Eu resolvi usando o pacote i8kutil que é usado para controlar os ventiladores em laptops Dell.
Use esta solução encontrada nesta página da Web: Como resolver problemas de fãs de laptops Dell no Ubuntu .
Eu usei essa configuração para meus gráficos dedicados Dell 3521 com Radeon HD7670M de 1 GB.
# Sample i8kmon configuration file (/etc/i8kmon.conf, ~/.i8kmon).
# External program to control the fans
set config(i8kfan) /usr/bin/i8kfan
# Run as daemon, override with --daemon option
set config(daemon) 0
# Automatic fan control, override with --auto option
set config(auto) 1
set config(nouserconfig) 1
# Applet geometry, override with --geometry option
set config(geometry) {48x48}
# Report status on stdout, override with --verbose option
set config(verbose) 1
# Status check timeout (seconds), override with --timeout option
set config(timeout) 5
# Temperature display unit (C/F), override with --unit option
set config(unit) C
# Temperature threshold at which the temperature is displayed in red
set config(t_high) 80
# Temperature thresholds: {fan_speeds low_ac high_ac low_batt high_batt}
# These were tested on the I8000. If you have a different Dell laptop model
# you should check the BIOS temperature monitoring and set the appropriate
# thresholds here. In doubt start with low values and gradually rise them
# until the fans are not always on when the cpu is idle.
set config(0) {{-1 0} -1 60 -1 65}
set config(1) {{-1 1} 50 70 55 75}
set config(2) {{-1 1} 60 80 65 85}
set config(3) {{-1 2} 70 128 75 128}
# end of file
Após a configuração, execute este terminal no início de cada inicialização:
i8kmon --auto --verbose --nouserconfig