I dug out my old Taz 5 that has been in my closet for years. Hooked it all back up and when I power it on the screen lights up but nothing comes on screen. I started testing cables, as I had done some modifications to the cables years ago. When I unplugged the one going to the head everything comes on. I tested all the motors, all good but the second I plug that cable back in it goes blank.
Thoughts?
BTW that particular cable I did NOT modify years ago.
Start disconnecting individual components of the toolhead. It’s probably easiest at the control board.
Disconnect heatbreak fan, attempt power on. Shut off if still failing. Plug back in.
Disconnect part cooling fan, attempt power on. Shut off if still failing. Plug back in.
Disconnect thermistor, attempt power on. Shut off if still failing. Plug back in.
Disconnect extruder motor, attempt power on. Shut off if still failing. Plug back in.
Disconnect heater cartridge, attempt power on. Shut off if still failing. Plug back in.
Disconnect X-axis limit switch(es), attempt power on. Shut off if still failing. Plug back in.
That should help you narrow down which component or wiring to the component, has failed.
My frustration level is showing for not doing this first. 
The one with the red arrow is the culprit.
Ok appears to be the blower pins 9&10
Yes, is a fan:
2 Pin Fan wires connected
Check continuity and against ground, if it’s just the fan, it’s easy. If you have to replace wiring, slightly more involved.
Was fan.. I’ve ordered replacement on slow boat from China. Will get here near the end of the month (hopefully)..
Did you verify it was actually the fan and not the wiring to the fan? Usually a failing fan just doesn’t spin, but a short in the wiring to ground would cause power failure.
