The reason that the overboost protection valve on the firewall wasn't connected is that there is NONE on a 240D! No turbo.You need to get it from the donor car. You'll need to cobble an electrical source; the sensor should still be on the "new" engine (intake manifold, rearward of air cleaner). I recommend doing it right, although some just bypass it and hope that the wastegate doesn't stick closed.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)