Hello, does it matter when i have a buck converter and i take the ground from the in- or out- ?
Or if i have a 12v load and a 5v load poeered by lowering the voltage with the buck converter , can they share the same ground from the psu on the in-pad of the converter, does that matter ?
Unless the buck converter is specially designed to have an isolated Ground, the output Ground is usually connected directly to the input Ground. If the buck converter is not inside a case you can just look for a circuit-board trace connecting the two grounds. If it is in a case, you can use a multimeter to check for (near) zero resistance between the two grounds.
Thanks for the answer, i checked with a multimeter and i got 11 ohms so it have continuity but i believe it doesn t read god value anyway
It should not matter, except if you were using a CVCC buck converter.
(current sensor is usually (always?) on the negative side)
There are also real isolated buck converters.
Physically they are connected together (your 11 ohms is hopefully you meter being in error or you had a bad connections)
However I would connect the load to the output side. The reason is the output ground is closer to the output filters. By using the output ground you may have less electrical noise sent to your load.
Personally I would connect to the output ground. And I would ground everything to the output ground. See "star grounding"
To add to what @JohnRob said, think of the buck converter as a power supply in its own right, with its own 2 output terminals. Think of the output as being entirely separate from the input, even though it isn't really separate. The output current flows between the positive output terminal and the output ground, the input current flows between the positive input terminal and the input ground, these are separate, different currents and they flow in opposite directions. Using the correct grounds keeps them apart.
Vaguely possible there is a current sense resistor in series with the ground line, but 11 Ohms clearly sounds like a measuring error.
Remember at 1 Amp current draw, there would be 11 Volts across 11 Ohms! 
Hi, @alex0512343
Can you please post a picture of your DC-DC converter and a link to data/specs?
When you short the DMM leads together in OHMs range, what reading do you get?
Thanks... Tom... 
Hi,
Thanks for the picture.
From your DMM readings the -ve on both ends are connected together.
What DMM do you have, or is it a moving coil type meter?
Can you post a picture of your DMM?
Tom....
What reading do you get if you connect your two meter leads together at the clip ends?
You could have a lead thats gone high resistance.
It varies betwen 8- 20 ohm
Issue with ur meter. I use these boards. These generally share ground and are not isolated. Should not matter which gnd you use. Just be careful with the positive side.
Try switching leads. Check battery level. Or your meter might be out of calibration
Hi,
Try cleaning the probe tips, also unplug and plug the leads back into the meter.
It sounds like you have a connection problem.
Tom...
In other words, that exact same "8-11.5 ohm" you quoted. 
Yeah,
I can attest to those converters not having isolated ground. They're also pretty shitty as far as buck converters go. Because they don't use a synchronous rectifier, just a regular diode, they'r'e inefficient, compared to more modern devices, This is a large enough effect that it has real ramifications in projects (for example, my string lights use a buck converter at the start of every section to generate 5v for the lights from the 19v I route alongside the LEDs. With those converters, I had to cap the total brightness at ~90% to avoid the powersupply shutting down from overcurrent. Now using a better buck converter module that really wasn't much more expensive, I can go all the way to 100% Plus, their response to short circuit on the output is rather dramatic - the IC pops open with a puff of smoke and brief flash of flame presumably from the die of the IC combusting...
You should figure out what it wrong with your meter and get it to read closer to 0 ohms when you touch the leads. That's inaccurate eough that it makes life harder ffor arduino-level stuff.
I only need the buck for powering some modules and an arduino that i won t expect going over 1A that i believe the converter support, but if i have a 12v 2A psu and the buck outouts 5v i can get more amperage out of it but only it s maximum rating limitation