Only hot air from vents - Mercedes Benz SLK Forum
SLK R170 General Discussion SLK 200, SLK 230K, SLK 320, SLK32 AMG

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#1 Old 04-18-2013
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Only hot air from vents

99 SLK230:

Right now I am getting only hot air from all vents/both sides regardless of the setting that I have the controls on. I saw something about the "duo valve" needing to be cleaned or replaced so I was able to replace that with a known working unit very cheap and it still did not solve the problem. I'm not sure what else to look for at this point but as spring/summer nears here having to defrost or defog the windows with only hot air on full it gets very uncomfortable to drive unless you roll a window down. With the heavy rains we are getting here in the Midwest at the moment that is not a very "dry" option. On top of that I do not think the AC is kicking on which I assume is what the "EC" button is for. Regardless, with the settings all the way down to cold and just the fan blowing, I would think I would get outside temperature air instead of blast furnace hot.

I've done some searching here and on Google etc. but cannot find any good tips on what to look at or try next. Any ideas to point me in the right direction?


Thanks!
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#2 Old 04-18-2013
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Question, did it work last summer or you just purchased the car and do not know?
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#3 Old 04-19-2013
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Sorry, I just got the car in the winter from my dad and started going through the pains of getting everything in working order. It was stored in a garage for a year after my mom passed so neither of us know if it was working. I'm just crazy enough to try and do something to get this all working enough to keep or sell.
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#4 Old 04-19-2013
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Okay, good info. I would start with engine running have someone to turn on the AC as you watch the AC compressor to see if the clutch on it works. If it does let it run a few minutes and feel the lines near the firewall are they getting cool. Is one cool and one hot signs of low coolant. Both cool signs of control doors not working.
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#5 Old 04-19-2013
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Thank you, I will certainly check that when I get a chance this weekend. However, why would the car not at least get outside temp air through the vents when both dials are set to cold whether the EC button is on or not? Perhaps I just don't understand how the HVAC systems work in these cars.
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#6 Old 04-19-2013
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It may be recirculating the air.. check the switch selections. The air is also warmed somewhat by the heat of the engine near the air box/vent system. But if you're saying it is really warm, it may indeed be a valve/controller issue.

Follow Chuck's advice for a start (it wouldn't be a rarity for the coolant gas to leak out of a car of that age over a year). Is the light in the control unit going on and off when you select AC (now..bear with me...light on is AC off in a 170 I believe). A check of the fuses may be in order as well.


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#7 Old 04-19-2013
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As I described before it is always "blast furnace" hot regardless of the setting on the temp dials or if the "EC" button is on or off, the fan is blowing or just the vents are open or whatever selection it is on for the different zones (ie upper, dash, feet or some combo of those). The only time I can get anything but hot air is to turn the fan dial all the way off at which point you hear something closing which I assume is the main vent. Of course, at that point, there is no air flow other than opening a window. The heater or dual valve was replaced with a tested and working part so I'm fairly confident we can rule that out.

Thanks for everything so far.
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#8 Old 04-19-2013
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Also to mention a few other things, there is a recirc button below the EC button that seems to work fine but then it is just recirculating the same ridiculous heat without the outside air mixed in making it even worse.

The EC seems to work but I'm not sure yet until I check this weekend where the clutch for the AC pulley is engaging.

I believe I have found all the locations of fuses and checked them all so nothing seems blown unless you can tell me there are other locations for relevant fuses other than on the side of the drivers side dash and under the hood on the driver's side. Oh there was also a row of fuses in that larger box that houses the K40 that I checked also but again they all seemed to be in working order. Aside from the #9 fuse for the alarm and the PSE pump which is future me's problem (please don't tell me that is somehow related to the air issue!).
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#9 Old 04-19-2013
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Sure sounds like a duovalve problem. I suppose it could be the electrical signal going to the duovalve. Would you be able to figure out how to check that? The temperature of the air and the air conditioning have nothing to do with each other. You can have the ac on and blow hot air (most cars do this automatically when the defrost is on). Having full heat means either the duovalve is stuck, or the signal is telling it to let full flow of the coolant.

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#10 Old 04-22-2013
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Ok, I confirmed that the clutch is not engaging on the AC compressor motor when the "EC" button is pressed so there's a problem right there. I have not been able to test the connection to the Duo Valve to see if there is any juice flowing to it.

How much of this stuff is actually connected to that area under the big plastic cover on the passenger side? That's really the only area I haven't messed with because it seems a little intimidating after you remove the cover. Other than that I have been through every fuse in the other two locations several times and they all appear to be fine.
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#11 Old 04-22-2013
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I think its a problem with a valve which shuts of coolant around the heater matrix. the duo valve does this but not sure if there is another valve somewhere.
Just to confirm with the EC Button ON (red light on) the air con is OFF. Also the compressor won't kick in if the refrigerant level is low. If the aircon is working one of the pipes from the condenser should be ice cold, the other warm. On a UK car they are on the top n/side in the engine bay.
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#12 Old 04-22-2013
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Helllo there!

Had the same question just 2 weeks ago when our weather turned slightly warmer.
This is what I found out about the "EC" button.... from the manual found online.

The "EC" button is a economy mode switch and the air-conditioning compressor will not engage if it's switched on, i.e. red light is on.

Dial all your dials to blue, switch off the "EC" mode (red light off) and you should get some cold air. Else, the refrigerant gas level is low and you'll have to top it up at a garage...

Hope this clarifies...
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#13 Old 04-22-2013
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#14 Old 04-22-2013
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Does your duovalve have a connector with 4 wires? If so its 2 wires for each side of he heating. Connect a multimeter between the 1st two and check the voltage. With the heater control set to hot it should show zero volts and as you turn the heater control to cold it should increase to 12v. If you are not getting the 12v the controller is probably at fault.
There's a good description of the duovalve here.
http://www.mbclub.co.uk/forums/engin...-duovalve.html
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#15 Old 04-22-2013
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Duo Valve

The duovalve controls the quantity of coolant flowing through the heater core.

At low heater setting the pushbutton control module connects the solenoids in the duovalves (A31y1 and A31y2) to ground, the solenoids pull in and block the coolant flow through the heater core separately for the left and right sides.

At the maximum temperature setting the pushbutton control module Interrupts the ground connection, current no longer flows through the solenoids and the coolant flow through the heater core is at its maximum.

In the regulated mode the pushbutton control module switches intermittently to ground.

The closing and opening times for the duovalves (A31y1 and A31y2) depend upon the deviation between the temperature measured by the passenger compartment sensor and the temperature set on the pushbutton control module.

Set your temps to lowest and measure to see if you have voltage across the duo valve electrical connections. At cold you should have voltage. At hot settings you should have none. This is a "fail safe" system, no voltage or dead valve = max heat.
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#16 Old 07-31-2014
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Hi, I know this is an old thread but my issue is similar. With a/c on full cold setting it blows out warm air. If I turn it to full heat it goes to full heat so that seems as it should but I'm not getting the cold stuff.

I have not yet tested for leaks but I was looking in the engine bay and touched the two silver bits on the top of the duovalve and they were red hot to touch. Is this normal? I've not seen any mention of whether they should be hot or cool to touch and I'm wondering if this could be a signature of a problem.

I may just need a re-gas, booked in on Monday to an aircon specialist but interested to know regarding how hot the top of the duovalve should be.

Thanks
Simon

Last edited by Gee23; 07-31-2014 at 07:00 PM. Reason: predictive text!
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