Apparently no current limiting...
What exactly happened? You accidently shorted the fan output and converted the PCB trace into smoke?
Yep. I ran the fan test to check things were working right. Heard a spark and smelled something bad. Pulled bottom off and track is lifted right off motherboard (and extruder board). Checked wiring harness in case something happened by moving around but found nothing, so I think extruder pcb must have touched metal on stepper.
Ouch! As some politician once said, I feel your pain.If so maybe you were lucky and the smoked trace acted as a fuse and protected everything else (I've had it happen). If so, and if you can figure out which connection is missing, you might try a piece of wire wrap wire, soldered with care, to replace it.
If everything is fried there's not much risk in trying.
The fan went to full on briefly. I think the transistor is in short internally, but Ill know more tomorrow when I get to a rework station so I can check things after removing q12 and r31 Also, I'm having trouble locating the transistor q12, marked UDSW3, which I thought was a BCR151 but not sure and the part looks to be discontinued. I hope I can bodge fix it because I don't think it's possible to get a new motherboard in 2 weeks in US without paying like $500. If the fan runs at constant speed (anyone know?) when on, I might just wire it up to be always on temporary.