Hi! Im new to this forum and I see there is a lot of guy with experience on the w123! Im the owner of a 1983 MB 300td. I have done a lot of search, found many other ACC related problems, troubleshooted my system, and dont know what to do now!

Here is my trouble! It is starting to be cold outside ( -1, 0, +1 C ) and I cant have hot air blowing in my car. Last month I had really hot air for the first 5 min drive, then it was turning to cold, and stayed cold for the rest of the ride, no mather wich button I press. Since the la 4-5 days, its just always cold air blowing.

My engine is heating good, always around 80 F, and I know I can have burning hot air inside when the system permit. I tought it was my control unit at first. I removed it, opened it and re-soldered every connection. Nothing changed. Then I tought it was the monovale, I disconnected it ( I think its supposed to be normally open/always hot) and ran the car like that. With no current going to the monovalve, I can get really hot air when the dial is set to COLD! If I dial it to hot, then its blowing to my face only cold air again.

Seems like something is giving a wrong signal to my monovalve?

Did some of you guys ran into something like that before?

Winter is coming ( -30 C ) and I would like to have some hot air

Please specify what year and model of w123 you have, so we're sure which system we're talking about to start with (some people use the term monovalve when they mean the servo on the earlier system).

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Sorry, I edited it.

I have a 1983 300TD with the aotomatic climate control ( Horizontal buttons with Temp dial on the left and Fan speed buttons verticaly installed on the right )

If your monovalve is disconnected, how do you reckon that it is getting any signal, wrong or otherwise?

Remove the glove compartment and make sure the aspiration tube for the temperature sensor on the dash is still intact. If the tube is broken, the sensor does not know the temp. inside the car.

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'83 300D, 126K miles.

The monovalve is connected. I disconnected it just to see if it was stuck close and wouldn't let any hot fluid came in. It is normally open ( no current = hot fluid in )

So I asume something is giving current all the time to the monovalve so it wont let the hot fluid in. When no current to it, I got hot air.

I will be looking for that. Thanks

The monovalve works on a switched ground signal. That means it should always have +12v and the ground is opened/closed in order to cycle the monovalve. Continuity to ground = a closed valve and no heat; an open to ground = an open valve and heat. Also NO power = an open valve and heat.

Since you get heat when it's unplugged; and it responds to the wheel (change from hot to cold) then the problem probably isn't your monovalve.

I assume your description is backwards: you should get heat when you set the wheel to HOT, and no heat when COLD. If it works the other way (as you described) then it might not be correct.

The monovalve-ground connection is independent of the temp setting on the wheel in either full HOT (edit: it appears there's a fixed value resistor - R3 on the diagram - but it should stay open) or full COLD positions (it's connected to ground therefore closed). Outside of those two positions it's based on temp inputs into the ACC and the push-button positions.

You also describe the air as blowing in your face. I forget off the top of my head, but either heated or cooled air doesn't come out of the center dash vents. The vacuum pods also route the air differently based on the temperature inputs.

Checking for a rotten/missing foam tube by removing the glove box is a good idea.

There are also a few threads that detail how the pods route the airflow based on calling for heat or defrost, and some specific electrical troubleshooting.

Also, welcome to the forum. Ignore OD, and check out the link below for some info on your ACC - or search from the URL and you'll get the manuals. All the info is also available via Mercedes on their StarTek site. I believe you can access from Canada, but you may have some issues verifying.

Defrost should also = full hot.

http://mercedes.thatchermathias.com/w123CD2/Program/ETM/83_acc.pdf

Thanks a lot!
As for the dial, I got hot air when I select Cold only when I disconnect my monovalve. When its connected, no hot air is available, dial set hot or cold )

Did you checked the the soleniod assembly yet? The rubber diaphram usually goes bad. Check if it has any tear.

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

Your problem is similar to my problem.
Here is my post: http://www.peachparts.com/shopforum/diesel-discussion/329549-what-little-vent.html#post3045950

I plan on removing my glove box I suppose but first will see if the tube is plugged.

Thanks for posting. When I originally posted I accepted the heat being wacky. At least my old car had heat. My vw went two winters without heat. It has it now though.

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85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.

I didn't check that. In fact, I dont even know where it is . What is the fonction of that solenoid? I will do some search on my side.

I will remove the glove box liner today and take a look at the tube and the sensor. I'll keep you informed if I found something.

Also, can I remove and check the diaphram of the switchover valve without removing the entire dash? I think I can only remove the climate control unit and they will be behind right?

Thanks again everybody for the help!

Its inside the monovalve. Remove the 4 flathead bolts and pull out the soleniod. It should look like this: Pelican Parts - Automotive Parts and Accessories - Porsche & BMW . If you have the money, buy the original otherwise its $15 but click the 'more info' button and read the rating for the part.

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

OK guys. I checked behind the glove box for the foam tube. It was missing. Nothing was connecting the 2 tube end together. I replaced it with confidence that it would solve my problem, but no. Same thing as before. I got hot air for the first minute and after it's switching to mild/cold air by itself. The dial is always set to max hot. On automatic, the fan is always blowing high, like it knows the temp is far from what is asked...

What exactly command for hot or cold air in the system?