I was doing some (what I thought was) preventative maintenance on my 1995 c280. In the process of replacing the aforementioned belt i ran into a little problem. What I did was to remove the fan shroud, loosen the locking nut on the tensioner and then proceeded to loosen the 13mm nut which slackens the belt. The belt got a little slack but I didn't think it was quite lose enough so I kept turning. After a bit I noticed that the belt wasn't really getting any looser.
I noticed that the although the top of the nut was turning the threaded rod wasn't turning at all. Now that I have the belt on I can't tighten it. Before I go and rip this thing all apart I was hoping maybe someone had a suggestion. Isn't the nut supposed to turn the threaded rod, Its hard to see and I don't really want to take this all apart if I don't have to. I'm stuck without a vehicle at the moment so if anyone has an idea please respond asap.

I can't make a firm diagnosis from here, but I expect that the tensioner ist kaput. It is actually all too common and that is what happens when you go to retension the belt you can't.

Hope this helps,

Larry thanks for your help. It turned out the tensioner rod and nut were stripped. I thought i'd get creative and run a tap through the tightening nut and grind off the bad end of the rod. I was feeling all good inside until I discovered the rod is reverse threaded. So tomorrow i will fabricate my own rod and put it all back together.

fabricated a rod by grinding an oversized bolt down and attached it to the cam. Then I bored the adjusting knob out and and tapped it out to fit the bolt. Worked like a charm. Evidently the prior owner had broken the damn thing and got by with the tension being somewhat loose. Makes no sense to leave something like that but oh well its that kinda world today. I'll order a new one and put it on once the water pump goes south. Until then this will be a suitable fix.