No, don't attempt to adjust that nut until you have loosened the 19mm Bolt that locks the tensioner in place. If you feel around or look with a mirror you will see it in amongst the pulleys.

Be prepared to find out that when you adjust the tension nothing happens. It is very likely that your tensioner is toast. They fail and the only remedy is to replace them along with the small tensioner shock absorber. If the tensioner has been replaced recently then maybe all will be well.

There are posts on the internet about how to adjust the tension correctly. Do a search in this forum on something like 300E Tensioner or word like that.

Steve
'92 300E Sportline 242K