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Old 09-09-2014, 02:31 AM
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Join Date: Sep 2014
Posts: 9
Question 1984 300d w123 a/c

so i took my car to the mechanic for an a/c diagnoses and he told me that everything works fine and i need a a new mono valve, the car doesnt blow cold air only hot. is it the mono valve or can it be something else?

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Old 09-09-2014, 06:21 AM
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Join Date: Oct 2007
Location: Greater Metropolitan Beaverdam VA
Posts: 2,860

The monovalve usually fails in the open position with the result being uncontrollable full heat. This can overwhelm the ac resulting in warm air at all positions.

Try turning ac on as soon as starting the car, before the engine warms, to see if you get ac at that time.
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Old 09-09-2014, 05:59 PM
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Join Date: Sep 2014
Posts: 9
yah i get cold air as soon as the car turns on but as the car gets warmer so does the air in the car once its fully warmed up i get nothing but hot air, its also like this during winter i only get hot air in winter time as well
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Old 09-09-2014, 06:22 PM
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Join Date: Mar 2009
Location: San Diego
Posts: 3,026
Did you check the fuse which supplies 12v to the mono valve? It happened to me with a blown fuse. Can't recall the fuse number.
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details.

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.
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Old 09-10-2014, 11:15 AM
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Join Date: Jun 2011
Posts: 240
I had a similar problem with my 300sd a few years ago, the cars history showed a replacement 10 yrs prior so I spent $20.00 and been fine.

While under the grill for the mono valve be sue to check them little hoses chances are they are original to.

Also as side note I replaced all the vacuum pods for the climate control main flap etc,etc makes a world of difference having the air come out we're it's supposed too.


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