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When used with the new Multi Material Upgrade 2S, the new IR filament sensor needs to be calibrated before first use, in order to work properly. The calibration of the IR sensor is based on the position of the chimney (IR sensor holder) on the extruder. This calibration is purely mechanical. There is a warning in the assembly manual and the exact procedure is also described in the MMU2S Handbook you have received printed in the MMU package.
Even when there is no filament inserted in the MMU2S extruder, it is possible the sensor assumes the opposite. In a similar way, even during a print when the filament is in the extruder, it is possible the sensor suddenly stops registering it. It might then trigger filament change, or the 'Spool join' feature if you have it enabled.
Since the 3.7.2 firmware version, the printer is able to test the functionality of the IR sensor by itself during the Selftest.
We need to calibrate the position of the chimney that houses the filament sensor. The MMU2S modification of the MK3S extruder consists of 3 plastic parts: the FS-cover-mmu2s, the IR-sensor-holder-mmu2s, and the IR-sensor-cover-mmu2s. During the following procedure, move only with the IR-sensor-holder-mmu2s, for simplicity, we will continue calling this specific part "the chimney".
Unscrew the Festo fitting of the PTFE tube leading from the MMU to the extruder.
Open the LCD menu -> Support -> Sensor info and focus on the IR value(Fil. Sensor). In firmware version 3.10.0 and newer this value is referred to as Fil. Sensor.
The IR sensor (Fil. Sensor) value has to be a stable 0 when there is no filament and a stable 1 as long as the filament is present in the extruder.
Loosen both M3 screws on the chimney. Move the chimney as far left as possible until the IR value (Fil. Sensor) is 0.
Tighten the M3 screws on the chimney.
Insert the smallest 1.5 mm Allen key (ideally the long one included in the MMU kit) instead of a filament into the extruder (without preheating), push it in between the Bondtech gears to move the lever (don't be afraid to apply a fair amount of downward force in order to get the key in between the gears). If the IR sensor (Fil. Sensor) value turns to 1, the calibration is done correctly.
If the IR sensor (Fil. Sensor) value stays at 0, pull out the Allen key from the extruder. Loosen the M3 screws on the chimney and move the chimney a tiny little bit to the right. Return back to step 4.
Check the chimney for little strings left after printing. These need to be cleaned out before attaching the IR sensor as it needs to have a clear field of view.
What if it does not work
Keep in mind that the idler screw tension is important too (idler screw is the one with the spring on it). The head of this screw should be flush with the extruder body, or slightly protruding from the left side.
If the value is stuck on either 0 or 1 and it does not change, you may have overtightened the M3x40 screw that serves as the axle of the idler door. Try to loosen it a little bit.
The same applies to the M3x40 pressure screw with the spring on it - the head of this screw should be flush with the Extruder body.
Another screw that must not be overtightened is the M2x8 screw going through the IR sensor. Try to loosen it a little bit if it's screwed in tight, or, alternatively, tighten it if the sensor board is loose.
Also, check if your IR sensor PCB is not loose. If it is, it can slightly rotate around the screw which can influence the readings. The IR sensor board must be perpendicular to the lever.
In case the sensor keeps showing 1 even though the chimney is pushed all the way to the left, you first should make sure the filament was unloaded and changed correctly. Make sure the Bondtech gear is aligned.
It might be necessary to cut a small dent (roughly 0.5 mm) at the top of the lever with an Exacto knife. Do not remove more than 2 or 3 filament layers! It is very easy to clean up too much for the sensor to be functional, in which case you must print a new idler. Consider having a spare before doing so, or you must disconnect both the power and signal cable of the MMU2S to use the printer in single filament mode.
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