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Old 08-11-2003, 10:12 PM
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Join Date: Sep 2002
Location: Puerto Rico
Posts: 103
Question Anyone solve the problem of hot air fromside vents ona W124

I have search the forum for this problem and people offers many solutions but never see if the problem was fix it or not and what was the real cause of the problem. I have a W124 E320 1994 with this problem. First it fried the CCU and now with a new one the side vents blows hot air and the center cool air with hot air too. Please I need help on this matter. Thanks in advance

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Old 08-12-2003, 02:42 AM
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Join Date: Jun 2001
Location: San Francisco, CA
Posts: 3,097
I just helped a friend with a 91 190E 2.6 doing the same thing: Hot air out the side vents and windshield while with the AC on, cool air out the middle vents.

There are many posts regarding this and similiar problems. The first and easiest (which fixed the 190E mentioned above) was a cracked vacuum connector tubing, driver's side, right next to the fuse box area. Very obvious. Trader the rubber tubing for another piece and instantly cured it.

Split vacuum hose

Also check out:

More vacuum line problems



Keep us posted,

'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)
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Old 08-12-2003, 11:28 AM
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Join Date: May 2002
Location: Mass
Posts: 1,127
I wonder if it could be the mono valve.
I recently unplugged mine for debugging purposes, and found hot air blew through the defog and side vents even with the climate control totally off. Open may be its 'default' state.
I suppose you could test this theory by crimping off the heater hose temporarily. I have special pliers for this purpose, but vise-grips with suitable padding works too, particularly the needle-nose type.
Good luck.

1986 300E 5-Speed 240k mi.
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Old 08-12-2003, 11:36 AM
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Join Date: Jun 2000
Location: Plano, TX
Posts: 2,533
Open is the default state of the monovalve. Applying 12v to it causes it to close, blocking the flow of hot water.

- JimY
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Old 08-13-2003, 07:11 AM
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Join Date: May 2003
Posts: 20
It is your monovalve. There is no way to get hot air if the monovalve is closed. And there is no way to get cool air if the monovalve is open.
The vacuum system has nothing to do with temperature. Just with direction to the different air vents.
If you get hot air in the center vent, it is due that the vacuum system ignores that there is a problem with the monovalve and assumes there is no hot water in the heater radiator and tries to drop the cabin temp asking the fan for more and more speed (in "Auto" mode), opening the center vent that is designed to deliver only cool air (but the heater radiator has hot water, meaning hot air).
Next week i will post almost everything about monovalve, pictures and how to service it (yes, you can let them like new for less than $ 30. Besides, a little description how the heater system works. Meanwhile, disconnect the monovalve and apply 12 volts directly from the batterie (polarity is not important). You must hear and feel a notorius click.
If not, i'm shure you will not, try to give it a couple of knocks to release the piston inside the selenoid (the monovalve is a selenoid), fixed with the coil together due rust. Of course, if you get only cool air, the problem may be the same. A fixed piston inside the monovalve. Be patient. Next week i will post something can help you.
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Old 08-13-2003, 07:32 AM
Posts: n/a

It sounds as if you are talking about the monovalve as found in the 123 and 126 cars. acha is working on a 124 car. On the 124 car the heater valve is not a monovalve even though many folks call it that. It is a plastic heater valve that is not rebuildable.

I'm sure that your information will be helpful for those with a true monovalve though. The $30 repair kit is a five minute job to replace.

Have a great day,
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Old 08-13-2003, 08:28 AM
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Join Date: Sep 2002
Location: Puerto Rico
Posts: 103
In my car E320 94 the monovalve and the aux pump are together as one unit, I will check today this, I will let you know later, thanks for the reply.
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Old 08-13-2003, 03:48 PM
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Join Date: May 2003
Posts: 20
I'm talking about the valve in the csnow picture in this page, the same that is present in my CE 300-24, 1991
Is this the monovalve?
Unfortunately, not everywhere you can find the repair kit (what includes?). I'm in Chile, Southamerica and i'ts not in the market.
I had two problems with mine. First, the inner piston stop its work due rust. And, cause a little leak of coolant, not through the o-rings, i disassembled COMPLETELY the solenoid itself just to find a broken plastic core. I make this plastic core, roll new wire on it and replace all the o-rings, for less than $30. The cost of an original new valve in Chile is US$ 360. See you next week.

Last edited by Sprop; 08-13-2003 at 11:40 PM.
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Old 08-17-2003, 09:27 AM
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Join Date: Sep 2002
Location: Puerto Rico
Posts: 103
I check the monovalve and it is okay, it works, check the climate control on another car and it is working perfect, check the fuses, they are okay. Still hot air is coming out from the side vents. This give me no other choice than block the heater. For my luck I live in Puerto Rico where it is a tropical island (heat all year). Now my car is working fine. My question is , by blocking the heater it would affect the car in any way?? Thanks for your help
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Old 08-18-2003, 02:11 AM
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Join Date: May 2003
Posts: 20
Dear acha3:

How did you check the monovalve?
I ask you this because there is no way hot water comes inside (really suctioned through) the heater radiator if the monovalve recieves the "close" signal (12V) from the climate control and it works ok. All this, assuming all the hoses are well connected and no one is bypassing the MV.

Thinking about damages after blocking the heater radiator, the answer is no. Blocking the waterflow is exactly what the MV does when it is closed. Maybe you will accelerate the wear of the heater pump due the extra effort to suction hot water (coolant) with no results . Could be a good idea disconnect it. But i think too you can solve your heater problem before doing all this. Let me know.

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