I was testing for a project I am working on and found that when I lock the doors I get 12 VDC on the lock wire that stays there until I unlock the doors. When I turn the key to unlock the doors it sends a momentary signal to the pump and then goes away. It seems like it should be a momentary pulse and not stay on in the lock mode so it would act like the unlock mode but does any one know for sure about this? The manual doesn't say anything about this and I think it may be an adjustment on the lock linkage in the door that is holding it that way.
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1983 300SD
200000miles

also something seems missing in your explanation, don't take this the wrong way, because I frequently do that when I am in a rush also--just type away with grease on my hands...but sometimes I forget that the reader is not privy to the information and visions in my head ( Thank the Good L--- for that...)

Its right there at the bottom of the page. Best bet is add it to your sig so it always comes up although if you have a lot of MBs that could still be a problem.

With this car there is a vac/pressure pump in the trunk and depending what way you turn the key in the drivers door it tells the pump to produce vac or pressure. The pump has one ground wire and two power wires. If you lock the doors the first wire has power and the other does not, if you unlock the doors it is the opposite.

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1983 300SD
200000miles

Until early/mid MY87 the 124 had the same. The problem was that a vacuum leak would cause the pump to run and draw down the battery. Later cars had only a momentary.

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Gone to the dark side

- Jeff

The switch in the door is hot at all times. One of the two wires (either lock or unlock) from the switch to the pump is hot at all times. When the switch is moved to a "new" position, the circuit to the motor is completed. Then the pressure sequence switch breaks the circuit by flip-flopping a switch between the door switch and the motor. The motor circuit is then broken until the door switch is moved to the opposite position.

But wouldn't the door switch break all contact when it returns to center? Where is the pressure sequence switch located? The wires from the door switch to the pump are not connected to anything else in between the door and the pump so the switch that you refer to would have to be in the door it sounds like.

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1983 300SD
200000miles

Thats wierd I added some more to this and it didn't move it up the list.

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1983 300SD
200000miles

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.

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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.

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1983 300SD
200000miles

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