The MB fuel shutoff is vacuum actuated (the Cummins is an electric fuel solenoid). There is a metal/plastic can on the top of the pump with a vacuum line running back towards the brake booster and into the dash. Another line from the dash comes back out around the strut tower to the brake booster line. This is the vacuum shutoff circuit. Likely points of failure are the shutoff switch on the pump or the vacuum unit behind the keyswitch or both. But the caution about the runaway diesel is true. Be sure to vacuum test the shutoff with the engine off. The stop lever should move up and down. I've done this on my 1987 300D.

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Brian Toscano