I had my '84 300D in the shop last week for a cooling system flush/change. Now that the car is back, I note that the ACC blows only heat. I have read here that this may be a bad monovalve. Is it possible that the cooling system work could lead to a bad monovalve?

Silly question from a newbie: Is the "monovalve" the same thing as the "heater control valve" shown in the Haynes manual on pg 112?

Thanks all. Great forum here.

Mark

It's probably coincidence. Sounds like the diaphragm in the valve tore. It is common. I don't own any Hanes books, but the monovalve is the heater control valve in your system.

Peter

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Auto Zentral Ltd.

It's also possible that a flakelike chunk of crud in the cooling system floated loose and lodged in the monovalve. Either way, it's easy to take it apart and just replace the diaphram and see that the valve moves freely again.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

You can remove the active parts of the monovalve without taking the whole thing out of there. Just unplug the electrical and then the four small screws and lift it out. You will spill a cup or so of antifreeze, but so what. The diaphragm will be obvious to you.

I drove around with flush in my system for about 2 weeks or 400 miles, system came out looking like new, no damage was done. As others have suggested take a look at your monovalve.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k