I am not sure I understand the above comment.

"So, if timing has changed, how do you time it properly? I've got an AB light and set it to 13 ATDC."
I have not used the AB light or ever completely read the instructions. However, if you follow the FSM instructions your timing should be correct.

If the timing is not correct you have to loosen all of the IP mounting bolts and I also recommend loosening the Hard Line Nuts at both ends (so you can rotate the pump easily) and rotate the IP and recheck it.
If you have a newer IP that has a screw to rotate the IP follow the FSM directrections.

"How do you actually know when the pump is opening the injector to set it right?"
If the Injector opening pressures and the IP Begin Injection/Timing are both with in specs ignition should happen when it is supposed to.

It is sort of a prediction that if the Injectors are set within in a certain Opening/Pop Pressure range and the IP begin Injection is set with in the specified range of degrees that Ignition will occur at the proper time.

I believe the AB Light uses a spot on the IP Camshaft to trigger the Light and because that spot is not in the same place as the begin Injection of #1 if you used the Drip Method the Light triggers and a differnt degree mark. Mercedes has calculated which degree/s that is and gives you a different degree spec to compensate for that.

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84 300D, 82 Volvo 244Gl Diesel