need help on fan for extruder (no replies)

Hi there

im trying to get the fan for extruder to start when the extruder hit a temp of 40 degress. (NOT the fan for cooling PLA)
i got a mks gen 1.4 and all is working(so fare :) ), i just need this last thing to work before i start melting some plastic

the code i found and where i need to change it:

#define E0_AUTO_FAN_PIN -1 
#define E1_AUTO_FAN_PIN -1
#define E2_AUTO_FAN_PIN -1
#define E3_AUTO_FAN_PIN -1
#define EXTRUDER_AUTO_FAN_TEMPERATURE 40
#define EXTRUDER_AUTO_FAN_SPEED   255  // == full speed

BUT when i set it to 7 (in E0) the board freezes and i cant find anything that might conflict with it :(

or is it way to complicated, and just set it to the 12V?