Vast simplification: It's the device that tells the injection pump to send more fuel as boost pressure increases, and vice-versa. The pumps tend to lean out as they age and if you tweak the ALDA setting it can return power to normal.

how do u do that? b/c i have similar symtoms, 91 300e 3.0.
if i gun it off the start it accelrates very slowly. but if im driving and gun it , it goes. and when it shifts, its shifting very hard. if i stop and turn a corner, and giving it a little gas, u can har the tires squeel when it shifts hard.
aany ideas?
thanks

Your 300E has a totally different setup than the diesels. but if you are getting hard shifts, I'll almost guarantee you have a vacuum leak between the engine (intake manifold) and the transmission. There is a single vacuum line, check that and make sure it's intact, and the rubber fittings on both ends are not falling apart. The fluid & filter need changing every 30kmi, so do that also if necessary. (Make sure to drain the torque converter also - half the fluid is in there.)

The 300e is a gas engine, no ALDA.

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True. However you can tweak the EHA on the M103. The effect is similar to tweaking the ALDA on the OM602/603. However you'll need to adjust the idle mixture afterwards, which requires an ammeter and home-made wiring harness. Kind of a pain, but if you have a doggy 300E it can help. This won't do squat for shift quality on the gas engine though. The gasser sends the shift signal based on engine load, the diesel sends the signal based almost entirely on throttle position, which is why the two respond differently.

What does ALDA stand for? Is one of the A's "aneroid"?

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It's an acronym for a long German term (or terms). I don't recall offhand what that is. I know the non-turbo models have an ADA (no "L") which compensates fuel delivery based on altitude only. The ALDA adjusts for both altitude and boost pressure.