Hello Everyone,

I have wonderful new-to-me 1984 300D. The previous owner took great care of the engine and everything else but there's some issues with the vacuum system that he ordered the parts for but never got around to.

Unfortunately, he never drove in Arizona during the summer so the A/C issue never came up.

I was hoping it just needed a charge, took it to the shop and had them test for charge and vacuum, all is well so they filled it with R134. The lines get cold, fan comes on, great stuff.

So the AC just blows super hot (read hotter than external air) no matter which switch I push. I've read elsewhere that people that couldn't get the right heat had a problem with the monovalve.

Now before I go digging around in the dash could this be a simple byproduct of the vacuum system or something else?

As of right now I have the parts to replace the ignition 3-way vacuum valve and the two rear door locks. I'm hoping that fixes it but if it is indeed the monovalve I'd love to hear an internet diagnosis so I can get the parts ordered.

Thank you!

Vacuum would just affect how the air comes out the vents. It is supposed to come out the center vents in "cooling mode" (and only then), so that seems to work. I think all air flow always goes thru both the heater core & AC evaporator, i.e. there is no "diversion door". There is a door that draws in outside air though.

Probably the "heater servo-valve" (or mono-valve) is open, letting heat overpower the AC. It requires 12 VDC to stay closed (commanded from "TempRegular" box), so an unplugged connector or broken wire could be your problem. But, more commonly the valve internal diaphragm failed. Many posts w/ photos. I kind of recall seeing them affordable lately (~$25 ebay vs $100 before). A quick test might be to clamp off the heater hose, but note that it splits to 3 going to the heater core, so don't get confused. The servo-valve is on the firewall in the engine bay. Just follow the heater hoses.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Sometimes mine will lose its mind and spit out hot air when the thermostat is rolled up into the blue. I just roll it down to the red for a minute or two and then roll it back to the blue. Most times it will adjust itself and put out cool air again. 81 300SD

Another thing - read the Owner's Manual, since the climate buttons are not intuitive. The "EC" button disables AC, but probably not your problem since your evaporator gets cold. Normally, use just the center button and fan on "auto". The other buttons are sort of "emergency use only" per the manual. They force the fan to max speed (can melt fuse holder).

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Sure sounds like you've had a control failure.

System is arranged to give you max fan, max heat, to the defrost vents, in a total system failure. Better to roast the passengers than not be able to keep the windshield clear on the Autobahn.

I would get the online service manual and go through the troubleshooting procedure. It's on startekinfo.com, MB workshop resources, page through to 123 service information.

It will be most beneficial to have a good multi meter and a MityVac.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

I would suspect a faulty mono-valve. It has 12Vdc at all times and the Pushbutton Controller and Temperature Controller apply a ground to regulate the temperature. Usually, the report is no heat. However, it can mechanically fail allowing unwanted heat as well.

Here are some links that should help you:
Untitled Document
Mercedes-Benz Model 123 - Disc 2

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Sam

84 300SD 350K+ miles ( Blue Belle )

Thank you everyone. When I get some time tonight I'll find my multimeter and give that a go. The online service manual is a great find. Thank you for the links!