If I had to guess, it would be one of two things. Normally when this happens either the motor could be shorted. If you try and turn the motor, does it have a cogging effect, or does it feel like there is something rubbing? If there is a rubbing feel, then check the solder posts to make sure no two are touching each other, even if it is just a small little piece of solder.
The second thing might be the wire harness. Check the wires to make sure they are all intact, and check the harness to make sure all the prongs are correctly in place.
If this is a motor other than a Novak, I am guess the sensor wire connecting the motor to the speedo may be bad. I have seen it may times where people buy a sensor wire and it will not work. They buy a new one and all is fine.
Former RC fanatic just trying to keep up......
BSR Tires / Spektrum RC