On the ACC II and ACC III systems you get full heat, with no way to modulate the temperature, when you select defrost (DEF).

I wonder if anyone has ever explored hacking in a temperature control to defeat the full heat?

The reason I ask is that I live in an area that sees high humidity in the summer months, and it's getting a bit worse every summer. There are days when we get nasty thunderstorm conditions when you have to keep the windows rolled at least partway up, even in 90-100 degree heat. At those times the windows fog up badly, and you really have to use DEF to clear the windshield.

Of course you then get roasted as well as steamed.

I generally don't worry about keeping the AC functional for the length of our hot periods during the summer. But I seem to recall that, back when I did have the AC working when I did way more highway driving, it wasn't up to coping with the problem I'm talking about here.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Hack it by supplying power to the monovalve to shut off coolant flow?

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Or just install a ball valve in line with the heater core, turn it off, no heat. Also, the bilevel setting is supposed to be used for "defogging" the windshield. Have you tried that? The manual says that the defrost setting meant only the melt ice.

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The defrost blend flap has two stages, i believe when the solenoid opens up vacuum to the center vents it should open half way. It will open more when the defrost solenoid is opened. You might check that your center vent pod is seeing vacuum, and that both the defrost and center pods are holding vacuum. The logic that controls when each flap is actuated is explained in the FSM (poorly explained, that is).

The second button (arrows up and down) from the left on the 123/124/126 is the function that is intended for rain storms. It allows conditioned air at the selected temperature to be deilvered out all openings- defrost, dash and floor. Of course this assumes you have a working air conditioner- you have to get the dew point of the air inside the car lower than the outside air to keep the windows from fogging. And the only way you can do that is by refrigeration.

The first button (universal symbol for defrost) is the "I'm going down the Autobahnn and its below freezing, I just crested a mountain pass, and I'm starting to ice up, and I got to get the windshield clear NOW because I'm going 175 km/hr!" mode.

In the older vacuum/analog systems, this drops power to the mono valve (fail safes to full hot flow), drops all vacuum solenoid power (flaps fail safe to closed except the defrost which fail safe in the open position), and puts the fan on Max (shorts all the series resistors in the 123/126 1st gen, or puts zero reference voltage on the blower drive in the 124/second gen 126, which forces full output).

Even in the newer models, (personally I can speak for the 210 and the 164, which appears to share the climate control from the 211), hitting the defrost symbol has the exact same effect.

So this is clearly the design intent of this function. I wonder if it is driven by the German vehicle safety standards to cover the scenario I described above.

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both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Thank you everyone for the comments.

When I had my nearly new Dodge Magnum, with a fully modern AC system, I wasn't able to quickly defog the interior windows in the extreme moisture conditions I mentioned. Perhaps our weather here is weirder than I had imagined. And the Dodge I do believe gave me full airflow up to the windshield on one setting.

Just for giggles I had the system recharged in the 300d yesterday, and to my surprise it all works. For now, anyway.

I did note the comments on the pods.

__________________


Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Or, use the "some defrost" button that on the 81+ system is one to the right of Max Defrost

-J

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