Hmmm, I have a '91 300SE with the 103 engine - I suppose that is the same motor you have. My tensioner is not oil pressurized, just spring loaded.
The way it works is: there is a pin inside the plunger that is spring loaded. It has little grooves in it that allow it to move inward only, when it reaches the next groove it cannot move back outward again (sorta like a plastic tie-wrap). As the chain stretches, the tensioner plunger clicks inward, tightening the chain, but cannot move outward again to allow any chain slack.
if you removed the chain without taking the tensioner spring out, the tensioner plunger clicked all the way in, and cannot be forced back.
The only way to reset the tensioner is to remove it, remove the plunger, and reinsert it again from the other end.
Hard to describe, but very obvious when you get the tensioner out and apart.
Just take a big allen wrench and loosen the tensioner outer bolt, it will all come out easily. getting the spring basck in is not fun, but can be done with some patience.

This all assumes you have the 103 engine with the spring loaded tensioner.
If you have a big allen-head cap, about 10-12mm, holding it in, then it's probably like mine.

DG