Unable to locate the vac switchover valve for the defroster flaps. There is a vac switchover valve on the driver's side fender well; this runs the turbo. Vac diagrams show several other switchover valves which control (among other things) the defroster flaps. My defroster flaps are stuck open and I believe it might be the vac valve. Where is this valve (and the others)? Are they behind the glove compartment or someplace under the dash?

Hint on the vac switchover valve for the turbo. Replacement part looks different; has three potential connectors for vac tubes instead of two. Just cap off the single connector and do not use it. Attach vac tubes to other end which has two connectors. Turbo will work fine and engine will have pep.

On my 85 there is a stack of them on the center consloe, pass. side. Remove thecapret cover and there they are righ near the CCU.

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83 300SD Dark Silver Dark brown inside

Do you only get defrost?
If so, the problem is likely in the ACC pushbutton unit. Do a search for "resoldering" and "pushbutton" and you will likely find a description with photos. What happens is the circuit boards are arranged in an "H" pattern and the solder joints crack along the edges where they meet.

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Sam

84 300SD 350K+ miles ( Blue Belle )

No, I get heat, defrost, and AC through the vents. In the summer I put a rolled towel on the vents to prevent fogging the windshield; this gives full air through the ac vents. In winter I remove the towel for heat and defrost.

Mercedes tech told me to try to reset memory. Step 1 - roll thermostat to highest heat and punch each button from left to right starting with defrost. Step 2 - roll thermostat to highest AC and repeat punching buttons in same order. Did not work but my flaps have not closed for over five years so maybe memory is not the problem.

So I must assume the vac switchover valve is not operating to open and close the defrost flaps. But not sure.

Mine only blew from the defroster outlets when I got it last year. All diaphrams were good. Had 3 bad switchover valves.

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Sharing my partner's 2012 Forte 5dr SX til I find my next 123 or 126..
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Do I miss being a service advisor ???

On your 85SD where are those three vacuum switchover valves located? Earlier post indicated passenger side of console under carpet. Mercedes tech said it might require removing the glove compartment.

You need to remove the carpet on the floor, the pannel on the side by the door, the pannel under the glove box, then there is a screw in the carpet up front. Remove that and slide the carpet forward and pull it out.
I removed the glove box when I wias in there. there are like 5 or 6 keepers the pop out and the box came out the door. It helped testing the pots but not to the valves.

I then took a meeter and check voltage at the plugs for the valves. Output to all of them. None working. replaced all of them, and now everything works except the floor one. It holds vac with the mighty vac, but does not open, and is dificult to push open.
Not as hard as it sounds. Just do not pull too hard on anything so you do not break it.

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83 300SD Dark Silver Dark brown inside

If you don't find any problems with the switchover valves, then it's likely to be one of the rubber diaphragms in the actuator pods. The defrost pod is located to the right of the main plenum and can seen by pulling the glove box and looking to the left and up a little. Fortunately, on the 126, most of the pods are very accessible. You will have to order a replacement through the dealer as it is considered a "long throw" actuator and there are no replacement diaphragms that I am aware of.

(BTW, I am not aware of any memory in the pushbutton controller for this model. I've had mine apart a few times and it's relays, switches and resistors technology.)

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Sam

84 300SD 350K+ miles ( Blue Belle )

Have a meeter?
When I was looking at mine, I gained access, then unplugged each one after labeling them. And checked Voltage output DC between the two wires, in each button mode. You should get a change as the buttons are pressed. If they are changing then pull a vacuum with a mighty vac tool and see if the switchover valves are switching. then pull on each line and see if the line pulls a vacuum, and holds it. Record it in a chart, and you will see which is the issue.

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83 300SD Dark Silver Dark brown inside

Will try it. Thanks for the hint.