I have read the previous threads here on the topic, but still have a question that didn't seem to be answered.

Obviously I'm having issues with the system. When I purchased the vehicle two days ago, all door locks did work, but it took awhile (perhaps about 5 seconds or so) for all the locks to operate once the driver's door switch was activated.

Now a day later, none work, and the pump runs continuously. My question is this: Under normal operation, what causes the pump to turn off? Are there not switches on all the individual door locks that sense when they've popped?

When I unlock the drivers door manually with the key, (hear the pump turn on) then unlock all the doors from the inside manually, I would assume that (because I have assisted the locks open) that the pump would now turn off. But it does not. This is after I have plugged the line to the trunk, so I have ruled out an issue in the rear.

The door lock pins seem rather tough to pull up and push down. And so I plan on eventually opening up each door panel to access each pod to see what I can do to lubricate them to make them operate without so much force. Is this possible? Maybe this will fix the problem?

But because they worked so recently, I am suspicious that there is a leak somewhere.. Perhaps the pump is weak and needs replacing too.

Any advice on how to proceed would be greatly appreciated.

The driver door actuator should be the first one to inspect after you have determined your pump is OK. I think beginning 1985 Mercedes made a change with the vacuum pump to assist the operation of the lumbar feature for the driver seat. If your 190 has this feature, there should be a dial similar to the speaker fader switch on the right side of the seat connected to a vacuum or electric connection line. Under the seat are vacuum lines to inspect as well.
But first, remove the rear seat bottom to inspect your vacuum pump. Look for vacuum lines that are crimped or unplugged. Unplug the vacuum line from the pump. If you can make the pump work while you are inspecting it, plug the vacuum port with your finger after the pump begins to work. This should cause the pressure switch inside the pump to shut it off in a couple of seconds or so. If not the pump is bad. If the pump seems to shut off quickly, begin following the vacuum lines. Remove the passenger carpet. There are several vacuum line junctions here. Inspect for damage or unplugged lines. Open the trunk and inspect the vacuum actuator for the fuel door. Test it's operation manually if you can. Best to remove it to inspect.
Unplug the vacuum line to it, plug the vacuum port on it and see if you can move the plunger in and out with the port plugged. Yes? It is bad. No? It is good. The trunk lock, and each door has a similar actuator. If the problem is not with the pump or in the trunk, best to begin with the driver door actuator. Don't forget to inspect the vacuum lines as they pass from the doors to the car. Could be a pinch in these areas too.

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Selling used Mercedes parts on Ebay...always changing...
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Solid advise by cdplayer.

Don't think the standard '85's had a fuel door actuator nor a lumbar support so you probably won't have to worry about those.

If you hear the pump running, I'd suspect a vacuum leak. Remove the rear seat as advised (can be harder than one would think). Start following the vacuum lines which will require pulling up some of the floor carpet. Find the places where it splits off going to each door. Disconnect to isolate each door and hook that line up to your mightyvac to determine if that door is working correctly before pulling the door panels off as those are a pain and the plastic anchors on the back tend to come off when messed with (hot glue reattaches). Hopefully you will find the leak in the lines feeding to the doors.

Although this is for the W124 body, I think most of these system cross over to the W201 (190D)
80 Central Locking System

Good luck.

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