Did you check the rubber line from the boost control module to ensure that it was fitted tightly to the module itself? Also check to see that there is nothing in the line plugging it up. Its very rare that these modules go bad and it could just be a loose line that is causing the problem.

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2005 SL65
1999 E300
1995 E300
1994 SL320
1988 560SL
1987 300TD
1982 300D
1955 300SL