MMU2S Selector not moving | Prusa Knowledge Base

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.

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.
Pull both of the small smooth rods out using the Allen key (right picture). Then remove the selector (green circle).

Remove the holder's screws (blue arrows) and position the blade back in place and tighten its holder properly.
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

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.

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

The selector motor isn’t working

First, try powering the printer off and moving the selector by hand, by turning the threaded rod.
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.
Push the smooth rods out with an Allen key. You can now easily remove the selector.
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.

2 comments

Follymaker

•

I have been having a similar issue. The symptom is that the selector won't zero its initial position successfully. It tries 3 times and then quits. This has been a problem off and on for some months now. Sometime the selector bounces back and forth (chatters) at the right (motor) end.
I have had some success disconnecting and reconnecting the cables at the MMU2 circuit board. But the symptoms seem to return in hours or weeks.
Now I have tried also lightly spraying the connectors (both on the board and on the cable) with a little Contact Cleaner. I am hoping that will provide a longer lasting fix.
BTW the incoming power connector on the board is the 2nd from the right (with 2 red and 2 black wires).
I wonder if there is something unusual with the cable connectors or the board connectors that causes resistance to increase over time?

David

•

Hi! It is not usual that the Molex connectors loose conductivity over time for no reason. A cold joint on the board is more common. You might want to check if the cables aren't damaged on the way, for example where the Einsy-door meets them. Try to re-tighten the fork connectors at the Einsy. Check if there isn't a mechanical obstruction that would prevent the selector from moving. And please, let us know what you found out! ;)

Follymaker

•

David - Thanks for the suggestions. I checked the Einsy contacts again and they continue to be tight. Wires are undamaged. I don't see any obvious cold joints on the MM control boardboard (version 2019-09 rev 0.4.0). The power socket has much less solder on the middle two connectors than any other Molex connector on the boards. But it appears to be a reasonable contact. The contact cleaner seems to have helped. Over the past week I have conducted several dozen load-to-nozzle tests. There have been some failures that appear to be due to non-electrical problems (filament jams, etc). I haven't tried any multi-color prints yet. But I do keep wondering if there is an intermittent glitch in my MMU2 controller board that causes the MMU2 to forget what it is trying to do. I suppose replacing the board is the only way to find out.

David

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Hi! Please, contact support via the prusa3D.com page if you don't mind. They might be able to help you with this ;)

Michael

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For me, it was just a simple sensor calibration issue. It was wierd to see the selector not moving when manually using the selector buttons on MMU and the printer was refusing to perform "load filament to nozzle" operation when chosen and during a print. If this happens, check your sensor calibration. The IR sensor is probably misaligned and telling the printer that filament is loaded thttps://help.prusa3d.com/en/article/ir-filament-sensor-calibration-mmu2s_2245

David

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Hi Michael! You are right. Another cause may be the steel ball stuck in the upper position or being pushed up by a leftover piece of filament or dirt.

CreepShow

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This was my issue exactly! I had taken the selector off and when I reinstalled it I had mistakenly placed the sensor at the lowest setting. I read your post and confirmed by looking at the light on the sensor that it was too low. Thanks for the hint and tip! It got me back up and running.