My understanding is that the entire vacuum system is interconnected. The engine shutoff mechanism uses vacuum to my knowledge.

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Saying that the vacuum system is interconnected is like saying that the electrical system is "interconnected"

To put it in abstract terms "a wiper switch shouldn't open the windows"

Yes the engine shutoff uses vacuum and when it gets vacuum it shuts the engine off.

The vacuum shouldn't be getting from the locking circuit to the engine shutoff.

I think what is happening is: When I turn the key to lock the door, the valve in the door switches to the circuit without a leak and therefore the vacuum in the entire system can build up again and has enough pressure to actuate the shutoff mechanism for the engine. I think you are implying that the two circuits are isolated. I do not know. I am just presenting the symptoms and trying to deduce the problem. This is all new to me.

Since you mentioned "a wiper switch shouldn't open the windows", when I move the switch to adjust the passenger side rear view mirror in my 84 300D, the wipers turn on for one cycle. My understanding is that this is a grounding problem, but electricity is not my forte.

__________________
85 300D 75K Anthracite Grey 0-60 in 13 seconds **For Sale**
84 300D 333K Black (The Velveteen Rabbit) 0-60 in 14 seconds
00 Toyota Sienna 208K (Sold)
15 Subaru Outback 43K
11 Subaru Outback 67K
98 Ford Taurus 100K (Gertie - Was Grandma's - drove it to church and shopping - really) Daughter's car now.
30 Model A Ford 2 Door Sedan (Sold) 0-60 in . . . Never reached 60

Ok now that makes sense.

With the ignition key off and switching to lock closes a vacuum leak.

My bad

That would mean that: there is a massive leak in the unlock circuit and that there are no vacuum "restrictors" to prevent that from happening

It's