I had a similar problem with my 8.5 pro combo abd it also worked out to be the motor, it was only 2 weeks old, I returned the motor to schumacher and got a replacement within a week and it has worked flawless every since, in fact i bought another combo this time a 7.5 for my 4wd, and that one setup first time out the box.
Try a different sensor wire then a different brand of motor to rule out the speedo.
Raymond
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