You will need to know what chassie number your car has perhaps one of the Members will post as I am not sure what chassie a 77 has. As the Manuals are classified by the chassie number. Example mine is a W123
For the Factory Service Manual.
Go to the below site and you will see "Search by Literature Type"-- click on the arrow to show the menue and select Service Manual. $19.99 + $7 shipping.)
http://www.classictechlit.mbusa.com/MB/default.asp?catalog_name=Retail