is there a way to find out the temperature of the CPU? I'd like to change the fan on the PegasosII by a quite big passive one, but I won't want to damage my system, so I'd like to be able to monitor the cpu temperature before.
There is a motherboard sensor accessible through the via southbridge, but it may need some kernel work. The G4 powerpc used has a broken TAU unit (as do all the G4 processors), while the G3 have a working unit, altough it may need calibration :
$ more /proc/cpuinfo
processor : 0
cpu : 745/755
temperature : 27-29 C (uncalibrated)
clock : 601MHz
revision : 51.17 (pvr 0008 3311)
bogomips : 1199.30
machine : CHRP Pegasos
There may be place for a sensor chip on the cpu module on a future chip, but this would need an additional cpu slot line, which we don't think we have.
Furthermore, the 7447A and 7448 cpus have a temperature sensor, which needs some external logic, but again using that will need some small redesign of the board as i understand.
So, your best bet is to use an external solution. The CPU is supposed to dissipate 8-12 W, and you will notice unstability and crashiness well before you burn the CPU, even if you will be able to do that.
So i recomend an external temperature sensor like Aaron suggested.