[Solved] Belt test fails on X Axis length 50% of tests (alternating tests) - Layer shifting.
My Prusa i3 mk3s has been running for a couple of weeks with good results.
I switched to using PC blend and so upgraded my firmware to prusa3d_fw_MK3S_3_9_1-RC1_3512.hex for PC blend support.
I then noticed some intermittent X axis layer shifting.
I switched back to PLA in case this was a PC blend issue, but still saw X Axis layer shifting.
I downgraded the f/w to 3.9.0 but again still have the issue.
I have checked everything I have found on line and every things seems to move freely and withing obstruction.
Interesting, when running the Belt test, it fails with "Selftest error! Please check : Axis length Axis X" exactly 50% of the time.
Eg it passes, then fails, then passes, the fails ... (you get the picture).
What am I missing? All was going so well for the first week or so.
I was completely thrown by the alternating reports of pass and fail on the belt test. It turns out that the troubleshooting advice general given was spot on.
The X-axis pulley orientation was incorrect every other test.
I had [failed to] tighten the two grub screws in such away that it ended in the correct position after every other stop (which was when I looked at the position) and then twisted a degree the alternate time.
Correctly tightening the two grub screws resolved the problem.