I agree. If only the very tip snapped off, it can be adjusted with some difficulty, but still workable. Worst case you can get a used ALDA from Pick-n-pull for a few bucks and just replace the whole thing.

Or if there are enough threads he can tighten two nuts together.

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UPDATE:
Well, I took the half of the screw (well, it's actually a shaft) and just held it on the screw, and by the way it fits, the alda was NEVER adjusted, it's still in the factory pre-set position. So, everything is fine. It takes me 16 seconds to do a 0-60 run, but I can live with it.
-Joe

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

Actually, 16 seconds is pretty close to factory spec for a W126...! I'd try to tweak it a half turn or so CCW and see what that does. Or pull the ALDA and install a shim underneath, which has the same effect as turning the screw.

I'm afraid to touch that ALDA anymore! How exactly would you pull it and install a shim underneath? Also, I tried tweaking it anyway, but with a broken screw head, it's pretty unlikely I can get it turning, I even tried with pliers.
-Joe

__________________
'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

You hold the ALDA and unscrew the large nut underneath, and the ALDA just comes off. There is a washer underneath. You can add another one, and vary the thickness to adjust the fuel enrichment. Photos of the procedure, and the washer, on a 603 engine are here:

http://www.meimann.com/images/mercedes/OM603_injection/

look for the "ALDA removal" links. Your engine is much easier because the intake manifold is not in the way.

How does adding more washers increase fuel enrichment?
-Joe

__________________
'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

The ALDA presses down on a spring-loaded shaft. Boost pressure makes the ALDA internals move upward, letting the shaft push up. Inserting washers moves the whole thing up, simulating boost pressure - same as turning the shaft CCW, which also moves the shaft up.

Maybe if I can take the ALDA off like you said, instead of using washers to increase fuel enrichment, can I still turn the shaft CCW somehow once I get in there?
-Joe

__________________
'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

where can you get those washers?

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Nope, you can't turn the shaft with the ALDA removed - the ALDA would need to be disassembled. See photos at the link above, that should help clarify how it works.

for the washers, pull the ALDA and remove the washer that's in there. Then visit the local hardware store and try to find something similar in size (same ID/OD). Then you can file the washer to gt the thickness you need. I think something like 0.030" is equal to 2 turns of the setscrew, or something? I forget now...