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  #1  
Old 07-10-2020, 02:34 PM
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HVAC only putting out heat, even when off

I posted last year about my HVAC system putting out only heat no matter what the setting was. When I pressed some different buttons after a few minutes it would go start working properly again.

Yesterday my AC was working fine. I stopped the car and a few hours later went back out to drive it and all it would put out was heat no matter what setting it was on. Even in the off position, it puts out heat. In AC mode it puts out heat. In economy vent mode puts out heat. None of the buttons are doing anything to make a difference. Even pressing all the different buttons doesn't make a difference this time.

1991 300d 233k

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Old 07-10-2020, 02:50 PM
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Start by diagnosing if you have 12V being applied to the monovalve. Make sure it is connected to the coil when you're testing (it's a transistorized circuit). If you do not have 12V being applied, you have a pushbutton unit problem. If you do have 12V being applied, you have a bad monovalve cartridge or a bad coil. The coil can be tested for resistance, it should be somewhere around 14-15Ω if good.
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Old 07-10-2020, 03:37 PM
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Correct, the system will fail safe to heat mode if the power to the valve fails (either just the valve itself, or the whole climate system is lost).
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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
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Old 07-10-2020, 05:24 PM
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Quote:
Originally Posted by Diseasel300 View Post
Start by diagnosing if you have 12V being applied to the monovalve. Make sure it is connected to the coil when you're testing (it's a transistorized circuit). If you do not have 12V being applied, you have a pushbutton unit problem. If you do have 12V being applied, you have a bad monovalve cartridge or a bad coil. The coil can be tested for resistance, it should be somewhere around 14-15Ω if good.
This sounds like a diagnosis that would also apply to a 1980-something to 1985 300 series W123.
Correct or no?

Thanks-
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  #5  
Old 07-10-2020, 09:12 PM
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Quote:
Originally Posted by 5cylinder View Post
This sounds like a diagnosis that would also apply to a 1980-something to 1985 300 series W123.
Correct or no?

Thanks-
Similar. 1986+ W126 and all W124 use a common setup that has all of the logic in the pushbutton unit. The pre-1986 W126 and 1981-1985 W123's have a control box and relay box that handle all of the logic.
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Old 07-10-2020, 09:29 PM
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Quote:
Originally Posted by Diseasel300 View Post
Similar. 1986+ W126 and all W124 use a common setup that has all of the logic in the pushbutton unit. The pre-1986 W126 and 1981-1985 W123's have a control box and relay box that handle all of the logic.

Ahhh. Okay. Since you mentioned it, I recall skimming over threads here about the automatic climate control system's "control box" and "relay box". I'm curious to learn a little more about those two since I have a 1985 300D...I'll search the archives.

Thanks for the clarification.

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