I'm sure this has been covered, but I can't find an answer to me specific problem. When I bought my '82 300D a couple of months ago the heat was full blast all the time - I could not adjust the temperature. When the cabin got to warm I would turn the heat off. Just about when it got cold enough that I would not mind having maximum heat I stopped having any heat at all....well once in a while for about 3 seconds a little warmish air comes out, but otherwise it's just cold air....maybe almost lukewarm air at best.

• The fan works fine and blows plenty of cold air wherever I want it too.
• The auxiliary water pump seems to work fine.
• The pushbuttons on the ACC seem to do what they are supposed to.

From what I have researched it does not seem like it's the ACC and that the thumb wheel temp control does not usually go bad. So, where do I start?

__________________
Mercedes Benz

• 1975 280 SLC
• 1982 300 D

Land Rover

• 1976 109 FFR Carawagon
• 1989 Range Rover Convertible
• 1991 Range Rover (getting a TDI Transplant)
• 1994 Range Rover
• 2003 Discovery 4.6 HSE

Others

• 1975 MG Midget
• 1977 Mario Andretti Signature Edition Alfa Romeo Alfetta Sprint
• 1983 Bertone X 1/9

Remove the electrical connector from the monovalve and see what happens.

Sounds like a bad monovalve.

__________________
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've been told that if both hoses on the monovalve are warm, then it's no the problem. They are.....I can't feel a temp. difference from one to the other.

Right now I would be happy to just have lots of heat tomorrow. If the problem is a bad monovalve can I just bypass it as a temporary solution without harming anything else? Seems like that should be OK.....

__________________
Mercedes Benz

• 1975 280 SLC
• 1982 300 D

Land Rover

• 1976 109 FFR Carawagon
• 1989 Range Rover Convertible
• 1991 Range Rover (getting a TDI Transplant)
• 1994 Range Rover
• 2003 Discovery 4.6 HSE

Others

• 1975 MG Midget
• 1977 Mario Andretti Signature Edition Alfa Romeo Alfetta Sprint
• 1983 Bertone X 1/9

I don't think that is necessarily true. If there is a hole in a monovalve diaphragm, enough coolant might be able to get by to heat up both sides but not enough to produce heat.
Unplugging the monovalve might give you heat, but as someone pointed out the other day, it might not if the diaphragm has a tear in it.
Bypassing the monovalve with heater hose shouldn't cause any problems.

__________________
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

Why is the monovalve there anyway?

__________________
Charlie

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'66 VW 1300 96K miles
'97 E300D 239K miles
'85 300D 203K miles (sold Sep 2012)

It was designed to increase postings, thereby increasing advertising dollars, for the Diesel Discussion. It has been a far more successful product than the previous device intended to do the same thing since unlike the initial product the monovalve has never been labeled 'evil'.

__________________
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 wanted to ask the same question but was afraid I would not get an answer as logical as Kerry's

__________________
Mercedes Benz

• 1975 280 SLC
• 1982 300 D

Land Rover

• 1976 109 FFR Carawagon
• 1989 Range Rover Convertible
• 1991 Range Rover (getting a TDI Transplant)
• 1994 Range Rover
• 2003 Discovery 4.6 HSE

Others

• 1975 MG Midget
• 1977 Mario Andretti Signature Edition Alfa Romeo Alfetta Sprint
• 1983 Bertone X 1/9