Why look for the FSensor  

RE: Why look for the FSensor

I believe that both filament sensors are still being used. The filament sensor at the extruder is activated when the bondtech gears are separated indicating that the filament is present. This signals the MMU bondtech gears to release and stop feeding the filament.

Here is a very good article of things you might check including calibration of the extruder sensor:

https://help.prusa3d.com/article/mmu2s-setup-and-inspection_2233

RE: Why look for the FSensor

with the MMU fitted the filament sensor on the extruder has a printed part shoved between its legs so it is permanently "engaged". 

The issues I, and it seems others, am having seems to stem from the firmware "upgrade" rolled out recently. I had no loading problems at all until then, 

RE: Why look for the FSensor

I can't speak to the latest firmware update. I have not installed it yet due to concerns expressed by the user community. If you were not having problems prior to the firmware update, I agree that this is a potential cause.

When the load fail occurs, what exactly happens? Do the MMU2 bondtech gears continue to grind on the filament?

I have experienced a bunch of load errors over the years. Here is a summary of my laundry list:

1. Not cutting filament at an angle to create a sharp point prior to initial load.

2. Deformation of PTFE tube in hot end.

3. Heat sink coming unscrewed a little creating an out of tolerance gap in the hotend path.

4. Bad contact in the control wiring harness for the MMU. Most of the time fixed by reseating connector on both ends. Have replaced cable once.

5. Loose set screws on the roller assembly in the MMU causing bearing to not line up with selected bondtech gear.

6. Failure of the hotend filament sensor. I understand that parts may have changed, but it seems to me that you must have this sensor to tell the MMU bondtech gears to release. Most of my errors were fixed by following the guidance here: https://help.prusa3d.com/article/ir-filament-sensor-calibration-mmu2s_2245.

I hope this helps.

RE: Why look for the FSensor

Hi

avoid it like the plague!

downgraded both printer and MMU last night. First print using slot 3 on the MMU - straight in and off it went. Yet another Benchy 🙂 printed out near perfect. 

Big thing was I stripped down, cleaned and rebuilt both the extruder and MMU so that was a bit of a bonus. I've ran the MMU for two years without the blade in it but I've put it in now. Never actually had any issues without out and it would load, unload and reload just fine. 

I'll stick to 3.12 and show massive distrust towards any more upgrades. 

RE: Why look for the FSensor

I am glad that it is working for you again.

My MMU3 upgrade kit should be shipping out at the end of this week. I wonder if the firmware upgrade will break it as well. 

RE:

It seems as though you might have disabled the Fsensor on the old-version firmware, or it just doesn't care and operates the same way anyway. It loads the filament based on the load distance calibration first, and it then checks if the Fsensor is active. Any blockage in the nozzle will result in a failure since it will retract back past the bondtech gears of the extruder during the load process.

The new firmware uses the Fsensor during the loading process:

• MMU pushes filament, checks that Finda activates within set length
• Continue pushing filament to pre-set bowden tube length (set in firmware now, 360mm default)
• If it detects Fsensor active during this push, it will move to the next step, otherwise it will continue pushing slowly after the intial 360mm until Fsensor activates
• Fsensor active, MMU idler disengages
• Extruder motor rapidly pushes filament to nozzle and retracts back to verify no blockage
• If Fsensor deactivates, it fully unloads filament and retries the process up to 3 times

I'm not sure if there's an option to disable the Fsensor for the new firmware, but it sounds to me like you may need to calibrate your Fsensor, or possibly check the installation of it. The tab that goes in between the optical slot on the sensor should *not* be actually in between while there is no filament loaded. There are directions that say you may need to snip off a bit of the corner of the arm if this is the case (it was for mine initially -- since then, I installed a tower mod so that there is absolutely no failure condition any more).