I keep the interior blower off, b/c the air coming out is like the inside of an oven...my dash even FEELS like an oven. The radio, knobs, and vents radiate crazy heat. And with high humidity and 95+ temps today in Detroit, I was sweating like a horse.

My mechanic buddy says that the valve that controls the hot fluid (water?) in through the dash and out the vents in probably stuck in the open position. As a temporary fix, he recommended me undoing the two hoses (sorry, not sure what they're called...maybe 1" black rubber hoses going into the back of the dash from inside the engine bay) and hooking them together with a barbed coupling.

That's fine and dandy, but I want to understand what the problem is, and how to really fix it. Having air conditioning would be awesome (an entirely different animal), but NOT having constant heat would be a step in the right direction!

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1984 300D: 275,000 miles (soon to be for sale)

In search of my 1987 300D in anything but white.

Search "monovalve"

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'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E

The heater valve is electrically operated. It remains closed when energized and opens when power is removed. It's not uncommon for the electrical connector on the top of the valve to fail to make a good connection. You might remove and clean that connector. If you have a voltmeter, check for battery voltage at one terminal and continuity to ground at the other. (The valve is controlled by a switched gound, so the presence of battery voltage does not tell the whole story.) The key should be in the "run" position.

As an interim solution, you could plug or clamp one of the heater hoses.