Need to know how to replace sepertine belt on a 1986 Model 300. Where is the tensioner. Also photo of belt routing.

Do a search in this forum. Betting there are a lot of posts.

You can find a picture easily, by looking it up in Buy Parts above.
Also look up the adjuster rod and nut for a picture.

Look just to the left (facing engine compartment) of the Power Steering reservoir. You will see a brass adjustment nut angling upward to the right. Shine a light straight down and you will see the adjustment pointer. You can't see the tensioner as it's buried.

You have to get tension off the belt. Note: Important. There is a locking
bolt that goes through the tensioner from front of car towards back of car.
Think it's a 19MM. This bolt must, repeat must, be loosened before you loosen the adjusting nut at the top. Note: The adjusting nut is a reverse thread. Clockwise loosens the tension.

Also beware. If the tensioner is old, when you get the new belt installed and try to adjust the tension the tensoner may fail. You adjust but you can't achieve the proper tension. Happened to me twice on two differenct 300E's.
Then you have a mess on your hands and a fair amount of wrenching to get the old one out and the new one in. If it were me and the tensioner has not been replaced in the last 75K miles, I would buy the new tensioner ahead of time.

Steve