560SEL Won't Turn Off... Sometimes
 

My 560SEL recently started having this issue where it will stay running after removing the key. I figured that it was just a worn ignition switch and replaced the whole assembly with a new one from the dealer. Yet, the car will still stay running sometimes. Now, when I turn the key to the off position it will stutter and then rev up like it would if you were to restart it. Almost as if I am inturrepting the power to the engine for a moment and then it continues to run. Everything in the car acts normal otherwise and it only happens sometimes and it is a difficult problem to recreate.

By the way, when this occurs I unplug the fuel pump relay to turn it off and I unhook the negative battery terminal. Also, I check the diodes in the alternator and they test fine. Any help would be appreciated.

BE:

Try unplugging just the D terminal from the alternator when the symptom next occurs.
The field exciter circuit has its own set of three diodes, separate from the six diodes in the main power circuit.

Forgive my ignorance, but which one is the D terminal? Is it the smaller of the two wires that are attched to the studs on back of the alternator?

It's acting almost as if circuit 15 and circuit 30 are shorting, but I can't find any damaged wiring. Is it possible that the factory alarm module could be shorting them? It does connect to both circuit 15 and 30 and it is intermittent. Maybe I'll unplug my alarm module.

I haven't had the problem ever since I disconnected the wires from the back of the alternator to check them. Perhaps they didn't have a clean connection until I reattached them this last time.