starter won't spin

[w123amg]

I did search but couldn't find an exact answer.
On one of my benzes starter would work once in a while.
when I would turn the key, I would just hear relay clicking by the driver's fender and I would have to do it for good minute to get the starter to spin.
On another benz that I have starter worked fine every time.
When I swapped it, it did the same thing on the first car, just relay clicking.
So i put jumper cables right on the starter while it's in the car. positive on the small wire and neg on the ground wire of the car - nothing happens, just relay clicking.
I pull starter out, put positive on that small wire and neg on the bracket for the mounting bolt - the gear engages but won't spin. Got another starter from the junk yard - when applied power to it while it's outside of the car - engages but won't spin. I have tested starters before and all of them spin after engaging. Is mercedes different? is it not suposed to sping while not mounted? should i supply positive or neg to big bolt on the starter, that normally has wire going onto it? or i have 3 bad starters?
thank you.