OK folks, I have a 1982 Mercedes 300D turbo, 205K miles, and climate control issues. I have read most (if not all) of the entries but need some advice. Here goes:

I am getting hot air from the A/C vents. The car usually starts with cool air (A/C on) but then turns to hot shortly!

I did have a siezed auxilary water pump, and it has been replaced and now has a 1 A fuse. It is working properly (I think).

I have put in a new (inside) to the monovalve and I think it is working properly. The wire to the top of the monovalve is giving 13 volts when on max cool (engine running).

I have removed and checked the in-car temperature control sensor and found that it does give the correct resistance readings according to the MB service manuel on CD. (That little thing was a B**** to get back in and connected!) The hose from the guage to the blower motor is wrapped foam and has just been replaced.

I have cleaned the temperature control wheel and it seems to be working fine. The fan control is good, all buttons work as they are supposed to.

When set a max cool, I hear the compressor start but still get hot air. When first started in the morning, the air is cooled but warms up quickly.

I can block the aux pump hose to the heater core with vice grips and get cool air from the vents. I say cool because it runs in the mid to upper 60s from the vents; outside air is mid 90s. It should probably be colder but for now at least I know the compressor is running. I do not see bubbles in the sight glass - but probably would not recognize them if I did.

I think I have fried my CCU, but am reluctant to buy a new one. The local junk yard has none. I am considering taking it apart to check from burned areas and/or crack in the solder. However, since I don't know what I am doing I havn't done that part yet.

From these symptoms, does it appear that the CCU has been fried and if so, would trying to re-solder the unit be of any benefit? I maybe not, what else should I check.\?

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How old is your compressor and dryer? You may be have a slow bleed of freon .Buy a cheap a/c pressure gauge from a local parts store,I got one for $8.Then check if your charge is low.You can see by adding a can of a/c 134 with leak finder also. This will give you an indication were to start with the leak.

The compressor and dryer are about a year and a half old. They were installed by a specialized AC shop. I had the coolant checker earlier this summer by them, and was told it was fine.

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Have them check it again ,did you see them check it or did you leave it with them and came back to get it .Heres the problem ,if youve got a leaking system ,it only gets worse .This being on a warranty they might have just thrown some sealer in the a/c system to patch it , this buys them sometime til the warranty runs out.These type of compressors on a large amount are rebuilt ,with very high pressures being loaded on a rebuilt unit ,you can see the conflict here.Their work is complete as long as you have working a/c .Their is some investigation on your own you can do, if the system was leaking they would have added a dye like liquid to the system to find the leak.Now ,if you go to the compressor and look all around the hoses and compressor with a white towel you should find some residual dye from the freon leak.Hope this isnt the case but if it is ,your now aware.

Thanks Chasin..
Actually I did take it back to the repair shop this morning. The owner is a good guy, and I don't think he would intentionally mislead or try to cut corners like that.

Also, I tried a little experiment. I disconnected the aux pump by removing the NEW fuse, and put a pair of vice grips on the heater hose going to the monovalve. The result was much better cooling in the car. The vent air was down to the mid 50s, which is not too bad since the system has been converted to R-134. It also tells me that one definate problem is heated water leaking through the monovalve to the heater core. Wether the heated water "leak" is a bad magnet in the valve (a new inside part has been installed) or to a bad CCU. At the moment, I am leaning toward the CCU not sending the correct signat to the monovalve. Possibly due to either a fried system or just a broken solder point. If I am very lucky, it will be a broken solder point and I will be able to find and fix it.

If you have any other suggestions, I would like to hear them. I am a true novie when it comes to electrical stuff.

Remember that "Life is a learning experience. When you stop learning, ..."

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Today I took the CCU out and apart. By apart, I mean that any part that comes apart is apart. All of the solder joints I could see with a magnifying glass look solid and robust. I checked each solder joint that looked like it was connected to another with an Ohm meter and didn't find any discontinuity. There is a darker area on the green circut board, but the CCU is marked as "rebuilt" so the darker (burned?) area could be from a previous reapir. Is there something I am missing, some test for the CCU that I can do? All the parts are spread out on a sorta work-bench.

I also checked the electro-magnet circut on the monovalve. The circut tested as good from the two connections at the top, but it also gave a resistance of about 20 ohms. Is that the right amount or should there be no resistance?

This morning, I drove around with full-on air conditioning by clamping the heater hose exiting the aux pump. For Texas summers that would probably be OK, but I am somewhat retentive and would like this thing to work right (or as right as I can make it).

Assistance from anyone that knows about climate control stuff would be greatly appreciated. Thanks.

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I had similar head scratching on my 300SD and it turned out to be an intermittent short on the coil inside the monovalve assy. I think the insulating varnish was damaged. As it's a part internal to the monovalve assembly, voltage tests on the electrical connections coming in to the monovalve did not show any issues.

In my case I got a used part - exactly just the small piece I was after - from a parts supplier in Washington State that everyone loves to hate on this bulletin board! I have no wreckers with older Mercedes in my area of BC. Given the reliability of the climate system on these cars I am wondering how many weeks will pass before something else breaks

Thanks 300sd. I was considering getting that from a local junkyard because I can't find anything else that is wrong. The local here has about 10 or 12 older MBs, so if you need something I can try to find it. Just let me know before I go there. I have been often enough so they recognize me. :-)

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