MMU2S Prompting filament change randomly
Hello All,
Thank you for taking the time to read this post. I added the MMU2S to my MK3S a few months back. I had it running pretty smoothly on small multicolor prints and running it in single mode for smaller prints. I recently tried to print some taller objects in single color mode, and that's where the issues have begun.
There is not specific layer where this begins, but at some point above around 45mm the printer will stop, unload the filament and prompt to be reloaded. It the asks if the color change was successful and continues. It is showing a M600 code, but I have tried and retried slicing different files, and verified no color changes are in the slicer and they are not showing up in the gcode file. I have tried different settings and defaults. The Firmware is on the latest version, 3.14. I have tried to disable the filament sensor. I have made sure there is slack in the buffer, I have removed slack from the buffer. I have checked the logs and it is not reporting these as "run out" events. If it were only once of twice I in a print I could live with it, but once it starts with this issue it becomes impossible to finish a print. I will restart it and get anywhere from 5 seconds to 5 minutes before it stops again. I haven't tried printing something in multicolor that is tall yet, but its on the troubleshooting lineup. I am happy to answer any question and I appreciate any guidance you can provide.
RE: MMU2S Prompting filament change randomly
adjusting the filament sensor at the extruder correctly is vital. With only the on-screen display it is fiddly. It can be made a lot easier by adding a simple LED. see:
loads-filament-but-doesnt-notice
extruder-laedt-filament-nicht-korrekt
There is also a variant with an SMD-LED + 2 SMD-resistor. I have that on my MK3s. Can't find the photo of it here in the forum right now. Please search for it.
Low overall friction in the filament path is also relevant. Meticulous smoothing can help, especially at inlets and outlets. Adding internal chamfers to the ptfe-tubes.
RE: MMU2S Prompting filament change randomly
Thank you for the reply. I appreciate the suggestions. I’ve actually already tried disabling the filament sensor entirely, and the issue still persisted, which makes me wonder if it’s not strictly a sensor alignment problem at the extruder. That said, I’ll definitely look into the LED mod you mentioned and check out the posts you linked.
I’ve also done a fair amount of work already on reducing friction in the filament path—chamfering PTFE tube ends, making sure the buffer lines are smooth, and testing with different tensions/slack. So far no luck. I may revisit the sensor adjustment and try the LED method just to rule it out completely.
Thanks again for the ideas. I’m still hunting for the root cause, so any other tips or oddball fixes you’ve seen work would be greatly appreciated.
RE: MMU2S Prompting filament change randomly
the version of filament sensor tower used on MMU3 extruder setup is easier to adjust than the version used on the MMU2 extruder
May I suggest that you try that option
I added a 5 volt LED to my filament sensor to make adjustment, easier. 
I connected it like this
And I got the LED From Ebay
Zaribo sell a filament sensor with a built in diode
https://zaribo.com/sensors/546-zaribo-smart-prusa-filament-sensor-for-3d-printers.html
I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility.Location Halifax UK