This is a 1980 300SD.
I tried doing a search on this and came up with very little. And I know this is a common problem.
The engine will not shut off when the key is turned off. I put a hand vacuum pump on the shut off valve on the back of the IP and it shut off. I am getting about 18-19 lbs. out of the vacuum pump on the engine. When I turn the switch off I am getting about 4-5 lbs. of vacuum to the shut off valve from the ignition switch. So, my guess is the vacuum stuff at the ignition switch.
What vacuum stuff is in there and what needs to be replaced and how difficult is this?