I have never owned a 602/603 turbo engine that did not need ALDA service or adjustment at some point in the first 150k miles (and thereafter). There is an adjustment procedure in the FSM, which in simple terms tells you to back the screw out until there is just visible smoke off-boost at full-throttle acceleration.
With proper adjustment and tune, your car should jump off the line.
As mentioned above, there are three common issues with the boost and boost enrichment, two of which take less than 10minutes to test and correct.
1) Boost fitting, boost line from intake plenum to switchover valve plugged or broken, line from switchover valve to ALDA plugged or broken, orifice or switchover valve plugged or broken.
The hose from the intake plenum on the 603 comes from a fitting threaded into the driver's side around #4 cylinder IIRC, crosses over to the switchover valve, through a restriction (orifice), and over to the ALDA. You should be able to disconnect this at the plenum and ALDA and blow air through it. Even if you can, disconnect it and shoot some cleaner through it (I have used MAF cleaner, and WD-40, either one works). This will once cleared, shoot black crap all over and stain your garage floor. You should also be able to blow into the intake through that fitting, if not then clean it (if you shoot anything flammable into the intake, it can cause the engine to run-away or damage glow-plugs, best to remove the fitting if this is necessary, ... I just stick a paperclip through and give a quick squirt of WD-40).
2) The switchover valve has three connections, top is vent (should have a cap) and vents the ALDA when the switchover valve closes (overboost protection). Bottom connects to the intake plenum, middle goes to ALDA. The diagram should be on the car, and I believe that I did not confuse the bottom and middle.
3) Alda condition. If the ALDA leaks air pressure, it will not work. You will need to reseal:
https://www.youtube.com/watch?v=i6TZ2bnlAjI
If it does hold air pressure (remember we're only talking about 1bar), then it might need adjustment, but this should only follow 1 and 2 above. It
can be adjusted on the 602 and 603 with the intake plenum in place, I've done it on a couple of my cars, ... but it is painful. Your better method is to remove the intake (especially if the seal and cap are still installed) to do this. The post above is correct, 3/4 to 1-1/2 turns CCW is all I've ever needed and usually goes a little over the original off-boost enrichment to give a little black puff and lots of off the line acceleration improvement.
True that Bosch Engineers put this there for a reason, mostly emissions and Mercedes did not want smoky un-refined cars. These cars had emissions standards to meet. The ALDA compensates for atmospheric pressure (lose 3% of your air every 1,000' of altitude increase for example), and also for boost. However, giving the engine a shot of rich fuel at the drop of the pedal does increase initial heat out of your exhaust, and guess what spins your turbo, ... yep, faster spool-up and better acceleration.