You have probably disconnected the vacuum line either to or from the vac switch that is mounted on the steering lock/ignition switch.

The diesel uses no electrical circuits for running OR shutting off. The shut-off is done by connecting vacuum to the fuel shut-off servo on the back of the injection pump (you didn't say but I assume you have a 77 or newer 240D with vacuum shut-off; not the manual cable shut-off of the 74-76 240D). The fuel shut-off servo uses a diaphram and vacuum to pull the pump rack to the zero delivery position; no more fuel, no more run.

The lines going to and from the switch are brown. One of them might have a second color tracer.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician