My CPU fan does not speedup

Dead Lazyweb: I'm having problems because the fan of my laptop seems to always be on "slow mode" and my CPU is getting to 80/90 ºC and sometimes the laptop just shut downs itself. I know the fan can spin faster because in those cases the laptop shut downs, upon reboot the fan makes more noise while booting, but once it gets to Linux it turns to that slow mode again. Any idea how do i make it smart so that it actually goes faster when the CPU temperature raises?