After installing Manjaro XFCE 0.8.10 anytime I boot or reboot my system the LCD backlight reduces to lowest. I can still increase this using the brightness control, but the situation remains the same after the next boot.
I found out from the Manjaro Wiki page that some hardware manufactures automatically disables the backlight when Windows is not detected at boot and can be fixed with GRUB.
To fix this all you need to is open your terminal and run the command below:
sudo sed "s/\(GRUB_CMDLINE_LINUX=\)\"\"/\1\"acpi_osi=Linux acpi_backlight=vendor\"/" /etc/default/grub -i
The above command simply add this line:
Reboot your system and brightness should be maximum.
Hope this helps.
Credit. Manjaro Wiki