I posted a few weeks ago about a climate control problem with my 85 300td. Heat would be fine for about a half hour, then cold air would blow. Sometimes I could push different buttons and get heat for a while again, but it would always start blowing cold eventually. I disconnected the monovalve as a result of the advice from that thread and thought the problem was solved. Monovalve open all the time, hot coolant thru the core, lots of heat. Worked fine for the last couple of weeks, except it got too hot sometimes. I was waiting to pick up a new control unit at the Pick and Pull.
Today on the way to work after the car was running for about half an hour, it went cold again. What could be causing the air to cool if the monovalve is not getting voltage so theoretically ought to be letting hot coolant flow freely thru the core?? The engine temperature did not change on the gauge. Is there a flap of some kind that might be allowing cold air to enter the system, overwhelming the hot air from the heater core? I can't imagine that the monvalve is closing since it has no power at all anymore.

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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

It's supposed to fail open but sometimes it doesn't. When I didn't know much about the car I actually PAID SOMEONE ELSE TO FIX IT!! As a result, I don't know exactly how it looked, but it's a cheap fix with a rebuild kit!

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

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

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

Look at a couple of things.
The mono valve may be the problem. Voltage closes it. but with no voltage it may allow it to move freely.
To check this completely remove the internals of the mono valve, this may require cuting and modifiying.
The Ac compressor may be trying to engage and cool the car. Are you running the EC mode or the other modes?
Fix the mono valve first, with new internals. Then resolder the aC control panel. The other problem may be the temp sensor foam tube is gone and causing the Climate control amp to vary the temp based on the messed up signal at the temp sensor in the center of the dash.
Pull the glove box insert and look at your foam. if bad replace it. Do the typical search for the foam fix.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

I'm pretty sure I replaced the foam when I was working on some pods a few y ears ago. I will recheck. I wish the AC was producing the cold but it doesn't work and it happens on all button settings that should produce heat.
I had assumed it was the climate control unit, that's why I was planning to replace it. But now that the problem resurfaces with the monovalve disconnected, I'm flumoxed. I guess it could be a bad monovalve.

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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

maybe the Aux water pump is intermittantly working.....IE failing.....

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Pulled the monovalve today and the diaphragm was torn. Ordered the repair kit.

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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

Anybody have any good theories why these things fail? I never get more than 1 or 2 seasons out of one. Different symptoms nearly every time.

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Palangi

2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg



TRUMP .......... WHITEHOUSE
HILLARY .........JAILHOUSE
BERNIE .......... NUTHOUSE
0BAMA .......... OUTHOUSE

This problem confirms what I have observed with the monovalve:

1) If the diaphragm is intact and functioning properly, you get full heat with no voltage to the monovalve and you get no heat with full voltage to the monovalve.

2) However, if the diaphragm fails, you get little or no coolant flow past the monovalve, no matter what the voltage.

Therefore, a diagnosis with a failed diaphragm is always false.