Hey guys,
As some of you may no I recently repaired the monovalve in my 85 300SD. The heat has been great, however I have noticed that no heat seems to be coming out on the floor. When I would start the car in the past I would hear the thunk of a vent opening near the floor. No I don't hear the thunk and no matter which button I press there isn't any floor heat. Do you think I perhaps have a vacumm leak? Maybe it is something else? The heat works great through the other vents. Any help would be greatly appreciated. I searched but couldn't find much of anything. Thank-you

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1985 Mercedes Benz 300SD, TMU
2015 Chrysler 200S

Yep, there's a failure in the vacuum system controlling that floor vent. Probably a leaking vacuum pod but it could be a leaking fitting or a bad switchover valve. I don't know where the pod is on a 126. On a 123 it's pretty simple fix which I wrote up in a post about 4 yrs ago.
Temporarily, you could do something to hold the vents open. I'm not familiar enough with that system to know exactly what could be done but the center vents in my TD are held open with a piece of aluminum flat stock stuck down in the vent tube.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

The swtichover valves are in the center console right next to the passenger footwell.

Pull back the carpet on the center console side (passenger) and it reveals the switchover valves. Put a MityVac on each one and diagnose by pulling vacuum through the valve and then after the valve. If the pod or valve are bad it will not hold vacuum. I forget off hand which numbers control which units. I think #1 or #3 are the foot vent pods.

I think the foot vent pod is the hardest to get to and you may have to pull the dash. So if this does indeed turn out to be the culprit you could always wedge the vent flaps open with something.

See if you have any floor heat on the passenger side. My SD had the same issue on the drivers side only. There is a door that the wrist pin fails on, I have propped mine open.
I found a use for a spark plug!
I wedged it in there blind, and tied a string around it in case it ever falls in, I may be able to retrieve it.
The last two pics are of the duct work under dash and the back of the lower kick panel. Make sure you hook everything back up. Also, there is a plastic 90 degree screw near the trans hump, highlighted, it does not come all the way out.

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83 SD

84 CD

Thank-you for all of your replies. I will look into resolving this issue this weekend.
Thanks again!

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1985 Mercedes Benz 300SD, TMU
2015 Chrysler 200S