Probe, noise problem??

Hi alls. This is my first post. A pleasure to post and share with yours.

I’ve recently finished to make a mini CNC mill for PCB routing with GRBL 1.1f firmware version. Last week, I’ve installed an aluminium block on it for automated Z zero tool lenght. The block has two wire, one corresponding to the aluminium block (A5 probe pin in Arduino) and the other end, to GND in Arduino and motor chasis.

I’ve tested continuity between the tool and motor chasis and all is ok! I tested the probe and all works well.

The problem comes when I send M3 command to activate the 12V DC motor spindle throught the rele. All 3 steppers begin to rotate, with any signal sended by me. If I disconnect the GND wire on the motor chasis, all works fine.

My question is? Must I disconnect every time I use the probe the GND wire attached to motor chasis?
Is a way to leave the wire attached to DC motor chasis without these steppers movements? Must I connect something between A5 and GND on Arduino??

I hope you can help me with this. Thanks in advance!!