Mercedes used a unique and clever way to adjust alternator belt tension during most of the mid to late 1970's. This covers 123 240D, 300D, 300CD 1977 to 1979. The head of the outboard mounting bolt actually has teeth machined into it. When you turn the head of the bolt either direction it will move the alternator in or out to loosen or tighten the v-belt tension.
Problem & Solution
Common among these chassis:
This works great as long as the adjustment procedure is done properly and apparently more often than not it is done incorrectly. You must loosen ALL three bolts holding the alternator to the engine BEFORE turning this adjusting head. If not you will strip off the teeth as you can see in this picture. Once these teeth strip you will have a very hard time getting your alternator belt tight enough.
Symptoms will be a squealing belt especially when the engine is cold. When you try to tighten your belt you may find it just won't adjust. Look closely at the teeth on the head of the adjusting bolt. If striped or rounded off order our new bolt replacement.
This part is no longer available new from Mercedes. If yours is worn out try to find a used one from a recycler. If you can not find one then grind all the teeth off and pry out on the alternator with a pry bar while you tighten the bolt. This may be a two man operation!