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MMU3 regular maintenance

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Last updated 2 months ago
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It is recommended to run maintenance on your Original Prusa MMU3 upgrade kit. For that, see the images below and try to remember the name of each part of the MMU3 unit.

  1. Idler motor: rotates the idler.
  2. Idler: Contains five pulleys. By rotating the idler, a single pulley gets directly in contact with the
    filament and applies pressure on it. Then, the pulley motor can push the filament forward into
    the selector.
  3. Left hardware button: moves the selector left.
  4. Middle hardware button: preloads filament into the selected position.
  5. Right hardware button: moves the selector right.
  6. LED indicators: indicate the current status of the MMU (selected position, errors, etc.).
  7. SuperFINDA (or FINDA for short) - MMU3 filament sensor built into the selector.
  8. Selector: selects a filament slot by moving to the requested position. The filament is then pushed
    through the Selector into the nozzle by the Pulley Motor.
  9. Festo fitting: there are two Festo fittings on the front PTFE tube, the other one is mounted to
    the top of the extruder.
  10. Reset button: triggers reset of the MMU3 unit only.
  11. MicroUSB port: for flashing MMU3 firmware via PC.
  12. Pulley motor: rotates the pulleys that move the filament inside the MMU3 unit.
  13. Selector motor: moves the Selector left and right.

Nextruder main plate

This part guides the filament from the drive gears into the nozzle tube. Repeated loads and unloads of filament cause wear. Without the MMU, this wear is insignificant. However, a single MMU print can have thousands of filament loads and unloads, increasing the wear rate. Once this part wears out, the printer cannot properly push filament into the nozzle. Unlike the rest of the MMU parts, this is printed with Multi Jet Fusion so that the part lasts longer.

Useful tools

Besides the usual tools, such as Allen keys, needle-nose pliers, and IPA + paper towels, you might need:

  • Compressed air: also known as canned air. Be sure not to hold it upside down while spraying - otherwise, you could damage any electronic components while trying to clean them.
  • Brush: a small brass brush is ideal for cleaning the pulleys. If you don't have a brass brush any brush with hard bristles will do the job. Alternative: You can use the acupuncture needle that comes with the printer or a toothpick.


The pulleys may be grinding the filament and its dust may accumulate around them. The best practice is to clean the pulleys with a mini-brush or a similar tool every time you run the maintenance. The pulleys also must be perfectly aligned with the filament holes, as shown in the photo below, and well tightened by the small black grub screw against the flat part of the Pulley motor axis.



Even a properly calibrated selector can be occasionally blocked with filament strings or dust or by a broken piece of filament. By pushing the right MMU button, move the selector fully to the right. Use an interdental brush or any thin wire (⌀ 1.5mm/0.06”) to push the waste through (still in service position). Use compressed air to clean any leftovers.

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