When I built my Replicator 2X using a BTT SKR 1.4 Turbo I was disappointed to learn that only one of the three fans is controllable.
I really wanted an automatic extruder fan and with an external MOSFET it’s a really simple thing to setup.
All you need to do is connect a MOSFET to a spare pin (I chose the power detect plug).
Using a DFRobot GRAVITY MOSFET (other brands are available) I connected the red to 5v, black to GND and green to pin 1.00
I connected the fan to the output of the mosfet using the output from fan1 for the VIN but you could obviously use your PSU aswell (Make sure this is the correct voltage for the fan!!)
In configuration.adv I changed the extruder fan pin to “P1_00” from “-1” compiled Marlin, uploaded it and bingo!
Automatic fan on extruder 1!
Really simple and well worth it!