My 8 year old is just finishing the Bucket Wheel Excavator. He turned the motor on and there is a significant clicking noise. It looks as though the top gear noted in the picture gets stuck. Not sure where the problem is as he build it all himself.
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First of all, let's get some basic questions out of the way:
The surefire way would be to follow the "chain" of mechanical parts from the motor along the gears and axles. As far as I can remember the gearbox is not that complicated or hidden that it would be easier to dismantle and rebuild than to peek inside (maybe after removing some of the decorative panels). So turn on the motor and see if the gears and axles leading to the selected function are moving. Of course if the selected function is working correctly, you can assume that specific chain is complete and correct, so no need to analyze.
Or just operate all functions for a while each and any gear that did not move during this time must be disconnected from the motor.
Another way would be to locate the source of the clicking sound and seeing if that area is built up according to the instructions, as it would be the prime suspect. My model has once experienced a problem with the three parallel grey universal joints connecting the top and bottom halves (which are assembled quite late in the process) where the insert of one of the joints came loose and was turning in its sleeve without transferring any motion but producing a clicking sound. So maybe check that too.