The driver's door close assist wasn't working on my 1998 S420, so my excellent independent Mercedes shop replaced the following:

1. Actuator bellows in the driver's door
2. Rebuilt door closing assist pump

Now, all of the pneumatic door closers work again, but I think there's an issue. I can hear the door closer assist pump cycling on every few minutes when I'm driving.

Let me describe exactly what's happening. When I soft close a door, I can hear the assist pump humming while it provides pressure to the actuator to close the door. That stops after a few seconds. Then, a few seconds later, I hear a second, slightly differently sounding humming from the assist pump, which I assume is the pump providing vacuum to reset the actuator. Then, as I'm driving (or even if the key is in the on position and I'm just leaning in the trunk to take a listen), I can hear the humming sound of the vacuum pump (the second sound during a door closing action) cycle on for a few seconds. And it does this intermittently while I'm driving.

I don't remember hearing this pump cycle when I was driving before. It's not supposed to do that, right? Any ideas?

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Mark S.
Seattle

1998 Mercedes S420, Smoke Silver, 138K

i just solved a cycling vacuum pump on my dad's w140... but its the door lock pump... not the closing assist...
it turned out there was a leak in one of the lines, it seems there is a vacuum reservoir/buffer which the pumps maintain... not so sure which line it was but i still have all my vacuum functions(door locks... trunk lever...reverse assist antennas...) don't know what function i plugged tho...

to check just plug the nozzles at the pump 1 at a time... see which one "stops" the pump from cycling.

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85 190E 2.3(SOLD)
86 230E (-->300D) sold
87 300D (-->300TD) sold
68 250S w/ a 615 and manual tranny (RIP)
87 300TD (SOLD)
95 S280 "The KRAKEN" (Turbo 2.9 602 transplant) traded
86 190E 2.3... current project

You are correct, the pump will run to provide pressure to "draw" in , it will then reverse to reset the elements, you will hear the sound in the trunk. The pump will also run to pressurize the lumbars/ reservoir, and lastly it will again run backwards to build up vacuum assist for the HVAC/reservoir.
I think it also runs randomly to remove any moisture that has accumulated, its something I read recently!
And I found it, its on the 140 MB DVD
One minute after turning on the ignition, the pump runs for 2.5 seconds and supplies vacuum to all the elements, then twice at 5 minute icrements, and then once every ten minutes. After closing all the doors and things it, runs again for 2.5 sec providing vacuum to all the elements. Only MB would have engineered it this way!

So, are you saying that the door close assist pump runs at regular intervals while the engine is running and that's normal? The door close assist pump is the one that I'm hearing (on top of the left rear wheel well).

Aren't all of those other functions, like the HVAC control, central locks, and lumbar pillows operated off of the PSE pump under the rear right passenger seat?

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Mark S.
Seattle

1998 Mercedes S420, Smoke Silver, 138K

Well Mark, I am sorry for the confusion, and when re-reading, did get my info a little wrong, the "close assist" pump does the purging and resetting of the elements, I indicated.
The "locking pump" under the seat provides for lock and unlock and provides supplemental pressure for the lumbars, and supplemental vacuum for the HVAC when the engine does not produce any. You should only hear that when it is replenishing the two reservoirs due to a leak. It does none of the other purging stuff. The sound in the trunk is from the other one doing its thing, the vent hose is behind the trunk lining near the tail lamp.

Thanks. I won't be concerned about the door assist pump cycling while driving then. I took my car in to get the cluster backlights replaced, and my mechanic checked both vacuum pump systems again for good measure. He said they weren't registering any errors.

Thanks again!

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Mark S.
Seattle

1998 Mercedes S420, Smoke Silver, 138K

Yes, its normal behavior for the CA pump. Perhaps you only noticed now, because
the rebuild of the pump has made it to run louder due to motor being louder,
or different mounting that lets vibrations gets thru to your passenger area?

br,
syljua

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1998 CL500 W140
1984 230E W123