Does a 603 ALDA do anything at idle?
 

Does the ALDA have any influence at idle? Not clear to me from the 60 page IP functional overview in the FSM.

Thanks,
Sixto
93 300SD

Nope, it only makes a difference under acceleration, it determines how much initial fuel the engine gets before any boost comes on, and then how much it gets when the boost is building. 0 effect at idle.

So there's a transition from no influence at idle to full influence the instant the pedal moves downward?

Sixto
93 300SD

The shaft that the ALDA actuates is spring loaded. When the ALDA is installed, it pushes the shaft down (leaner). As boost increases, the capsules inside the ALDA compress, allowing the shaft to move upward (richer). So at idle, no, it's not "doing" anything until boost builds. There's no connection between the pedal/linkage and what the ALDA does inside the pump, meaning when you floor it the ALDA shaft isn't moving - it's separate. Kinda hard to explain... the Bosch injection pump books are somewhat helpful here.

However if you remove the ALDA completely, then it would give full enrichment off-idle. There's another thread going at the moment recommending ALDA removal from all turbo cars, which I don't quite agree with, but a number of people sure do like the idea. Go figure.

:dizzy2:

Back to the original question, does the ALDA pressing down on the shaft alter fuel metering at idle? Does idle go up with the ALDA removed or does EDS keep it in check?

Thanks,
Sixto
93 300SD

It alters fuel metering off idle, but the idle speed doesn't change - don't ask me to explain that, though. This is why an "ALDA tweak" makes the car so much more responsive... it's increasing the fuel delivery curve above idle. Maybe the idle circuit is separated, which is why idle RPM remains unchanged.

(If you twiddle the full-load setting internally, there is a different internal idle adjustment that needs to be tweaked, as the full-load moves the whole rack. The ALDA may also need to be adjusted *leaner* if you do this, especially if adjusted towards the max capacity of the pump, about 20% above stock settings.)

:kid:

Yes, it does. On the OM617, I can press the shaft at idle and it will alter the RPM by 50rpm or so. I'd imagine the EDS will keep the RPM's where it wants no matter what the ALDA is set at.

If somebody wants, I can post a video as proof.

Interesting. Never would have figured that. Goes to show that the mechanical system can't fully isolate one of its parts (ALDA springs).

http://www.youtube.com/watch?v=8K4ZzW1WS1w

Keep in mind, I'm not saying that an OM603 will do the same thing.

Nice video! No doubt about that result.;)