The W123 chassis covers 240D's, 300D's, 300TD's, 280E's, and several other models of Mercedes coupes, sedans and wagons from model years 1977 to 1985.

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Climate Sensor on Dash not "clicking"/AC blows HOT


1982 Mercedes W123 300D Turbo Diesel

1982 W123 Turbo Diesel

Recently, my AC started blowing extremely HOT (hotter than the normal heat). No matter what settings I change and which buttons I click, it blows HOT. It blows HOT with all buttons.

I also noticed that the climate sensor on the dash that usually "clicks" when I rotate the temperature wheel is not "clicking" anymore and is silent.

Note: for a long time now (years and years), in order to get any of the buttons to work, I usually have to click the defrost button first and then the other buttons will engage and work. Not all the time, but most of the time.

I have a newer compressor (less than 2 years) and my blower motor is working and I can hear the compressor engage when I click the AC button (it just blows extremely hot).

I checked all the fuses and they seem fine.

Coincidentally (or not), this started happening the day after I did two things to my Mercedes: 1) I took apart and replaced the rubber bumper fasteners (mounts) under my air filter housing unit. 2) Replaced the front shock absorbers.

Could either of these two things have anything to do with this problem with my AC? Could I have jarred or bumped anything in the same proximity to any AC stuff causing this extreme heat?

Many thanks for any suggestions, help and guidance. I've already done a lot of research online, but I just don't know where to begin (aside from taking it to a shop for diagnosis, which is the next step if I get stumped here).


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Good news to report. I have driven the car for 4-5 days now and it's still blowing cold. Whew. Problem solved. If your Mercedes is blowing hot air no matter which button has been selected, or no matter what position the temperature wheel is in, start with your Mono Valve. It worked for me.



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Here's the excellent guide by @nsiemsen on repairing it: Mercedes W123 Climate Control Unit v2 Replacement

Hopefully he will get a notice of your question and chime in ;-)

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All heat, all the time, is usually a sign that there is an electrical problem. The monovalve is opened (to allow hot coolant to flow to the heater core) by the ground side of the circuit. If you are getting very hot air all the time then the monovalve is seeing a ground at all times.

Before anything else, check the connector on top of the monovalve. It is a small black plastic plug that could have been knocked off while you were changing the front shocks or air filter mounts.

If the plug is still attached, the problem MAY be in your climate control push button panel or it MAY be in your temperature regulator (silver box behind the glove box). Both can be replaced for units that have been rebuilt.

One quick thing to try that may give you some relief in the meantime - if you have not already, try turning the temperature wheel on your dash until it clicks in to the MIN setting. By design, this cuts the ground to the monovalve entirely. One of my cars has the all heat issue, except when in the MIN setting, so I use that during the summer to get me by until I got a chance to look it all over.

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Thank you for the replies. Having been guided on another forum to first look at the mono valve, I was able to find a Bosch mono valve on Ebay and I installed it tonight. I went for a cruise and (for now), I'm happy to report that it's blowing cold again. When I removed the old mono valve, I could not see anything obviously wrong with it, but I replaced it anyway.

I don't know for certain if it needed a new mono valve, or if just the action of pulling the electrical connector off and on caused it to work again, but so far, I'm hopeful it working properly again. The true test will come tomorrow, during the heat of the day when I drive to work. Fingers crossed.

Thanks again for the guidance,


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Brook will be eternally grateful.
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