Nees help configuring RRD fan extender in my i3V8....

[printbus]

Yeah, the instructable is configuring firmware for fan control other than the print fan. Leave CONTROLLERFAN_PIN set to -1 as well, since you're not trying to add a smart fan control for the RAMPS stepper drivers.

I haven't validated it is still the same in newer versions of Marlin, but motherboard type 34 used to be in the Arduino Mega section of pins.h:

/****************************************************************************************
* Arduino Mega pin assignment
*
****************************************************************************************/
#if MOTHERBOARD == 3 || MOTHERBOARD == 33 || MOTHERBOARD == 34 || MOTHERBOARD == 35 || MOTHERBOARD == 77 || MOTHERBOARD == 67 || MOTHERBOARD == 68

If you have configuration.h already set to motherboard 34, pins.h should already have FAN_PIN defined for D4, through the "else" part of this...

  #if MOTHERBOARD == 33 || MOTHERBOARD == 35 || MOTHERBOARD == 67 || MOTHERBOARD == 68
    #define FAN_PIN            9 // (Sprinter config)
  #else
    #define FAN_PIN            4 // IO pin. Buffer needed
  #endif

That else branch is where you'd move the print fan between outputs D4 and D5 as desired for the geetech extender, assuming it is installed two pins up on the RAMPS servo header.