Something strange happened today.
I was working on battery (both main 6-cell and additional 3-cell optical drive bay) starting at 100% capacity. I could work 3 and a half hours until the capacity dropped to 18% and left me with a remaining 50 minutes battery life. I plugged the power cord in and kept on working.
After a while I had to move. So I unplogged the power cord, moved around and when I settled down and plugged the power cord again, the system fan stood still.
There was no reason for it to stand still. I didn't put the system to hybernate or standby, The programms were all running, etc... but there was no ventilation any more. I was happy with that.
After 45 minutes of fan inactivity, I worried about temperature thershold as the upper left corner of my T420s felt pretty hot. So I started AIDA64 exteme edition and could see that the CPU temperature was at 64°C! I kept on working. When I reached 1 hour without fan activity I tryed to lanch a system stability test in AIDA64. I started the CPU stress test that made the temperature climb at 82° C in no time. Still no fan cicking in.
Worried, I stopped the stress test, closed each program and did a reboot. As soon as the log off screen appeared, the fan cicked in at a decent speed and my machine rebooted as I demanded.
Since then I'm woking on and the fan behaves as from the beginning on: IT SOUNDS LIKE A HAIR DRYER!
Right now I've opened Chrome, one instance of Word 2010, one wireframe software (very light) and a MS PictureViewer and I'm writing a report. The fan keeps spinning like mad. AIDA64 reports 40° C CPU temperature and 100% GPU fan load.
When I put my hand next to the ventilation, the air feels cold, which is normal.
In my opinion, there's something terribly wrong with the drivers or the temperature sonde from the fan.
I really need some backup here, it's not a vital situation.
[EDIT 29.05.2011] : the behaviour described here at message 19 matches the above mensioned symptoms. I Think JameZ allready treated this issue...