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Recently my E320 coupe started blowing hot air through the dash vents despite the dials not being set to hot. I rarely touch the temp dials. I did a bit of research and found the duovalve might be the problem, so I got a used (confirmed working) one and swappped it in, but there was no change.

Upon closer investigation I found that when I first drive the car each day the vents blow air temp air and continue to do so until I turn off the engine. However once I start again they now blow hot (and that means not warm, but very hot). The duovalve supposedly has a default position of open (presumably so that if it fails you can keep warm in chilly Europe), but I'm in warm Australia so I really need the opposite. What I suspect is that when I shut down after driving the valve opens and hot water flows into the system which then translates to hot air when I restart. Does this make sense? If it does can anybody offer a possible solution?
 

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Easiest solution would to just close the center vent with the dial.

Beyond, I think you are looking at either bad HVAC control unit (knob & dial assembly) and/or vacuum pod or vacuum distribution block problem.

I'd try replacing the HVAC control unit first and observe any change(s).
 

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Discussion Starter #4
Easiest solution would to just close the center vent with the dial.

Beyond, I think you are looking at either bad HVAC control unit (knob & dial assembly) and/or vacuum pod or vacuum distribution block problem.

I'd try replacing the HVAC control unit first and observe any change(s).
I do close the vents, but hot air still seeps through and with hands on the steering wheel right nearby you can feel it (and it's annoying). Will check out the HVAC and see what I can come up with.
 

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If the duovlave is ok, I think the suspect would be the central vacuum pod which can not adjust itself to the proper position, causing the hot air venting through the central air duct.
There are two pods controlling the opening position, mixing the blown-in and interior air circulation.
If pods got stuck or malfunctioned, temp cannot be adjusted properly.
 

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It seems to me that the heater monovalve could be an issue here. When the heater monovalve, located in front of the battery looking like a vertical cylinder, fails, the default of the valve is open, letting the hot coolant flow through the heater core when the engine is warm, making the air hot, no matter what temperature setting you have. This monovalve is controlled by the temperature control unit to modulate the hot coolant flow through the heater core to regular cabin temperature.

Check out the post by evansdave, who provided an excellent writeup on how to check and repair the monovalve.

I assume that your initial drive in the morning is pretty short. If you drive long enough, do you get hot air too eventually? If so, the heater core coolant valve likely is at issue here. Unplug the connector and apply a 12V to it, you should hear a click. If not, the solenoid of the monovalve is bad. Again, read the post by evansdave for details.

Finally, sbaert, while trying to help, often suggests replacing parts as solutions, no diagnosis first and no cost spared. The suggestion of replacing the temperature control unit is such a typical example. This unit is an awfully expensive unit unless you can source a good used part.

jftu105
 

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It seems to me that the heater monovalve could be an issue here....
The OP has the semi-automatic HVAC system that was not offered on US spec cars. The semi-automatic system uses a duovalve. There is no monovalve.

So if you're going to get "Scientific" about it, at least break out the right book.

And never did I suggest to get a brand new replacement. Just a donor as HVAC panel replacement is a LOT quicker & easier than pod replacement.
 

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Discussion Starter #9
Problem solved at last! It was the duovalve that was the culprit. I purchased a second hand duovalve, but that made no difference. I swapped in a friend's spare climate control module with similarly no change. Checked all the various sensors and vaccum pods with no issues being apparent. People kept telling me it was most likely the duovalve but having a replacement make no difference I was reluctant to shell out over $500 for a new one in the hope that would solve things without any real guarentee it would.

Then I found a repair kit on the internet for $70 shipped to Australia. Seemed like it was worth a go, so I ordered it.

It is basically a set of O rings, a custom built seal and 2 plungers. I had it installed and my heating/aircon now works as it should. When the replacement duovalve was disassembled on of the plungers had a broken stem, similar to the original one. These plungers are made of plastic and clearly go brittle over the years, no doubt exacerbated by heat and fluid they have to put up with. The new kit has metal plungers (looks like brass) so hopefully will last forever.

Upon reflection I should have disassembled the replacement duovalve before installing it because that would alerted me to the fact it was actually no better than my original duovalve. But all's well that ends well.
 

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Pottering thats a good post :smile.With information that will help other members that dont know that the duo valve m , is repairable . Good and glad you have the heater up and running :smile:smile:smile
 

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Pottering

Nice post. When you can, post up a link to the source you used for the repair kit. I'm certain others will find it useful.

Jayare
 

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Discussion Starter #14 (Edited)

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I thought John went down this road once and got a kit that had some plunger parts that wouldn't fit/apply to his valve?

Anyway, this is great news....because it's been new or JY a long time.

Kevin
 

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No I had the mono valve issue, one of our members had the rubber seal sitting on his shelf that I needed and sent it to me for free! what a great forum! thanks again,still working!
 

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I got my repair kit from a company in Taiwan - BENZ W124 W220 R170 W215 W140 Heater control Valve Repair Kit -01 - Hong Mei Trading Co., Ltd

They make a number of kits for differing duovalves. I dealt directly with them and they were vey helpful and even sent me an email when they posted my kit out with a picture of the labelled airmail package.
I've emailed these guys, prompt reply seems good. But by the time I've paid shipping etc the kit will be £50. I can get a new valve for £130 so I may just go the new valve route.
 
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