84 300sd wont shut off with key
 

If I turn the key to off sometimes it wont shut off but other times it will and if it doesn't shut off with key if you wiggle the key or remove it from the ignition slot it will shut off, what might cause this? Where is the fuel cut located on these cars?

It sounds like the vacuum switch on the side of your key switch is bad. It is only 2 screws, but a pain to get at them. Last time I removed it, I ended up pulling the switch shaft off the column to get at them, and turned out my switch was fine and I had the 2 vacuum tubes reversed. I suggest that you test your shut-off valve first using a hand pump. The vacuum switch is fairly cheap (~$25). As far as how the engine shut-off works, there are maybe a gazillion posts, so search and read, nobody is going to spoon feed it to you.

Chances are you have a vacuum leak....somewhere....probably in the vacuum system for the locks. Here are a few articles to look over..

Mercedes Vacuum Troubleshooting

Video
Why Won't My Mercedes Benz Diesel Shut Off? by Kent Bergsma - YouTube

There are cheaper places to by parts...

I got it figured out by replacing some vacuum lines. Has anyone ever rigged up an electronic shutoff on theirs?

I have not read of someone installing one. Remember you are speaking of a part that likely lasted over 25 years.

You can buy Generic Fuel Shutoffs that would go in the Fuel Inlet Line and cut the Fuel off.
It is likely Boating Supply places would have them. Be sure It is rated for Diesel Fuel.

The Little Vacuum Valve on the Steering Colum Lock is about $28 each.

If some one thought that the Shutoff Valve on the IP was on its last legs the Generic Electrical Shutoff might be cost effective if the could do the Wiring themselves.

The vacuum fuel shutoff is one of the simplest vac systems in the car. Why try to complicate things by trying to fit some electric unit.

Mityvac that thing, find the leak and fix it.

No need to rig any electricity. You need vacuum for the doors and climate control. Plug those lines if willing to live without. Once the leaks are plugged or repaired which = plugging, your car will shut off.

I took a look on eBay and the Electric Shutoff that I saw that look the easiest to hook up was a Murphy Switch Gauge one at $95+$7 shipping.

And, I don't know if you need to have a Relay for that or not.

I did not take the time to look up any current draw specs on it.

I saw some others that were cheaper but you would need to modify them.

With the cost of the Vacuum Shutoff on the Fuel Injection Pumps rising it could be cheaper to install a Electric Fuel Shutoff when the time comes to replace it.
You could simply leave the old one on but disconnected from the Vacuum if you do not want to make a plate to blank off the Hole.

Cheap vacuum shutoff?
 

Bosch part:

Bosch W0133-1619965 Shut Off Valve | Auto Parts Warehouse

This is the one in the injector pump. Don't know the quality, but I bought one to have as a spare.

Can you tell us what Country it was made in?