3d printer extruder stepper jerking?

I have a wanhao duplicator 4 with 2 extruders.  The left extruder stepper burnt out so I replaced it but it only jerks doesn't spin even when under no load and only hooked up to the control wires.  The connections seem solid and I can't see anything short of tearing it all apart. 

Obviously the motor gets power since it jerks back and forth. I have tried multiple motors and all have the same result.  I believe that the jerking is what cause the other motor to burn up.  Other than the extruder stepper everything seems fine.  I have no clue what it is at this point. 

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