I have wired in a new extruder with its own cooling fan and I tried to change the Marlin firmware to switch it on automatically above a specific temperature (50°C). I did noting in the configuration.H I changed a line in the configuration_adv.H
#define EXTRUDER_0_AUTO_FAN_PIN -1
#define EXTRUDER_0_AUTO_FAN_PIN 8
as described in this article. But the fan is not starting when I bring the temperature above 50°C (manually). However it starts if I switch the fan on manually from Repetier. Am I missing something? How do I control the fan automatically on FAN0 output when the temperature reached the set limit?
The RUMBA board has two fan outputs. The primary fan output is switched by pin 7, the secondary one by pin 8. The reason your fan is not working when you set
#define EXTRUDER_0_AUTO_FAN_PIN 8 is because it's switching the secondary fan. If you switch the wires over to the secondary fan output it will work correctly.
If you do want to use the primary fan output then you can achieve this by setting
#define EXTRUDER_0_AUTO_FAN_PIN 7 but you will need to search for the
#define FAN 8 in the appropriate pins.h file and change
8 to something else (-1 or 7, for example).
I would recommend the first solution, because you should have a controllable print cooling fan, and an automatic extruder cooling fan.