heater core duo valve problems - does it always trigger a code? - Mercedes-Benz Forum
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#1 (permalink) Old 08-07-2010, 09:47 AM
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heater core duo valve problems - does it always trigger a code?

I've been reading through the archives/stickies/etc. on diagnosing the HOT-only AC/vent problems in my '96 E320 and I'm gathering the primary issues with my car are low ref. & a stuck duo valve. I've got other error codes but I believe none of the others would actually disable the AC completely. (interestingly enough, the compressor does kick on and defrost sucks the windows clear in seconds...but you can't stand the heat it pours out...)

The code that I do NOT have is a duo valve error. Is it common to have a stuck valve but no codes for it?
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#2 (permalink) Old 08-07-2010, 10:29 AM
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I don't think you'll get a code for a duovalve. Certainly not if it's stuck. There's no feedback to the computer to tell the valve position, so it won't know.
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#3 (permalink) Old 08-07-2010, 12:44 PM
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Noramlly, a duo valve will not give a CEL unless it fails electrically, and sticking valves are most often a mechanical problem and not a electrical'solenoid problem. Grainger sells a #10 treu magnetic torx driver for under $10, really-really helps in not losing a duo valve screw.

Please note that it is common for somewhat low refridgerant levels to cause uneven temp left and right from the dash as well.

Please do a forum search and do the climate control diagnostic - with 1that 96 shows low, I would recommend a evac/fill.
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#4 (permalink) Old 08-07-2010, 09:47 PM
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thanks. I figured as much. Having watched friends blend trouble codes with mechanical sense, this was what I expected, but thought I'd ask. I'm happy to report that with the help of the sticky threads on AC & the duovalve, I was able to piece together the information I needed to get cold air blowing in the '96 E320. Hopefully there's no huge leak in the system, but I guess I'll know if that's the case soon enough. Otherwise, I have some issues like a bad cabin temp sensor (where is it located, btw?) and sun sensor but nothing that I can't live with (or, more importantly, that my wife can't live with since it's her car)

thanks
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#5 (permalink) Old 08-08-2010, 07:54 PM
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Much has been said about the duovalve and some of the extrapolations are dubious.

When you go through AAC control for faults you may see 1417 and 1418. They are the right and left duovalve faults.

Most think that the valve is located in the inlet side of the heater coolant water. That is not correct. The valve is on the return side of the heater coolant line.

When you are in AC mode(Low cabin temp requirement),when you start the car voltage is sent to the solenoids to pull down the valve piston and thereby closing the return off so no coolant enters the heating system. If there is a short or if the valve gets stuck and can't fully close a fault is generated and because it is not apart of the emissions circuit the CEL will not be activated.

If you do not have the fault codes-----don't waste your time on the duo valve. Look elsewhere and by going through the ACC and reading the actual values will you know what direction you should follow. Good luck
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#6 (permalink) Old 08-08-2010, 11:07 PM
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Nobody said anything about

Cel codes. And most often a stuck duo valve does not throw an A/c diagnostic code.
Often a simple tap will free a stuck valve and if not the diy cleaning procedure will fix it.
Rare to see a code. Even rarer than a leaking evaperator
or seeing an error code for a bad evap temp sensor.
Why you think that the hundreds (or more) of people that have solved distinct variations in side to side temperatures and had no error codes for duo valve faults did so was a waste of time is what I find dubious.
But hey that's me.


OP you don't need the sun sensor and your cabin ambient sensor is in the little round vented unit up by the rear view mirror.


00-02 drivers side self dimming heated mirror
17mm special spark boot tool
O2 plug and play sensors
16mm spring loaded swivel spark socket
trans dipstick and magnet


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#7 (permalink) Old 08-09-2010, 01:50 AM
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I was in a rush on my earlier post.

I believe a climate control code (not CEL) "could" trigger if a solenoid fails - meaning incomplete circuit. I too have nver seen that happen.

Again, nothing can trigger from a stuck/partial valve - thats mechanical, not electrical.

That's my 2 cents..
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#8 (permalink) Old 08-09-2010, 06:36 AM
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Quote:
Originally Posted by fabbrisd1 View Post
I was in a rush on my earlier post.

I believe a climate control code (not CEL) "could" trigger if a solenoid fails - meaning incomplete circuit. I too have never seen that happen.

Again, nothing can trigger from a stuck/partial valve - thats mechanical, not electrical.

That's my 2 cents..

I agree with that, however MB offers many diagnostic tools with their cars and before I start to tap or replace a part, I trust the system to determine an approach.

With the duovalve you have two approaches, the AC fault codes and AC test 3 and 4 will tell you if either valve is open or closed. If open the coolent temp will be much higher on one or both sides. Run the test and post the data.
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#9 (permalink) Old 09-27-2010, 06:16 PM
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try this one as you seem to know what you are talking about. On my 1998 E300TD the AC works ok when the car is first started. However is I stop and start after heating up the side vents, both usually, will blow hot, while the center vents blow cool. Sometimes both side vents are hot sometimes only the drivers side. If I wait long enough it "sometimes" slowely corrects itself and starts blowing cold. I can switch to Defrost for a few seconds, allow the vents to shift and then go back to the normal AC postion and this sometime fixes the problem.

I do not think it is the duo valve as both side vents are involved and duo valve would be one side only as I understand it.

Where do I look next. Are the vent damper vacumn controlled. Is there a diagram somewhere. this is a desiel, and on my older diesel vacumn is always the answer to these type problems.

I did open the duo valve and looked clean except that stim seems to be slugish moving inside the solinod. also ran diagnoltics. below.

Here are reading with engine NOT started:Outside Temp 74
1=24, 2=23, 3=30, 4=29 , 5-31, 6 = 32, 7=05, 9=85 Engine at rest

Next: Engine running Outside temp 74 = inside temp set at 72 both sides after 3 min
1=27, 2=26, 3=39, 4=37, 5=39, 6=54, 7=23, 8 = 71 All vent not cold yet.


I then Shifted temp to 68 and noticed system engaged vents moved, center and driver side blowing cold but passengers side blowing hot.

Had 10 minutue commute ( welcome to SC) with everying blowing cool except passenger side vent. Outside temp = 75

1=27, 2=25, 3=23, 4=68, 5=28, 6=80, 7=25, 7=77

got any suggestion. should I go back and lubricate duo valve stem. ??

Still think vent flaps are involved... but will try anything now. have not added r134 as that has not been suggested and center vent blow very cold so do not think freron is problem.
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#10 (permalink) Old 09-28-2010, 02:52 AM
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Quote:
Originally Posted by shrumsjs View Post
try this one as you seem to know what you are talking about. On my 1998 E300TD the AC works ok when the car is first started. However is I stop and start after heating up the side vents, both usually, will blow hot, while the center vents blow cool. Sometimes both side vents are hot sometimes only the drivers side. If I wait long enough it "sometimes" slowely corrects itself and starts blowing cold. I can switch to Defrost for a few seconds, allow the vents to shift and then go back to the normal AC postion and this sometime fixes the problem.

I do not think it is the duo valve as both side vents are involved and duo valve would be one side only as I understand it.

Where do I look next. Are the vent damper vacumn controlled. Is there a diagram somewhere. this is a desiel, and on my older diesel vacumn is always the answer to these type problems.

I did open the duo valve and looked clean except that stim seems to be slugish moving inside the solinod. also ran diagnoltics. below.

Here are reading with engine NOT started:Outside Temp 74
1=24, 2=23, 3=30, 4=29 , 5-31, 6 = 32, 7=05, 9=85 Engine at rest

Next: Engine running Outside temp 74 = inside temp set at 72 both sides after 3 min
1=27, 2=26, 3=39, 4=37, 5=39, 6=54, 7=23, 8 = 71 All vent not cold yet.


I then Shifted temp to 68 and noticed system engaged vents moved, center and driver side blowing cold but passengers side blowing hot.

Had 10 minutue commute ( welcome to SC) with everying blowing cool except passenger side vent. Outside temp = 75

1=27, 2=25, 3=23, 4=68, 5=28, 6=80, 7=25, 7=77

got any suggestion. should I go back and lubricate duo valve stem. ??

Still think vent flaps are involved... but will try anything now. have not added r134 as that has not been suggested and center vent blow very cold so do not think freron is problem.
My problem is just about the same but with the drivers side not blowing cold air, it use to happen all the time until i got a refrigerant recharge as i did not do this ever since i've purchased the car. It cooled it down for a good 2 months or so but i've been experiencing it again every so often. Every time was during the day and it would last a good 30 minutes until getting back to the regular temp. My 2 cents would have to be not to worry about recharging your system if you feel like its still in good condition. The AC guys told me i had a full charge already but i insisted on a full vacuum and recharge. Do you hear hissing from the center vents?
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