My 1984 300D Turbo decided, that since we fixed the no-start problem related to a sticky 24yr old starter solenoid, it was time to have a no-stop problem!
Ironic but true.

The problem was vacuum leaks in the door lock circuit.

Unfortunately the links in this forum that have vacuum diagrams did not work for me. "not available". No problem. its easy enough to trace.

First check it to check for vacuum pump operation. I put a vacuum gage on the first T in the line nearest the vacuum pump. At idle the vacuum was approx 18 bouncing inches of mercury. High idle was approx 20" Hg. Good.

I pulled the brown with blue stripe vacuum line from the ignition switch valve and sucked on it and the engine shut off. This line goes to the back side of the injector pump. Sucking with your mouth will yield approx 7 " Hg.
Some people suck more than others so this is just a range .
The other line is just brown and had only approx 3 " Hg. Not good enough.

The one into three rubber fitting supplies main vacuum to the door lock system via a fat one into two fitting. I disconnected this and plugged the line.

This solved the problem and since our door locks need to be refurbed, this did not present a problem.
The car shuts off almost instantly.
I hope this helps.