Look on the firewall just inboard from the battery. It's usually brown plastic, two heater hoses attached and an electrical connector on the top.

Has anyone mentioned a bad monovalve?
Depends on the year of the CD. If you have horizontal as opposed to vertical buttons on the heater control panel, it does.

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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 is the monovalve. just replace the insert. Done

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1984 300D

Yup, I've got one then. Here's another question for you. I am taking the family for a couple of hour's drive in my 300CD on Thursday - going to my sister-in-law's for Thanksgiving dinner. Somehow I don't think I can get one of these repair kits and get it installed before then. Can the whole thing be temporarily bypassed to get some heat for the drive? Maybe with a plastic elbow and a couple of hose clamps for example?

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown

IIRC, rather than do a bunch of re-plumbing, all that is needed is power and ground. There should already be power there, from the fuse F10 and the ground is switched on and off by the Pushbutton Controller. So grounding the one side of the connector will cause full heat.

However, once you aquire a monovalve rebuild kit it's about a 15 min. repair.

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Sam

84 300SD 350K+ miles ( Blue Belle )

I guess I don't understand. From reading the previous posts in this thread, I understood that the problem is that pressure past a torn diaphragm inside the monovalve causes it to close - not an electrical problem. So I guess I don't understand how grounding and energizing it ("it" being what I presume to be an internal solenoid) is going to make it stay open.

And that is the plan - once I can get one. But as previously stated I don't see how I can get one AND find time to install it before around 10 am Thursday morning.

On the other hand, loosening two hose clamps, removing the hoses from the valve, and re-joining the two hoses with a plastic elbow - which is something I can pick up at any local auto parts store - seems more do-able to get the immediate result - until I can get the rebuild kit.

__________________
1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown

My monovalve was bad and here are some pictures of it.

Attached Thumbnails

     

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1983 300SD

The boot tearing is the most common cause of no heat. A 5 cent rubber part. The boot can be taken off via a clip on top and you just replace that if you have a boot that is not torn. Used monovalve with a good rubber boot will do the same thing. The cylinder and pin in the cylinder is not defective, just the boot for 99% of the time. Head to your local junkyard and pull monvalves until you find one with a good rubber part and you're good to go. Takes all of two minutes to remove and replace. Gas engines have the same part.

I believe another simpler solution would be removing the "innards" of the monovalve. The only drawback is no control over heat and you may need to open windows.

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Sam

84 300SD 350K+ miles ( Blue Belle )

DAMN you guys are GOOD! That was going to be my next question! I thought of that as a possible temprorary solution last night after I went to bed! If I did this, wouldn't I still be able to control the heat by turning the fan switch to high/low/off? No way to change the temperature of the air output, but if I can turn the heat to high/low/off, that will do for our trip tomorrow.

That's an easy one to test - I can just unplug it before I head home from work tonight and see what it does. If that doesn't do the trick (temporarily) then I'll pull the innards out tomorrow morning.

BTW, I did an eBay search on "Mercedes monovalve" and only got one hit for a rebuild kit - for 56 bucks!

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown

You might want to carry a clamp so that you can make a primitive adjustment (partially or completely clamping a heater hose) to the flow through the heater core. Full heat can be unbearable, even with the fan off.

I believe unplugging the monovalve gives full heat. It closes when it gets power I think.
However, if the diaphragm is torn, unplugging it does not always assure full heat.

__________________
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