84 300SD - Cool AC from front vents, COLD AC from rear vent. Why???

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All,
The AC in my 84 300SD has only cool air from the front vents, but ICE COLD AC air from the rear vent from the center console.
Clearly, I want to have the coldest AC from the front vents, but I am
puzzled as to how to solve this. Perhaps a vacuum issue under the dash on the recirculation door?
Thanks! Matt
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Matt ha scritto:

Most probably the gate of the heating system is leaking.
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Sounds like you got an actuator or vacuum problem. Does your defrost vent stay open regardless of what is selected? like the bi-level and only bottom vents?
Does your vent change position when you accelerate or climbs a hill? Like your defrost vents opens up during that time?
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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
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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 . . .

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Here are the links in a form so you don't have to paste them in your browser:
http://inyopro.com/switchovervalvetest.pdf
http://inyopro.com/testingvacuumactuators.pdf
http://inyopro.com/removedash.pdf
Also, if you need to check/replace the actuators for the fresh air and recirculate flaps behind the glove box, you will also have to remove the right kick panel.
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Heav, Thanks for the very detailed response. It is greatly appreciated. Looks like a project for my 4th of July when I'm not setting off fireworks with my great 13 year old.
Jeff
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My pleasure. It helps me to put the process in writing so I know what I am doing the next time I have to work on my 300 TDT's vacuum climate controls.
I just thought of another thing: If you have to remove an actuator, they are mounted in holes in a metal plate. The Actuators are held in place by "ears" that go through slots and then the actuator is twisted into place on the opening where the slots are not. Kind of like the way tail light bulbs are held in. So push the actuator forward gently and turn until the "ears" line up with the slots and then the actuator will come out. You also have to slide the hook at the end of the metal rod out of the bar it connects to that actually moves the flaps.
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My pleasure. It helps me to put the process in writing so I know what I am doing the next time I have to work on my 300 TDT's vacuum climate controls.
I just thought of another thing: If you have to remove an actuator, they are mounted in holes in a metal plate. The Actuators are held in place by "ears" that go through slots and then the actuator is twisted into place on the opening where the slots are not. Kind of like the way tail light bulbs are held in. So push the actuator forward gently and turn until the "ears" line up with the slots and then the actuator will come out. You also have to slide the hook at the end of the metal rod out of the bar it connects to that actually moves the flaps.
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On the 4th I will be running our Lions Club Sno Cone trailer at the "Kids Games" at the park. We have Giant Calaveras Frog Jumping races and other more normal things like gunny sack races for the local kids here in Independence, CA where I live in the eastern Sierra.
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On the 4th I will be running our Lions Club Sno Cone trailer at the "Kids Games" at the park. We have Giant Calaveras Frog Jumping races and other more normal things like gunny sack races for the local kids here in Independence, CA where I live in the eastern Sierra.
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On the 4th I will be running our Lions Club Sno Cone trailer at the "Kids Games" at the park. We have Giant Calaveras Frog Jumping races and other more normal things like gunny sack races for the local kids here in Independence, CA where I live in the eastern Sierra.
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On the 4th I will be running our Lions Club Sno Cone trailer at the "Kids Games" at the park. We have Giant Calaveras Frog Jumping races and other more normal things like gunny sack races for the local kids here in Independence, CA where I live in the eastern Sierra.
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On the 4th I will be running our Lions Club Sno Cone trailer at the "Kids Games" at the park. We have Giant Calaveras Frog Jumping races and other more normal things like gunny sack races for the local kids here in Independence, CA where I live in the eastern Sierra.
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On the 4th I will be running our Lions Club Sno Cone trailer at the "Kids Games" at the park. We have Giant Calaveras Frog Jumping races and other more normal things like gunny sack races for the local kids here in Independence, CA where I live in the eastern Sierra.
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On the 4th I will be running our Lions Club Sno Cone trailer at the "Kids Games" at the park. We have Giant Calaveras Frog Jumping races and other more normal things like gunny sack races for the local kids here in Independence, CA where I live in the eastern Sierra.
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Hev,
WOW! That is one heck of a response! I have been flooded with work this work, but will try this on the upcoming weekend!
You do bring up a very interesting I have forgotten about! I had a similar problem with a 85 300D I sold some time ago and one of the electric actuators was not working; I applied power to each one and you should hear the 'click' when it is activated. One did not and I changed it out and all was well with the system again!
Thanks!
Thanks
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The reason why the defrost vent is open all the time is because the actuator for the defrost vent is the most used actuator... every time you start up the car, the defrost vent shuts... every time you turn off the car, it opens up.
To fix this, you need to replace the defrost vent actuator... which is located right next to the glove box.... you have to remove the glove box's box and then you can see it right on the side on center ... very easy to replace and cheap... like $35.
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The actuator for the defroster flaps is on the driver's (left) side. It is accessed by removing the driver's side kick panel. It is the actuator with two hose connections.
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I don't know about you heav... my car was 84 380SE. And I actually replaced the actuator for the defrost vent myself... it was on the passenger side of the center console...right next to the glovebox.
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