'm no help at all on this one. 'Hot' chips used to be quite common in linux boxes, so being able to adjust the fan was critical. Mac used to be a Unix derivative so I thought it would be similar. This isn't the first time I've been wrong, though.
Oh, if I knew actual Linux code instead of a handful of commands, it might be possible to go into Terminal and do some mucking about. But there's no UI for adjusting fan control. It's one of those things that justifies sneering derision towards the OS, for its "Don't worry your pretty little head" attitude towards the guts of the computer's workings. Usually Mac "Just Works," but when it doesn't work you can't do a damned thing about it. It's as if they consider it an admission of weakness to provide users with tools to fix problems.
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(Edit: There is no built-in ability to control fan speed manually in OS10.6, because Intel chips aren't supposed to run hot and therefore you don't ever need to adjust fan speed, Thus Sprech Stevethustra.)
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