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So much infor i want to tell u  

New Member
So much infor i want to tell u

Every inch comes closer to a successful print, but it feels like the goal post is 100 miles away. My latest issue is that when loading, the filament isn't reversed enough - it at all on certain instances.

This happens at random. The unit loads a filament just fine, retracts it and moves to the next. Many times, however, the unit fails to reverse the filament enough. *pic attached in the next post (writing this on my PC, then will upload the pic via my phone).

Symptoms & Notes:
-The motors spin properly
-The filament is not getting stripped on the gears (more on this later).
-The filament holds both the selector wheel and carriage up and cause the next filament not to load.
-It can occur on different position #s (this particular one happened on position #2, where it's happened on all the other locations at one time or another).
-Multiple firmware images have been used. (I currently have v3.4.2 (V1.0.2) installed from October, but only for testing. All firmware exhibits the problem.)
-Everything moves freely - I have even slightly "lubed" up the innards of the PTFE tubes to ensure no friction is present. The filament slides perfectly without any issues. *That is, until the selector is engaged. The unit grips the filament exactly as expected and when it's active it's impossible to make the filament skip. It's very strong.
-I have adjusted the FINDA height up/down to each extreme and every 1/8th turn iteration in between.
-I have adjusted the spring tension on the MMU all the way from death-grip strength to barely holding on, and every 1/8th screw turn iteration in between.

As I mentioned, when this occurs - the motors are all spinning correctly, and the filament is moving (I can see it in the spool holders since I printed the auto-rewind ones from Thingiverse).

One painful aspect of this, is I can do a "load all" from the LCD, and it goes through the entire lineup. If I double-check to see and do the same "load all" procedure again, it can fail on one of the positions with this problem. I then verify the filament that failed to reverse completely, and its inspection absolutely proves that the filament was not stripped. *Pic also attached of a strand of this after the failure.

I'm seriously open to any ideas. Thanks for any help that can be provided.


Posted : 10/10/2019 4:47 am
Terry Sherman
Active Member
RE: So much infor i want to tell u

Not a solution but I believe the the latest MK3/MMU2 firmware is 3.8.0 (1.0.6). Please confirm you have the issue with this version 

Posted : 10/10/2019 11:15 am
Active Member
RE: So much infor i want to tell u

Your are not alone. I have been seeing the exact same thing.

I cannot give any advice since I never solved the issue. It went away by itself after

1) I removed a good 20cm or so of the filament (grinded ??)

2) openened and cleaned the teeth of the MMU2

3) made the filament sensor in the extruder rock solid (took ages)

I am now closer to a reliable setup. Caveat: I am using ptfe tubes with a larger diameter between MMU2 and extruder, a modified filament selector on the MMU2 , pass-through couplings in the back of the MMU2, the modified buffer

I have no idea why the issue went away. I am sure it will return.

Posted : 13/10/2019 7:45 pm
Trusted Member
RE: So much infor i want to tell u

By all the information given it is pretty hard to tell which of it is hard evidence (sorry, but it needs to give somewhere). Assuming all the electronics and the firmware is working correctly (as on many printers in the world) all the MMU does while unloading is this:

Retract till FINDA is not triggered anymore, then retract further to place the filament into its position behind the selector.

Hence, if the filament is not grinded, the gears are working, no huge resistance (and I push filament through narrow 2 meters of PTFE) the retraction can only be aborted by a triggering FINDA. Might not be noticed, might just quickly flicker. The PRUSA team might correct me if my perception of the procedure is wrong, but that's what I experienced. So, check the selector, adjust the FINDA and make sure there is no margin of error. As for the so often announced communication problem, as far as I can tell there is not much communication going on, hence seldomly the root cause.

Posted : 20/10/2019 8:27 am