As soon as I start the calibration process in the app, the gearbox jams. It doesn’t do anything, just clicks and clicks (stripping the gears). Any ideas what the cause might be?
Most likely a mistake in step 87.
The two most common malfunctions are
clicking gears and no motion for the shovel, ladder and plow but tracks are working: redo step 87 meticulously.
Wrong engine activation in the gear box often happens. (often ladder moves instead of shovel) and calibration fails. This is a bug, the engine mistook the placement of the gears, and calibration doesn't catch it in the beginning. To fix, Cancel calibration, press the app button to lower the shovel and remember to wait for the gear to be changed, then retry calibration. If that fails again, try the shovel tilt and redo. If that also fails, then lower the plow and redo. It cannot be fixed by the ladder unless the gearbox was built wrong, (in that case you need to do 87-120 again).