On my .970 the turbo spools very quickly, there's basically no lag.

If you floor the go pedal without pressing the kickdown switch, does the trans shift from 1-2 or 2-3?

Granted my car is around 700 lbs lighter, but this should give you an idea of the acceleration your car should have: https://www.youtube.com/watch?v=srXc2xV0fO8

Hooking up a temporary boost gauge is a good idea. You could also use a vacuum gage on the the wastegate to see if the computer is successfully commanding it to close.

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Yes, the kickdown works. A number of systems on this w140 dont, but the drivetrain is great.

No issues with acceleration, higher amounts of throttle, especially at speed, give the indication that boost is building.

Just need to get the gauge on...

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Easy way to check do you have a vacuum operated or pressure operated wastegate is to look (vacuum/pressure) hose that goes to the wastegate.If it is hooked to the intake than someone change it and you have a pressure controlled one.but if it connects to the valves that are under front right headlamp then it is stock and vacuum operated.
My advice is if it is not already changed to put a pressure operated one and block off the EGR.Then you wont have a limp home mode never again .

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Mercedes W140, S350TD, 95.
Mercedes W140, S350, 92.
Porsche 944, 86 N/A.
Porsche 944, 85 N/A automatic.
Porsche 944, 84 N/A.
Porsche 944, 83 N/A.
Opel GT, 73.
Toyota Supra MA61,84.

Sounds like a good warm weather project!

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)