Doh! You did say tensioner. There are a few more steps but I'm not going to remember what they are exactly off the top of my head so check the archives.

I've heard that you have to detach the power steering pump to replace the tensioner but I didn't. The trick is to blindly engage the threaded rod at the end of the hollow shaft onto the cam at the back end of the tensioner. I got it in 3 tries.

The water pump, fan and power steering pulleys have to come off to gain access to the tensioner bracket bolts. You might loosen the pulley bolts a little with the belt in place or wedge a screwdriver against the other bolts while you loosen one.

There are 3 bolts on a bracket that holds the tensioner in place. 2 bolts are close to the tensioner, 1 goes into the timing chain cover and requires a dab of sealant. The little shock absorber bolts to the engine lifting eye and the back of a bracket on the tensioner.

If you remove the 19mm bolt, the bracket comes off independently of the tensioner and the tensioner comes off on it's own with the shock... I think.

My flaky memory and questionable methods might lead you to more problems than solutions so I'll stop here.

Check the fan bearing and idler bearing when the belt is off. I'd be surprised if you didn't need a new fan bearing.

Sixto
91 300SE
87 300SDL
81 300SD