Long-time reader and participant here, though I admit it's been a while, which is pro'lly why my account got deleted. On with the show!.......

My father's 1982 300SD's locks stopped working last week. I hear about it now. So I go to the trunk-mounted vacuum pump and throw the meter on it. I did some reading here and she IS getting voltage at the right pins for up/down but yet the pump doesn't seem to activate/work at all. I also removed the single vacuum hose to it to bleed off any pressure that might cause this problem, but that didn't do anything. When does the pump actually activate(work)?

On the pump in my car I took the cover plate off next to where the wires go in and found a circuit board that connected to the motor by two long pin connectors. These connectors were not making good contact so I bent them slightly so they each had a little bit of curve to them and it has worked for the last 3 years with no problem. I picked up a spare pump and it has a different kind of connection inside so take a look to see what you have.

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1983 300SD
200000miles

Check pin 1 for ground and pins 2 & 3 switch power for lock/unlock.

There is a sequence pressure switch (diaphram-type with hose attached) that has to be at either end of its travel in order to switch power to the pump. If there is a leak in the rest of the system, the switch may not be making in either direction. With the door key in the lock position, you might try applying a little vacuum (.5 bar) with a mightyvac to the output. Then switch the key to unlock and see if the pump runs.

On mine the flex hose needed replacing and everything started working.

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Sam

84 300SD 350K+ miles ( Blue Belle )

I checked the pins and they are indeed about 13v when activated, one pair for up, one pair for down, so the switch in the door appears to be working. I'm going to go out and sand down the pins on my lunch break to see if that'll fix it.

I sanded the pins, but they looked clean, as was the female receptacles. Since the pumps getting proper voltage for up/down I guess it's the pump. Would a leaking line prevent the pump from activating?