Howto: Duovalve maintainance

Parrot of Doom

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Right, as I'm having problems with my aircon (blowing intermittently hot on driver's side) I thought I'd check out the duovalve to see if it was working correctly.

Feel free to point out any stupid errors I've made, but this is what I did:


1) The duovalve from what I understand controls the flow of coolant to the cabin. Obviously you have separate passenger/driver heat controls, so thats why you have 2 valves. On the W210, its located near the brake fluid vessel, offside at the back of the engine bay:

162901302_8824f87b85.jpg


See the two grey cylinders? Thats the duovalve.

2) Start by making sure the coolant isn't too hot. The last thing you want is to open it up and roasting hot coolant comes shooting up your arm, forcing a trip to hospital for burns! First, unplug the power connector. Its a bit stiff, so you may have a bit of a struggle removing it. Its clamped on either side, I used my fingernails to pull the plastic clamps back so the plug could slide free.

163106081_3cf23a1c02.jpg


3) When its reasonably cool, take a torx bit and unscrew the 5 torx bolts that hold the metal case together:

163106079_c2689ee4f5.jpg


Don't worry about losing the bolts in the engine bay. They're very long, the reason will be evident shortly:

163106083_59eb624bb3.jpg


4) Once the bolts have been removed, you can remove the two covers. They just slide off very easily:

163106084_7abdfc4221.jpg


You can now see 2 orange cylinders, not unlike cotton bobs. These are electrical coils, and they're what makes the pistons in the valve move in and out (its the same principle as a hifi speaker). I presume the position of the valve is controlled directly by the position of the temperature dial in the cabin. The two blue components are capacitors, very very cheap and easy to replace. Nothing exotic.

163106086_4a56043ec7.jpg
 
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Parrot of Doom

Parrot of Doom

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5) Obviously there are 4 large pipes into the duovalve assembly. 2 for water heading in, 2 for water heading out. The duovalve splits in two which makes sense if you have a blockage that needs clearing. Its fairly easy to split, there are no retaining plastic lugs, just plastic guides.

163106087_f4d58d117a.jpg


READ THIS BIT CAREFULLY BEFORE YOU SPLIT THE VALVE

At this point, I cannot stress how important it is to have a bit of help on hand. At the very least, a bit of cardboard sheet to stop things falling down into the engine bay.

163108162_6b048f3a70.jpg


When you lift the upper half of the duovalve away from the lower half, there are two small valves that are not held in by anything. So what will happen is that if you're not aware they're there, like me, they'll fall out and down into the engine bay! If this happens, to retrieve them either get a long screwdriver with a blob of grease on the end, or remove the cradle that holds the duovalve in place by undoing the bolts on other side of the firewall. You will have the devil's own job getting them out. They're brass and plastic and therefore not magnetic. Its easy to do with a grabber, but guess who didn't have one.... :(

Anyhow once I'd retrieved them, this is what they looked like:

163108159_9f73ed79c7.jpg


6) Theres not much else to do here. Check all the pipes and the valve body, make sure there are no obvious blockages. Give everything a wipe, remove any rust and crud. If you want to remove the rubber pipes its easy enough, just take a pair of mole grips and squeeze the clamps open, slide the clamp back away from the duovalve, then put a large flat bladed screwdriver in the end of the hose to free it before you tug it off the duovalve body. Might be an idea to get the help of a friend here since putting the clips back on can be tricky, there are a few bits of engine bay that get in the way.

7) I used an anti-fling spray oil to lubricate the pistons on the valve. The pistons are the only moving part visible unless you were to take the entire thing to bits. Its all pressed metal and plastic so I didn't think it would be a good idea to do that, especially when dealing with water and seals. Fortunately both pistons moved pretty freely on mine, they do make a slight grating sound as they move up and down, and they are spring-assisted, but I'm presuming thats normal.

8) To put the duovalve back together, the easiest thing is to put the two loose valves in the lower half on the duovalve body, and very carefully slide the top half of the body down so that the valves mate with the pistons correctly.

9) The rest of reassembly is the reverse of the above method. The power connector only fits one way, but even if you manage to connect it the wrong way all that should happen is that the heating controls in the cabin will be crossed over.

10) Use a hose and clean tapwater to carefully wash around the duovalve and any places that antifreeze spilled over. Its icky stuff and doesn't really dry too well, you don't really want it gumming up your engine bay. Take care not to get water down the numerous electrical connections that are in that area of the engine bay.
 

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Once again, very well done on this expensive piece of kit, and a regular source of problems in our cars.

malcolm
 

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I see that you have not, and that is to lubricate the solinoid, that should never be done just remove any corrosion if any but always assemble dry.
Excuse me adding this to your thread.

malcolm
 

C220GJS

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Thank you PoD, I've been intending taking mine apart, for no other reason than idle curiosity and that it is 11years old, this will make it so much easier.
Cheers, Geo.:D :D :D
 

sidmon

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I've attempted to complete this procedure but I have found one of the valves has snapped. Does anyone know if I can obtain the valves seperately? And if so where from?
 

benson09

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Thanks for the info

Thanks for the great info about the duo valves the pictures were a big help in understanding there use.
After reading your post could it be the recirculating pump that is the problem, if the water is not flowing on the heating side?
Would you happen to know if it's fused?
At least i now know what the valves are for.
Will try to get a look at it over the week-end
 

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We had one this week where it was the pump
 

bil

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hi, firstly that is a great detail explanation with pix. just want to know if it's similar on the w208? if i may ask just so i'm clear, you are taking apart the duovalve and cleaning the solinoid and putting it back together. how long did it take you from start to finish? i'm very tempted to do it myself, problem is i think i'm more mechanically inclines than i really am, usually can get it apart but cant piece it back together. my local mechanic said he can do it for 50 pounds so should i leave it to the pro's?
 

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Too hard a question for me,, they are all basically the same, just take car not to drop anything, easier to do off the car
 

bil

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thank you, guess i'd better part with my $$ and not have to worry about missing pieces from the duovalve.
 

bigshineybike

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I had a problem with my duo valves. I took them apart just for a look I didnt really think they were the trouble.
All looked good in side coils were rading properly and all the bits moved nicely for a 12 year old car.
Putting it back together though was a palaver,two of the fixing screws which are just like wood screws snapped under easy screwdriver torque.
fixing that led to a whole set of chalenges.
be warned my screws were only a little bit rusty.
I ended up punching the broken bits out then drilling the plastic housing and fitting extra long m3 screws and nuts.
 

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Strange how a job can go so pear shaped :(
 

Alex Crow

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i must add further duovalve information PLEASE READ.

a very common symptom on duovalve controlled cars is heating on all the time. there are a couple of common causes, one - sticking valves - has been detailed above.

the other problem is even more common. valves working sometimes, but when failing both sides always run hot at the same time. the control panel switches the earths to regulate heating and also monitors the valve current consumption. sometimes a solenoid suffers an intermittent or permanent breakdown in coil insulation and goes partially short circuit. this raises the valves combined current consumption beyond the control units threshold level, causing it to cease switching the solenoids - result, hot air both sides.

to test the easy way, unplug the three pin connector and test for resistance between pins 1+2, then pins 2+3. the measurement should be either 16 or 12 ohms, depending on model. if one reads eg 6 ohms and the other 12 ohms you have found your problem!!

remember that this usually happens intermittently so ideally you will test when the symptoms are current.

good luck and enjoy.
 

Number_Cruncher

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AP - is this monitoring of current something that is only true for W210s, or, is it also the case for earlier cars?
 

Alex Crow

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it goes for 126, 124, 201, 202, 210, 129, 170, 140 and probably loads more i forget about!! although there is a duovalve fuse, the controllers invariably have this circuitry saving feature.
 

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The circuitry for these controllers are only low current devices, should the loading become to high, then the unit will just shut down, with out damage in most cases. Sure the odd one can or will fail if persistently abused.

There are various ways of pulsing,,I must scope one up and look at the waveform, to see what they do,,heated seats work in the same way
 


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