I am having the same issue here. I am not using a BLTouch either and I have installed the correct firmware (without BLTouch support). The Z-axis readings work fine when moving the axis manually, but as soon as I press the button to home z-axis or home all axis, the Z-axis reading changes to -270 and continually runs into the bed. I’ve linked to a video of the issue here: https://photos.app.goo.gl/E4Pdr6tScLhAykrv6
(I power off the printer just before in runs into the bed)
Looks like you have some wiring problem. Which printer do you have?
I have the Aldi Balco 3D Touchscreen model and hence I flashed with firmware for the 5.1 mainboard. I flashed from the original firmware and did not have any issues homing before flashing.
To flash you have open the printer (at least the LCD part) so things may have moved. What is the version of your mainboard. Could be that you have a 5.2C version and thus it is not the same firmware.
Turns out I have a 5.2C version - thank you for the replies!
If it helps anyone, this was recently “refurb” purchase from the Cocoon Create website (01/05/2020) but branded as Balco. Being a refurb I assumed it would have an older mainboard, but alas it’s newer.
Please next time, create a new issue. As it turns out, your issue is not the same as the one on the top of this thread of messages (you do not have the same printer) so it is confusing.