The vacuum system seems incomprehensibly complicated at first, but it
really is not so bad once you start working with it.
The vacuum comes into the system via a rubber tube "manifold" that has
five connections that go to "switchover valves." The switchover
valves are just on/off controls for the vacuum. They have a solenoid
in each one. When there is power to the valve, it stays open. When
the power goes off, they close.
The vacuum is directed from the switchover valves to actuators that
are round plastic cylinders. The actuators have a metal arm that
comes out of them that pulls in when vacuum is applied to the
actuator.
The switchover valves are energized from the 5 button control unit
that is on the dash where you select which climate control setting you
want to use. To the left of those is the temperature wheel and to the
right are the fan buttons. The middle one is the automatic setting.
You may have to remove the dash, which sounds daunting, but is not
that bad.
If your defroster vents stay open it means that the vacuum is either
not being applied to the actuator on the defroster vents, or the
actuator has failed. You can test all of this if you have a vacuum
tool. They are little hand pumps with a vacuum gauge on them. I
have one called a "MityVac" and you can get one here:
http://www.performanceproducts4benz.com/search.aspx?word=mityvac
I just recently repaired two switchover valves on my system. BE VERY
CAREFUL WHEN DETACHING THE WIRE HARNESS!!! IT IS EASY TO BREAK THE
PART ON THE SWITCHOVER VALVE, TEARING THE WIRES FREE FROM THE
SOLENOID!!! I damaged two of mine and was able to solder them back
together, but be careful and don't break them in the first place.
The problem is most likely failed actuators. Be sure to check them
all and see that the system is operating properly before putting the
dash back in! If there are other vacuum leaks, your defroster vents
may not close even though the actuator and switchover for those are
working properly. The auto safety laws require that the defroster
vents default to open if there is a problem anywhere in the system.
It is also possible that a check valve has failed. I have not had
problems with any check valves, so I am not very familiar with them.
Here are links to the removal of the dash and the vacuum schematic for
a 1984 300D. My manual does not list an "SD," but I think the "D" is
the same thing.
http://inyopro.com/vacuum.pdf
http://inyopro.com/testing vacuum actuators.pdf
http://inyopro.com/remove dash.pdf
http://inyopro.com/switchover valve test.pdf
I notice that you only have to remove the driver's side kick panel to
be able to test the defroster vent actuator, so if you get lucky and
that is the problem you will not have to remove the dash or mess with
the switchover valves.
You can also get to the switchover valves by removing the electronic
control unit with the 5 buttons and the panel with the buttons for the
rear wiper and rear defroster, etc. You take out the electronic
control unit first, and then reach up and push the control buttons out
of the rear wiper panel from the rear, and then you can work the panel
itself out. It will look like a depressingly complicated mess of
tubes and wires at first, but if you go at it slowly you will start to
see that it isn't so bad after all. You have to remove the dash if
the problem is the center flap actuator. You can get to the other
actuators by just removing the glove compartment. The glove
compartment is held in place by plastic plugs kind of like the ones
that hold on the trim strips on the outside of the car. Take a butter
knife and pop out the plastic spreader pin that is in the center of
each of the plugs and then the ones with the serrations on them will
just pop out. Don't try to pry them out without removing the spreader
plugs first. You also have to take out the two phillips screws
holding the door latch in place. You don't have to remove the glove
box door.
The problem could also be in the electronic control unit, but I would
eliminate the actuators first, then the switchover valves before
worrying about the control unit.
If you end up having to remove a switchover valve, be sure to
magnetize your screwdriver heavily first because if you drop the screw
you will have to remove the center console and maybe the front seats
to find it again . . .
On Jun 29, 1:29 pm, Rooster <deet... RemoveThis @aol.com> wrote:
> Tiger,
> This is exactly what's happening with my 1984 300SD.
> Air blows out of the defrost vents no matter
> which switch is selected.
>
> What's going on?
>
> Thanks again for all your help!!
>
> Jeff >> Stay informed about: 84 300SD - Cool AC from front vents, COLD AC from rear ven.. 
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