MMU2S Selector not moving

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If the selector of your MMU2S upgrade is not moving smoothly, or if it cannot reach a certain position, the cause is usually one of the following issues:

The selector blade is loose

The blade that is supposed to cut the filament strings was not attached properly or tightened enough during the assembly. It may have moved out of its socket and now it is blocking the movement of the selector.

1. Release the three screws holding the selector motor in place (left picture) and pull the motor out by turning the threaded rod of the black trapezoid nut. You can also release the two M3x10 screws securing the trapezoid nut.
2. Pull both of the small smooth rods out using the Allen key (right picture). Then remove the selector (green circle).

3. Remove the holder's screws (blue arrows) and position the blade back in place and tighten its holder properly.
4. Reassemble the unit back to its original state.

Updated selector blade holder

The blade equipped with the selector, which will cut the tip of the filament unloaded from the extruder, if needed. Until firmware version 3.9.0 of the MK3S/MK2.5S the cutting function was not enabled.

To start using this function, make sure you have:

• Firmware 1.0.6 or above installed in the MMU2S unit.
• Firmware 3.9.0 or above installed in the MK3S/MK2.5S.

The printed parts for the selector (mmu2-blade-holder and mmu2-selector-finda) were updated to provide a better grip of the blade and are available for download here. Print using PETG and PrusaSlicers 0.2 mm QUALITY preset.

The filament is bent and stuck in the selector

1. Release the three screws holding the selector motor in place and pull the motor out by turning the threaded rod of the black trapezoid nut. You can also release the two M3x10 screws securing the trapezoid nut.

2. Pull both smooth rods out (use the Allen key) and remove the selector.
3. Remove the broken filament string from the selector and reassemble the unit.

The selector motor isn’t working

1. First, try powering the printer off and moving the selector by hand, by turning the threaded rod.
2. If that didn’t help or if you were unable to move the selector, release the three screws holding the selector motor in place (picture above) and pull the motor out.
3. Push the smooth rods out with an Allen key. You can now easily remove the selector.
4. Clean any stuck filament and try pressing the left and right control buttons. See if the motor’s shaft rotates. If not, the motor may be defective, or it is plugged incorrectly. Remove the MMU board with its cover and cross-check with the picture on the right to see if that is the case. Try also to unplug the cables and plug them back in to be sure.