Z not moving for first 3 layers

I have a CR10S with BL Touch added to it. I have been having intermittent hot end jamming problems. I printed a cube and found out Z was not stepping for the first 3 layers. It would start stepping at the start of the 4th layer. The cube is very simple: .2mm layer heights, 100% infill. I disconnected the Z shafts from the stepper motors and propped the Z up and ran the print. This way I could check the Z stepper motors for movement without anything connected to them (eliminating a possible mechanical problem). This is when I found out it was just the first 3 layers printing at a .2mm height. I also put a dial indicator on Z horizontal bar to measure distance to bed. This also showed no Z movement until the start of 4th layer.

BL Touch was disabled in the G code so it would not be making corrections for X & Y bed position. Problem persists with it enabled or disabled. However, with it enabled I was getting Z corrections as the bed moved.

Updated firmware from 2.0.7.2 to 2.0.9.1 with no help.

I get a great layer 0 (1st layer) print, next 3 layers are very messy due to running the hot end over it without Z stepping up. If it doesn't clog I get a good part except layers 1,2 and 3 plus overall height is a little low. Most prints I do on this printer have a larger layer height and I usually get successful prints. Sometime it jams the noozle.

I went to the test cube to trouble shoot it. One other problem I have noticed since installing BL Touch is the reported Z value on the printer's display always says .8 until layer height gets higher than .8mm. With test cube first three layers are .8 or less.

I have been using the BL Touch for a while and have been very pleased with it. This problem just recently started to show up and it really depends on the print if it will be successful, mostly succeeds.

Z positioning using manual operation works fine. I also verified this with the dial indicator.

Next troubleshooting step was to remove the BL Touch, putting the printer back to original condition, and see if the problem is sill there. It worked as expected with Z stepping between all layers.

I am hoping someone might have some knowledge of this problem and could clue me in as to what is going on.

Thanks for your help,
Dick

... this could be the case with microstepping and "weak" motors (not enough torque for the specific mechanical setup) -- the "fullstep-positions" have much more "holding"-force than the microstep-positions inbetween, so that (especially when combined with friction) the first microsteps didn't seem to move, until the summed up torque is big enough to force a "jump" ...

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Viktor
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Vikor,
Thanks for your reply. With the Z shafts removed from the motors so there was no load on them I could not feel or see any type of movement when the Z axis was suppose to step. Not even a fraction of movement.

I can manually position Z axis using microstepping along with using a dial indicator to measure movement. All seems okay when doing this.

Dick