Vacuum Pump??

[Nykam]

Hey guys,

I own a 94 C280
Shortly after starting and while running there is a loud intermittent buzzing noise coming from the boot of my Mercedes.

It sounds like
*Bzzzzzt* pause *Bzzzzzt* pause *Bzzzzzt*

After stripping back the lining I found a unit connected to vacuum lines.
Its the Black Box under the Spring in the following picture with the Bright Yellow vacuum line coming out of it on the right...




What Exactly is this unit?
Is it failing or is this due to a vacuum leak?

[mbdoc]

Besides locking & unlocking the doors, that pump helps the a/c system with vacuum assist.

Most likely a vacuum leak in the a/c system is causing that pump to run too muck.
You need to check for a vacuum leak.

[Nykam]

The yellow vacuum line splits into many yellow lines just above the unit and I assume these are the locks. Where does the uncolored line go to?

The unit stops buzzing shortly after A/C is switched on.

Also yesterday I stripped the rear deck, removing the rear headrest locking devices and the vacuum unit attached. Should the main line that attached to the headrest vacuum unit now be plugged??

The unit in the boot has been buzzing for a long time before I removed the rear deck equipment.

Also my car has a rough idle, which smooths out when A/C is in operation.
Jumping about 600-800 Revs at idle. Could this be related?

[Nykam]

BUMP

So I can get the Wrath of the da Bump back in my trunk...

[mbdoc]

The translucent line is for MVA. Helps the evgine vacuum & storage tank.

Try blocking off that vacuum line.

[Nykam]

So I bought me a MityVac and started testing my lines....

I followed the Light Grey line from my vac manifold back to the end. Its supposed to end at the intake manifold... and where do I find the other end??
Hanging in the engine bay behind the block

Where does this line belong? I cant see the connection
Should the end of the Grey line have a plastic connector on it??
Mines just the hose...

[Nykam]

So I pulled my vacuum manifold from behind the glove box today and tested my Bellow lines...

Line 2 (Straight Red) is my fail...

I also found the connection for the Grey line... It appears to be a rubber hose that the Grey line feeds into. I hooked it up and all my flaps bar the center vents work again! And the buzzing from my boot is gone.

So I have one failed bellow at the center flap.
Seen as I have to remove the dash to replace the heater core pipes anyway now I think ill replace all the bellows....
How many are there?

[Nykam]

Well it appears my center flaps only open when EC
is selected on the climate control unit.

Can anyone explain this odd behavior?

They open every time EC is selected and when you
switch back they close again...

[mbdoc]

The fresh air vacuum element is applied in every mode except EC & Defrost.

The dark red vacuum line goes to that element as well as the green/blue vacuum line.

That element is the most likely problem.
Have you tried blocking off that line at the vacuum block?

[Nykam]

I see. Thank you very much for your wisdom, this makes sense.

Will a fail in the fresh air vacuum element cause it to recirculate or take in fresh air?