pert:
There are evidently two different pins involved in this code:This code causes FanSpeed_RPM() to be called whenever pin 2 (interrupt 0) goes from LOW to HIGH. For more information, see:
attachInterrupt() - Arduino Reference
Note that it's recommended to use digitalPinToInterrupt() rather than just specifying the interrupt number directly in attachInterrupt(). That, in addition to not using a magic number for the interrupt pin likely would have prevented all your confusion.
Oh boy, gonna shoot myself with a gluegun.
Never realized the first variable passed to attachInterrupt() was the actual interrupt reference.
So much for "tweaking".
Thank you a whole bag full of happy, pert!