ok ... I recompiled incorporating acpi options in the kernel :
apparently, the CPU is well getting cooler and the ventilator stops correctly.
But here is some bugs :
my /proc/acpi/fan directory is empty
my/proc/acpi/thermal_zone/THRM/temperature file gives allways
temperature: 75 C
I don't have a /proc/acpi/processor/CPU1/performance file but have a /proc/acpi/processor/CPU1/throttling one.
The performance management I have on 2.4 is very useful : by setting lowest performance, the PC gets slower but the ventilator stays allways off. I'd like to do the same with 2.6 but no way