OK. I'm looking at my heater control unit and wondering waht to do. I'm not even sure that this is the source of my trouble, but with the fix described in this thread I can at least get started. The heater works; the AC works. But I can't control which will blow into the passenger when. For the most part, I can't keep the heat from blowing into the cabin. Will installing the valve and bypassing the HCU address this?

mmcan, I am not that familiar with these systems, but it sounds like the problem is your control unit in the dash, but again I'm guessing.

I am however certain that by putting a manual valve in the heater line that you can stop the flow of coolant to the heater core, and thus kill the heat in the cabin. That may be enough of a fix. I also have a '79 300, which is currently under the knife, so to speak.

Good luck.
Andy.

Is it possible that cutting off flow to heater core would allow fluid to settle in core? And will core be then more likely to corrode?

I guess that is possible. If you are running the proper mixture of antifreeze, then you could cycle the valve, monthly, weekly, whatever. That should help. If you flush the system, open the valve to flush the heater lines and core also. I don't think you would have a problem with all that.

As I stated before, I'm not a mechanic. This "fix" was recommended by an independant Mercedes mechanic in town, who was recommended by a friend.

This friend, by the way, is selling a pretty nice 300CD, '82 model, under 200K miles. I think he wants 5,000 bananas. I'd like to have it myself, but I got mercedes all over the garage at the moment.

Not really related to Mercedes but I had a 1972 MG Midget while in college and the heater was controlled by a valve under the hood. On in fall and off in spring. It worked fine, but not what you would expect in a Mercedes! It was not a ball valve but a gate, kind of reminded me of turning on the garden hose!

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Currently
1991 Mazda Miata 174000
1991 Subaru 189,000 Winter beater

got rid of:
1984 VW Vanagon 195,000
1976 350SE Euro 185,000 KM
1973 280SE 4.5 (boy do I regret that)
1980 280SE European W126 4 speed manual (miss it)
1989 BMW 735iL 119000 (good riddance)

Thats exactly what I put in mine. Just like you would have on a garden spigot. This seems to be a popular fix, probably due to the fact that the servo costs around $800, and the valve about $3. Pretty easy math on this one.

I'm puzzled by one feature of the fix. I believe the servo has a temperature sensing switch in the bottom which will not allow the fan to blow (and perhaps also not allow the aux water pump to run) until the coolant reaches a certain temperature. What happens with the switch when the servo is bypassed? Wont the blower not operate? How have people solved this problem? Bypassed the switch somehow?
Also does bypassing the servo cause any problems with the controls calling for some action which cannot be carried out since it cannot sense the temperature of the coolant? Could this burn out the amp or control panel?

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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