Diagnosed an apparent air conditioning failure and found a torn diaphragm in
the monovalve. This apparently allowed almost full heat all the time thus overwhelming the ac.

My maintenance records show that I replaced the monovalve insert with an MTC unit in August of 2011, about 64000 miles ago. The use of quality, OE parts instead of Chinese copies is a frequent topic on this forum. This site sells both monovalve inserts. An MTC is 25.50 + shipping. The OE is $130.50. I find it impossible to justify spending five (5) times the cost for an OE monovalve. I bought another MTC monovalve on ebay for $18.80 including shipping.

I will look forward to being forced to replace this Chinese crap in another seven years and 60k miles.

The bad news is that diaphragm rupture is not the only possible failure mode with the MTC part. There are plenty of reports of them not working correctly right out of the box.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

My MTC was better than a bad stock valve but not by much. Then I took the good parts off the MTC and put them on the stock valve and that helped a bit. I then found out that Mercedes makes the valves again...this was for a 603. Works great. I don't remember but I want to say it was just shy of $40 for the OE part.

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Dan
2005 E320 CDI - 246k
1987 300SDL TD05-16g, Herlevi pump, Elbe manifold, 2.47 LSD - 213k
Past: 1987 300D - 264k

My MTC got stuck and I had to pound it out destroying the whole thing. The OEMs are unobtainable for W123s. I ended up going with another system. In addition to leaving me with no AC, the MTC caused a coolant leak that needed to be plugged in the system.

Dkr.

Several report the MTC innards don't have enough force to close the valve against the line pressure. In my W123's, I have found that the climate box doesn't shut the mono-valve unless clicked to the "max cooling" dial, in which case the push-button box directly grounds the monovalve drive wire which does close it. Can't say if MTC innards or not since I have installed both those and guts from junkyard cars. I never paid $$$ for the M-B brand part. Anyway, proves the problem can be in the TemperatureReglar box.

For those who want to test, one way is to put a clamp on the heater hose. If your AC suction tube is cold, lack of cabin cooling could be due to the heater overwhelming it.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Replaced my monovalve insert today with the cheap-ass MTC unit.

Air conditioning will now freeze my cojones. Hope it lasts a while. If it doesn't, I'll just replace it with another MTC in a few years. I can do that five times before approaching the cost of an OE monovalve which by the way failed 7 years ago.

We'll be watching....... .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

The MTC part typically has more problem staying open than it has remaining closed. The solenoid that keeps it closed is external to the insert. The spring that keeps it open (or doesn't keep it open) is internal.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

? Is this because of the electromagnetic coil or something else ? .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Good luck. I had one last me about a week.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge

Mercedes OEM monovalve was $39 just 4 months ago. Prices on some parts are absolutely skyrocketing because the w123s are entering vintage status and being older then 30 years. I plan to buy a stock pile of all parts before they go up more in the next few years.