What's your oil pressure when idling, engine completely heated up? My is 1.5bar when N, and when D 1 bar. When engine runs 1200 rpm, full oil pressure. Just asking because oil pressure is used to turn cam timing gear.
I don't have clue how cam is advanced or retarded vs rpm. Do you have such graph? If timing is adjusted linearly, i think solenoid is driven by PWM signal, square wave voltage, duty cycle is adjusted to give certain force to let oil to turn timing gear.
With bulb you can see lightness change, but best tool is oscilloscope or multimeter with duty cycle or frequency counter.
I could measure from my car what kind of waveforms i got.
This is just a guess, but this i the way normally hydraulic servo valves are controller, and is probably used here also.
About your om 603, has there been any tuning around these diesel engines in US? It is increasing here, because of tractor pulling, diesels as well. You can squiz over 400 hp from this 3liter diesel engine for street use without major modifications to engine itself. sorry stepping side about subject.