blotch error on thicker build plate

I have a PULSE XE printer with BLTOUCH using UBL. The BUILD MESH works fine using a PEI BED(MAGNETIC) with z offset at -3.3; the bed mesh is an average of +3.9xx. When I change the bed plate to a glass bed I get an error:

QUOTE
Probing mesh point 1/100.
Errorrobing Failed
Errorrobing Failed

Probing mesh point 2/100.
Error:!! STOP called because of BLTouch error - restart with M999
Error:!! STOP called because of BLTouch error - restart with M999
UNQUOTE

The glass bed is about 1/8" thicker.

The printhead does not move and the probe continues to try probing.

I'm stumped; what can be causing this.

Bill

I take it that the BLTouch didn't trigger when the glass bed was installed. That's probably because Z_PROBE_LOW_POINT needs to be set more negative. Basically Marlin will abort the probe if it thinks Z is too low. Z_PROBE_LOW_POINT sets that point.

I was thinking the same thing so I changed Z_PROBE_LOW_POINT from -5 to -8 but the same thing happened.

Attached is video of the error occurring.
For information the PEI bed plate is .891mm thick and the GLASS plate is 4.678mm
Z_PROBE_LOW_POINT is -5
Z_OFFSET is -3.25

The glass plate is 3.787 mm thicker so if I understand the Z_PROBE_LOW_POINT correctly then -8mm should be enough the makeup for the glass plate but I'm not sure I really understand the Z_PROBE_LOW_POINT so any help would be appreciated.

Bill

It's a safety thing. The idea is to stop lowering the Z axis if it goes below the Z_PROBE_LOW_POINT.

Try setting Z_PROBE_LOW_POINT to -10. -8 may not be quite enough.

Adjusting Z_PROBE_LOW_POINT did not help.

I got the mesh probe to work by changing Z_MAX_POS from 215 to 210.

This is a PULSE XE from MATTERHACKERS and they have a different way of doing the Z axis. They home to max z( #define Z_MAX_POS 215); there is a proximeter at Z MAX that homes the nozzle to Z=215.(This is build plate dimension that MATTERHACKES advertises)
I modified the PULSE XE to a direct drive and this lowered the tip of the nozzle 4-5mm so now my Z build height is around 210 and not 215 I have no idea why this worked and there must be a setting in Configuration.h that make the probe fail when the probe trigger point is higher than the bed by xx mm.
The following info is all the data I have on Z settings.
1. Z is homed to top of gantry - #define Z_HOME_DIR 1
2. Z offset is -3.125 - #define NOZZLE_TO_PROBE_OFFSET {32, -12, -3.125}
3 GLASS bed plate is 3.787 thicker that PEI

Any help to understand how this works is appreciated.

Bil Lynn