Here in So Cal, we don't need the heats so much. So as part of my fenagling with the new SD, I swapped monovalve with the 300D which had itself been recently replaced (innards only). Both magnets tested good (.015 ohms). But both valves were also intact - the rubber was solid, no evident damage. From the manual, it appears that the monovalve is de-energized during vent operations at min temps. To test, I disconnected the electrical connection to the valve. I _still_ had great gobs of heat coming from the vents.

So what does monovalve control, if not the flow of water to the heater core?

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

1984 300d
the heater monovalve is on to turn off the hot engine water
off when heat is needed, pull the plug to test the valve for heat

If you disconnect the monovalve then it stays open and you have maximum flow of water! if you connect the monovalve directly to the batery it will close and you won't have the heat at the vents!

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Mercedes fanatic

1985 200D (w124) and totaled 190D

Disconnected=heat.

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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

i was told that for liability reasons all mercedes climate control systems will always defaut to a heat mode during major component failure or total vacuum or power loss.

the example i was given was if you are in freezing temperatures and the system fails and cannot provide heat or the ability to clear the windshield etc.. the customer could be killed or injured in an accident. they could also freeze if the system is not providing heat.

on the other hand it was said that if you were in the desert and the system failed, no a/c but if the heat were on it would be uncomfortable but would not under normal cicumstances cause an unsafe of fatal condition.

this is what i was told, i have never actually seen it in print

so by unplugging the monovalve you are removing the power and it opens allowing coolant to enter the heater core

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1967 250 S
1967 280SE
1968 250S
1971 280 SE
1973 220
1974 280 S
1975 450 SEL
1976 220 D
1979 300SD
1983 300 SD
1985 230 TE euro
1986 280SL
1986 500 SEL euro
1986 190E 2.3
1990 300 SE
2003 C230 K
2004 C230 K
1987 560SEL
1987 300SDL now in canada
2005 C230
1998 C43 AMG Sold to DON
1993 190E 2.6
300TD Former daily driver Totaled
current:
R320 CDI
Ram 2500 Cummins

Not really liability, just a "fail-safe" design that Engineers prefer.

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Gone to the dark side

- Jeff

Exactly....and also a very good way for it to default to for those of use in Michigan!

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

So I wired to solenoid to "energized" and voila, no more heat! Thanks for the explanations.

Then I took out the CCU, resoldered the connections, and voila, no more heat! Did I mention that I reinstalled the original connection? And still no heat!

Time for a new thread....

Thanks for your help. Nothing more to see here. Move along.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

True. My CC vacuum connections are all unhooked, I only get heat through the defroster and side vents(All I need this time of year)

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$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment