I have an 82 300D and have been having heating issues for a while now. It initially started when after driving for 20-25 minutes or so which I later figured out was just a matter of getting onto the interstate and driving faster than 65 mph the heat would go full cold. If I didn't drive faster than 60-65 the heat worked normal. It was only after going faster would it turn cold. Did some research and thought would rebuild the mono valve. After doing that the problem got worse. Now the heat turns cold by about 20 MPH. If I shut the car off and the restart it the heat returns until I go above 20. Once the car if fully up to heat sometimes the heat will continue to work good till I get above 60 again. I thought well maybe got bad mono valve so had them send me another and it is doing the same thing. I have checked the CCU for bad soldiers and it looks good. Even taking the wires off the mono valve which should give full heat all the time still does the same thing. Lose heat by 20 MPH till you shut car off. Any help would be greatly appreciated.

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1982 300D

I will still vote that it's the monovalve. The modern-day replacement MTC (Indian) versions are known not to be the best in the world. Perhaps your fix lies in the post below. I had the same issue you mention when i swapped to an MTC, so I went back to my original monovalve last year and, until I had to blow out the heater core recently, it worked fine, and is now again.

http://www.peachparts.com/shopforum/diesel-discussion/381589-mtc-monovalve-no-heat-issue-solution.html

Thanks for the info. That was my exact thoughts to just wasn't sure how to adjust it any to help. Great piece of info.

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1982 300D

If anyone needs assistance with fixing their heating system, specifically the heater valve; check out the below link to our site's DIY tech article for a guide on fixing it.

Heater Valve Replacement


-Dmitry

Just an update. I installed an o ring about the same size as the one described in the write up. With the first MTC valve I had received the heat worked until I would get up to about 50 MPH. So switched it with another MTC valve and it seems to be holding a little better. Today while driving it would sometimes lose heat around 73 mph or so but held well most of the drive.


The DIY write up mentioned by Dmitry basically suggests replacing the whole heater valve assembly at a cost of about $425. Not sure wanting to spend that much unless absolutely necessary. Not sure what the plastic housing has to do with this problem but am curious if anyone has any input on the Mercedes brand mono valve itself. Its 38 would try that if others have had good luck with them holding up and not needing an O ring. Thanks for any input.

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1982 300D

This is what I'm going to be doing this spring. No more monovalve.

http://www.peachparts.com/shopforum/diesel-discussion/361738-83-300d-no-heat-highway-speed-3.html#post3601285

http://www.ebay.com/itm/2-Way-Brass-Solenoid-Valve-NBR-Seals-DIN-Gas-Air-Water-Oil-Electric-Pneumatic-/351051293567?ssPageName=STRK:MESE:IT&rmvSB=true

Look back at the bottom of the first page of the thread to see photos by jaybutter. This is his rig and idea. I haven't tried it. One thing I would like to know is if I can use the existing power to the monovalve directly over to this rig, or if I should wire it through a relay. I don't want to burn out my climate control mystery box by drawing too many amps through a new rig. I don't know if the mystery box is responsible for direct power to the monovalve or if there is a relay involved anyway.

Haven't heard from jaybutter in a while. Don't know how it's working...

Jim

I had a similar problem earlier this year/late last year. I ended up replacing my aux water pump, my original one was beyond fried. I found one at PaP for $10 from a Durango (2002 I think) and it was a direct swap, harness and all. Works great now!

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1985 Mercedes 300D turbo, w123, 722.3, 315K miles
1987 Mazda B2200, Dana 44, 283 v8, th350, np203

Hi I haven't used the forum for so long I can't remember how to post a question. I was having a look at the blower motor which died and noticed this black hose coming from the right side(passenger) of it across to behind the centre console. I don't know where it plugs in behind the centre console so I would love to hear from anyone that might know. I am guessing this is where it goes because it seems to run straight and its that length. It is an air hose for sure. Thanks a bunch! Sorry if I am interrupting this thread... johann

Re the monovalve - I had the same problem with the MTC valve. No heat over about 10 mph. I think that because the diaphragm did not fold up neat like the original, the increased water flow at more than an idle sucked the valve shut. I got a genuine Mercedes one for about $70 from my almost local Mercedes dealer, and it works perfectly. Could see a huge difference in how the diaphragm material tucked away when the valve was in an open position. My impression was that probably all of the aftermarket ones are made by whoever makes the MTC.

Replacing the heater valve body won't do any good without a good valve. I would certainly try the original valve before trying to redesign the whole system.

RE black hose - If the car is a W123 (I'm not sure ab out an SD) and if it's a half inch or so thick, not a vacuum line, it should be attached to the plastic line from the sensor under the rear view mirror on the top of the dash. There was a foam pipe about 4 - 6 inches long connecting the two plastic pipes that may have rotted away. I used foam water pipe insulation on mine, and my climate control started working as it should.

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1985 300TD, 367,000 daily driver. EGR, ARV, trap deleted