.........what's the purpose of providing a vacuum.........???

The injector pump uses the vacuum to adjust for altitude.

Edit: operation of ALDA described at the bottom here http://mb.braingears.com/126_DISC1/program/Engine/617/09-450.pdf

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I haven't ever removed the ALDA from a Mercedes unit. I was speaking from my experience with a Bosch liscense pump on a Nissan SD33T. On this unit, the adjustment screw on top only changes the pre-boost max fuel. Again, on the Nissan unit, under the cap is a diaphram with a pretty stiff spring under it. By adjusting the initial height of the diaphram, this adjusts the max fuel before boost pressure moves the diaphram further down. On this system, if one wanted to increase the amount of fuel throughout the boost range, one would need to alter the spring rate. I don't know about the details of the Benz system, just figured they are similar (being both are Bosch units). I guess I'll have to dismantle one to see for myself how they work
BB

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The only use of vacuum is for the vacuum control valve for the transmission. The ALDA uses pressure to regulate the fuel quantity.

Put a Mighty vac on the ADLA and draw a high vacuum plan to buy another Alda. Thats a big no-no. Its designed for presure, vacuum will rupture it.

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Yes however it can also accept very small quanitites of vac (equal to altitude changes) for off boost fuel adjustment...IE...incorperates the properties of a N/A altitude device as WELL as enriching for on boost power.

There is a factory specification for the amount of vac that you apply to the ALDA when testing the altitude compensation. It moves the rack something of 1mm OUT or so.

Brian,

After reading through the operation more carefully you are correct. It does adjust for altitude/atmospheric pressure though. It just does it in a way different than I thought.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

http://ecomodder.com/forum/fe-graphs/sig692a.png

Yes, I can see applying 3" of vacuum or so to simulate taking the vehicle up to 5,000 feet. This would equate to 11.7 psia so the ALDA should have no issue with it.