060122-1237 EST USA
I can not really follow your question(s).
Do you think you have a bad motor?
Is the motor a DC brush type? Your description may imply this.
The following two questions assume nothing is connected to the motor electrically or mechanically.
If DC brush type, then what does an ohmmeter read for armature resistance?
Also if DC brush type, then assuming you are correct on BLK and YEL being the armature wires, then apply 12 VDC thru a 5 Ohm 50 W resistor to the armature and measure armature voltage, and observe whether the motor rotates. What are the results?
Given that the motor or its encoder has a fault, then it probably can be repaired.