Maybe you knew this but it's news to me. The key switch signals a switchover valve to actuate the engine shut-off valve. It's the same shut-off valve and vacuum source with a switchover valve bolted to the intake plenum. Might be the key to remote start for older Diesels.

Also the W140 does away with the vacuum amplifier for the transmission modulator. The vacuum controller on the side of the IP looks the same but there's nothing between the controller and the modulator other than the line that connects them.

Sixto
93 300SD