Call Garrett and buy a pressure operated wastegate valve (as opposed to a vacumn unit) and plumb it to the intake. Adjust it to 14 psi at 4000 rpms in third gear and you will be done.

PS - if you have disconnected the EGR, that will cause the turbo not to function on the 350

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1995 S-350
370K + SOLD
1952 220B Cabriolet
39K kilometers + SOLD
1998 E300D
310K +
2012 E350 BlueTec
120K