An AC shop may not be the best place to take it if they are not familiar with the older mb ccu operation.
You seem to be sharp enough to solve this problem with a little patients and research.
Do you know for sure the compressor is engaging?
At idle, have someone turn the ac on and off. Listen under the hood and you should hear the "click" of the ac compressor clutch engaging. At that point you can locate the hoses and see if it is cooling. If so, then the refridgeration part of the ac system is working. (the freon stuff)
Up in the dash you have ducting that the air from the blower blows thru.
There are flaps that 'steer' the air thru different parts. One part being the 'evaporator'. This is the part of the AC that cools the air. (or takes the heat away)
There is another part , 'the heater core' , that heats the air with engine coolant. In ac mode you do not want coolant flowing thru this. That is the duty of the monovalve mentioned in a previous post. I am not familiar with your "W116" car so it may not have a monovalve. But the basic system operation will be the same.
It could be the ccu is not steering the air properly. Its role is to send a signal to vacuum valves that send vaccum thru hoses to the flaps to steer the air thru the proper ductwork.
It could be something as simple as a vacuum hose slipped off.
Does the air come out of the proper vents when called for?

I hope my explanation makes sense. Its late, hot and my back is killing me so I just had a good stiff drink.
Might explain the rambling...

Good luck.

I had the same when I bought my 116 sd and ended up putting in a used servo(from a car on which it worked). then it worked somewhat, yet not really. it would stay on hot. that system is not ment to be working for longer than 5 years I guess. pretty weird engineering. so I put this valve inbetween the hot hose and that 'unit' ... voila; manual heat. pretty simple hack, and cheap. when shut, your air should work. mine did, for a month or so after having it recharged for $30 in mexico...
then at some point it leaked out.
in winter just open the valve.

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The "manual monovalve".

I have a 80 300D I "manualized" the HVAC. There's a switch for every air pod (defrost, floor, vent, recirculate...) one for the ac compressor, one for the heat valve... Basic.
I bet I pulled 50lbs worth of wire and hoses from the car when I took out the servo system the PO had botched up.

Someone suggested a good mechanic in Tarzana, which is great, but is 2000 miles away.... but thanks.

Regarding the above, I am fairly confident the compressor engages, yes. Litening under the hood is, as you know, a noisy affair, and didn't tell me much of anything. I could definitely hear the 'whoomp' of what I assume to be the compressor starting up, and last year after the $400 freon charge, compressor service, first go at this, the mechanic said the coolant system was working just fine, but that that was not the problem.

But you are spot on when you ask which vents work. The side vents are NOT dispensing air, as I believe they're supposed to in a/c mode. He told me "a door is stuck" and that it would be $600 more to get up in the dash and pry the door open. When I toggle the CCU from temp to temp and engage the a/c, the air flow changes neither path nor temperature, though I definitely do feel the compressor draw power. Is this a faulty ccu, though, or a faulty vacuum tubes? Could it even BE solved with the manual monovalve?

is the hose shown one of the two I'm supposed to touch to see temp? So if I follow that one back under to the compressor, I should be able to locate the one that's meant to be cold? Will I be able to locate these hoses from TOP of hood or UNDER car??

this looks like my setup, so this is the servo and I do NOT have monovalve? Correct?

I'd love to try this valve and will do so tomorrow if y'all think it'll relieve some of the misery.

I have a 300SD with a broken A/C (none of my cars has working A/C and we live in FL- go figure). It just isn't a priority for me so, I can't be of much help, except for this. You do NOT have a monovalve. Concentrate on the evil servo. It seems to me that money wasted on the servo would be better spent elsewhere. I would look into "manualizing" the system.

Rick

__________________
80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)

Can't resist this rather obvious comment. No offense meant.

If you are driving a 26 year old Mercedes Benz, and depending on it for a daily driver, then you had better be financially able to deal with the inevitable breakdowns. You also must be somewhat Do It Yourself or these cars will break the bank. Upset with $400? 600? Chump change relative what can and will go wrong with these cars. It's kinda like being frustated that your grandmother can go jogging. Things simply wear out.

A/C went out in my '92 300E for the second time in 5 years. Compressor and associated parts and labor $1,200.00. There have been lots of other maintenance items. However, it's at 245K miles. I have put on 145K of those miles. It's been cheaper than a new car to operate, but the repair bills are not linear, like a car payment. They pop up and must be paid. There have been a lot of times in my life, where this would have be really bad budget news.

Steve

I am driving the car my husband left me when he died slowly of lung cancer at age 48, bankrupting us as a catastrophic illness will, and trying to keep up with the bills it incurs intelligently, rather than have mechanics rip me off, because this car is one of the few things that makes me feel his presence and makes me feel like I am still honoring something of his memory now that he is dust.

So, you know, excuse me for giving it all a shot. Wanna be in my shoes?

topanga-

I hope my wife does as much for my memory when I'm out of the picture. In my case (8 cars) its gonna be harder for her....

Again, I have decided to sweat it out on mine so I have not gotten into the guts of the 300SD Climate control. If your compressor if known to be working, it follows that you must be getting heat from the heater. I would find a way to shut down the heater for the summer- perhaps a manual inline valve from Home Depot spliced into the line before the heater? Then find a way to manually open vents to get the cool air in.

As for regulating the cool once you find it- open the windows that work.

Good luck

Rick

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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)

oh, you bet, I do have the window that works -- that's right, window singular-- as faaaaaaar down as it will go and have ordered a new window regulator. I'm just going to have to have it installed, I have made junk our of the right passenger door and have no idea how to get the mechanism out/another one in. I've really messed it up. I hate having to quit on something that seems so simple, but I have to.

So the a/c vent: I am back where I started. My mechanic told me last year it was a $600 job to find the stuck vent door and pry it open. Is it really that bad? Is it really that far up in there?? I just think, again, this one is going to turn out to be beyond my capabilities.

It seems to me if I go the Home Depot route, I am only halfway there, ie I've stopped the heat but not enabled the cool. So I should attack the cold air vent first, because if I can't get it working, there's no point messing with my heat lines, right? Or if I stop the heat with a manual monovalve, will I get ANY kind of cool air? NO? Not without the air diverted properly, right?

Please note this is a Hack.....Just to get you through the Heat Wave.

Simple - quick fix is to Wrap the heater hose coming out of the Servo with electrical tape to protect it... and clamp it off with some vise-grips just tight enough to stop the flow of coolant to the heater core.

If the compressor is turning on, you may already be getting an air flow through the cold evaporator. If not then you can always play around with the vacuum hoses under the dash or coming out of the Servo. Sucking on one of them to open the vent to allow flow through the evaporator and into the cabin.

But I would at least stop the flow of hot coolant, simple quick test.. and no need to cut the hose.

topanga-

If you can turn off the heat some way and the blower is working, I would think finding some of the flaps should not be too hard ( again I have NOT done this on a W116). I believe that if you remove the under-dash panel on the passenger side you should see the evaporator and some of the flaps. You may want to take the inside of the glove box out too to get a better look. That's where I would go. This might take about an hour. On a W123, I am sure I could get at least SOME airflow by propping some of these vents open. I am not quite so sure on a W116. Again if the compressor and blower works, you should be able to get some amount of cool this way, IMHO.

Rick

__________________
80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)

My SD is of the next generation, but I believe that your flaps are all vacuum driven. So there is an arm attached to a vacuum diaphram that operates this flap that is supposedly stuck. Either the vacuum diaphram (pod) is still trying to tug that flap open, or the arm or attach point is broken (happened on my '80 240D), or it is not really trying to move that flap. Personally, I've never seen a "stuck" flap. It was either broken, or not commanded to open. Trying to pry open the flap will most likely break something.

We really need a manual for this climate control. I'm pretty knowledgable regarding the w126, but not the w116. I will try to look at it when I get home.

Is there any member in the St. Louis area that can give topanga some assistance? I applaude her effort. She is learning a lot and we need to help her get through this.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

This is the auto-temp system, used by Chrysler until 1971

http://www.imperialclub.com/Repair/Air/index.htm

Info can be found here

__________________
80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)