There is no adjustment persay on your 89 124
chassis car. When the belt is to be replaced the tensioner is releavied and then reset after the belt is reinstalled. The tensioners job is to maintain the tension of the belt. This is accomplished by a rubber type insert inside the tensioner. When this rubber material wears out tension is no longer maintained. If your belt had not been removed in the day or so pryior to this happening there is no other assumption to be made except for the tensioner to have failed.
Professional technicians are not in the business if repacing parts for no reason at all.