According to my infornation, the door switch has only two positions, "lock" and "unlock"; there is no center position. The pressure sequence switch is part of the pump unit.

I'll have to look at the door switch. The key returns to center to remove it but that may not move the switch until you rotate the key the other way. I did find that when I turn the key to unlock the doors it sends 12v to the pump but as soon as you turn the key back to center to pull it out the 12v signal stops but when you turn the key to lock it sends 12v even after you return the key to the center position.

__________________
1983 300SD
200000miles

Does that mean that you need to hold the key in the "unlock" position until the doors unlock? In other words, will the doors not unlock if the key is placed in the "unlock" position only momentarily?

I tested that and found that either when locking or unlocking if I turn the key and then turn it back to center right away the system still finishes what ever I turned it to do. Also I timed how fast and about 3 seconds is enough for it to work properly.

__________________
1983 300SD
200000miles

That would suggest that the switch itself does not have a neutral (center) position.