Okay, I wasn't clear - I was really talking about resistance, not voltage, with the machine powered off and across the limit switch, disconnected from the controller.

But, perhaps this is a good time to stand back and ask what you're trying to do? That isn't intended as anything other than perhaps an opportunity to provide perhaps a better solution to where you're heading.

Limit switches are useful to stop a large machine destroying itself, however, please understand that by the time you've hit a hard limit you're pretty much failed in planning your work. Most people will use "soft limits" - where the control software can recognise that it's going to hit an axis limit. A quick google suggests that grblcontrol recognises soft limits.

What soft limits does require, is resolving a known machine coordinate - and this is where your existing 3 axis inputs work as "home" switches, not "limit" switches. That would allow you to power the machine, select "home" for each/all axis, after which the machine will move each axis until each switch is active - at that point the machine knows where the spindle is, and you then configure the X/Y/Z constraints from that as "soft" limits (i.e. software-based offsets from the machine home position). You don't then need the additional switches, and you have more control over managing the machine (as the controller knows exactly where the spindle is at all times).

Suggest you read up on soft limits, homing switches, machine and work coordinate systems. I do believe that if well supported, that a homing/soft-limit set-up will give you a better experience than just hard limit switches. Going down that road will require that you disable machine limits (from what I've briefly seen of grbl, $21), and enable Homing, and that should resolve your latter question.

Note, although there are a few owners on here with GRBL-based controllers most of us are using somewhat different motion controllers/software - you might find better support for GRBL queries within a GRBL user group.