MMU2S Helpful Mods
Just to preface this, I've been printing countless hours with a MK3S for a while and it is pretty much dialed in perfectly. Excellent workhorse of a printer. Ever since I bought the MMU2S a few months ago, however, I've struggled with it nonstop. I spent all of this past weekend trying to get it working once and for all and finally got a successful test print yesterday with zero intervention. I wanted to make a list of mods that helped me. Perhaps it will help others as well.
As always, YMMV, first try a barebones stock set up as is from factory. Then try the mods one at a time until you find a combination that works for you.
My collection of these is available here, but I will explain the must haves one-by-one here, including ones not listed (please thank the creators of these, I am simply consolidating ones I've found):
1) 2.5mm ID clear PTFE tubes
Replacing the stock 2mm ID tubes with 2.5mm (4mm OD) ones make a huge difference especially with the fat tips. And if you get stringing, it has more room to just get displaced somewhere in the tube, instead of getting stuck. Also makes it easy to see what's going on. I've only been able to find them at China sites like Aliexpress or Ebay, like this. You can get 10m for only ~$15 USD.
Get extra tubing in addition to what you think you need and cut it down to the exact size you need to keep the filament path straight. Any tension at all and you will have tons of issues.
The selector I linked to does three very important things: opens the FINDA canal for filament debris to fall out and allow for easier troubleshooting of the ball/sensor, adds a magnet to keep the ball down even if filament tips are not perfect, and adds chamfers / fillets to the filament path to allow for easier filament load/unload. This is what should have been stock.
3) M10 passthrough plates on the buffer (10x M10s), MMU2S PTFE holder (5x), extruder (1x), and selector output (1x)
Finally, if you are mixing and matching filaments at different temps, @nikolai-r's postprocessing script is a MUST. Get it here. Otherwise, you will never achieve any multi material print. Trust me, I've tried without it and it just doesn't work.