A few weeks ago I posted the question

560sl motor won't turn off with key. Today I turned the key to my 86 560sl off and the engine kept running. I turned the key many times back on and back to off but no luck at killing the engine. ...

I have the answer to the question "what can keep you car running while the key is off?"

The answer is a shorted out window relay.

After several months of having this car fail to turn off once every 50 times I was able to get it to do it long enough for me to check enough stuff with a meter and find the path of voltage that was not suppose to be. Circuit 15 is the hot in run position contact on the key switch. It provides power to the ignition system to run the engine, power to the gauges, and some other accessories like dome light time relay, air temp meter, power window relay and others. The job of the power window relay is to provide power from the battery directly to all window switches when the key is on.

The relay closes by power from key circuit 15 and then sends power from the battery circuit 30 (HOT AT ALL TIMES) to the window switches. What happened to me was that a piece of loose contact metal inside this relay was bouncing around and occasionally would lodge between the coil terminal which is powered from the key and the terminal hot from battery. This was connecting every thing in the car that is supposed to turn off by the key to the battery and every thing would run whil you stood there and held the keys in your hand. The engine would die and all things would tur off in a minute to as long as 30 minutes. After the key was turned off all things using power are now conducting current through loose piece of trash inside the relay and in a while it probably gets hot and just looses contact the engine dies before you can get on top of this and it leaves you guessing because you have nothing to check out. I stated driving this car with panels removed, a good meter and a schematic on the seat covers.

I don't know how common this problem is but it is a bitch to solve and I think this will be the first posting of its happening in the groups. The lesson is this, any relay that provides power directly from the battery to any thing else and that that relays coil is powered by the run key terminal (15,) that relay can intermittently short and cause this to happen. My car 1986 560sl has window relay, air pump relay, and dome light relay, any of these could have caused this. In the end, I proved it was my window relay by just jerking it out as soon as the engine kept running and the engine would die. $19 later a new window relay has my engine turning off every time. So hope this helps some one else some day.

Bruce