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We've just finished 9398 4x4 Crawler. Everything looks good, the motors work but the front wheels rotate in the opposite direction to the rear wheels, they also turn in opposite directions. What could be the problem?
Likely you'll have the differential gears oriented wrong in the front or back section.
The differential gear is this piece:
Please check the instructions carefully and see if the orientation of this piece in the instruction book matches your build.
Required, but never shown