There seems to be some confusion. It sounds like you have a book that covers VW/Audi stuff. I seem to recall a method that used the fuse connection in the fuel pump relay and had 4-digit codes....but I've tried real hard (with some success) to forget it.
Though the fuel systems are similar and made by the same manufacturer, they share little in terms of diagnostics.
The "fault code" system used by Mercedes is very different. If you had an 89 model, it would be easy to get and check/erase codes. Early version Mercedes fault detection was done through the on-off ratio tester. I never had much luck using it myself since most of the faults were just as easy to find with a DMM. You need the MB manuals to fully explain the process.