Don't know about Mercedes - but on VW the injection pump shaft has a key on it so the pulley can only go on one way. On the outside edge of the pulley (and you really have to look) there is a little notch for the timing. On the top of the pump and on the mounting bracket there are two line markings. When you install the pump all three have to be lined up and a locking pin put into the pulley to keep it there. Then you set the cam and crank timing to top dead center and connect everything up. On the Bosch VW injection pump there is also an internal pump advance timing. There is a bolt on the back middle between where the fuel lines to the injectors come out. The injector pump is then timed using a dial guage screwed into the middle hole I described and the pump moved back or forth on the mounting bolts to get the correct timing then the mounting bolts are tightened. I don't know if both systems are similar - but hopefully this info will be of help to you.