[Firmware bug] Part cooling fan (M106) switches 2–4 s too early — not synced with motion

U1 Toolchanger
1

Hi all,

I found that on the U1 the part cooling fan reacts to M106/M107 noticeably earlier than the toolhead reaches the command: about 2–4 seconds, or ~one full 130 mm move at F2000 in my test. The fan turns on and off in the wrong places on the model, so slicer settings like first-layer fan, bridge/overhang fan and per-feature cooling are applied in the wrong spots.

Minimal test (no extrusion):

G28
G90
G1 F2000
M106 S0
G1 X10 Y10
G1 X140 Y10   ; should be OFF here
M106 S255
G1 X140 Y140  ; should turn ON here
G1 X10 Y140

Repeat the block a few times: the fan switches one edge too early.

Cause: in klippy/extras/fan.py, set_speed_from_command() schedules the fan at “current time + 100 ms” instead of using toolhead.register_lookahead_callback() like upstream Klipper. Klipper buffers ~2 s of moves ahead, so the fan fires before the move it belongs to.

Full analysis and a suggested one-line fix are in the GitHub issue: M106/M107 (part cooling fan) not synchronized with motion queue — fan switches 2–4 s before the toolhead reaches the command · Issue #18 · Snapmaker/u1-klipper · GitHub

Workaround to confirm: M400 before M106 fixes the position, but stalls motion, so it is not usable for real prints.

Has anyone else noticed cooling kicking in early on overhangs or bridges?

2

HI,

Since this is the community based forum, could I suggest you complete the below form with your findings so the engineers at SnapMaker can look into it further

3

…but please continue to post these things on the forum as well, so other users can be aware and also confirm.

4

yeah this tracks, klipper buffers ~2s of moves so a ‘set it now’ fan command lands early. the lookahead callback is exactly how upstream keeps it in sync :crossed_fingers: