also note that we are discussing a wagon here, it has got a bigger room to cool and more windows (more heat)

a sedan will cool down quicker.

Well the defrost vents stay open during A/C so I get a ton of airflow straight onto the windshield.

I checked all the vacuum pods with a Mityvac and they all hold vacuum (which they should since they are only a few years old). So if the vac pods are good, it has to be the vacuum bypass/control unit, OR the climate control unit. BUT in the winter my vacuum bypass/control unit it emits a high pitch frequent click that goes incessantly so I suspect its related.

I had a similar issue with my defrost vents open which is why I originally got the other switchover valve, but I tested it once I got it out and it was fine.....didn't find the real problem until the heater core exploded flooding my driver side foot well with hot coolant. long story short, once I had the dash out I discovered this part http://www.amazon.com/Mercedes-Vacuum-Element-Defroster-Nozzle/dp/B00D0H486W cracked. Ended up zip-tying the lever which moved the flap so that it is always closed. I mean really, when is the windshield ever frosted over in the southwest anyways? Once installed, see if those a/c readings change significantly.

Actually, just thought of something: If you applied vacuum to the defrost flap pod,wouldn't that actually cause the flap to close?? Did you notice or hear the flap close or reopen when vacuum was applied? Maybe the lever that connects the vacuum element to the flap, which causes it to open and close is broke or disconnected. That's how mine was, the pod still held vacuum, but the part that was responsible for pulling/pushing the flap open was cracked so the lever would just move freely.

I live down the road from Ismalley and agree that the older cars were not that great on AC. I've owned a '87, '91', '94, a '95 and now a '97. The '95 and '97 will freeze meat if it were hanging in the back seat. :) Recently I sat idling with the AC on and the outside temp indicated 117F. I sat there for about 10 minutes and the engine temp never increased and the cooled air was "cool". Didn't have a thermometer and didn't remember how to get the readings out of the Climate Controller, but I remember thinking: "Well the Germans have FINALLY nailed AC"
Anziani

Sending chilled air out the defrost vents will significantly reduce your comfort level relative to sending the same air out the dash vents. Had this occur on one of my cars and was surprised at how big a problem it was. You would think cooled air is cooled air and it doesn't matter that much where it goes - but it did for me.

My understanding is that MB designed this flap so that if it fails, it fails OPEN to allow you to keep the windshield clear.

Therefore, by default its open. So applying vacuum should close it. Its given vacuum via the control unit so since it holds vacuum from the mityvac, the control unit is the next culprit.

I do not believe the lever is cracked since I replaced it a few years ago and its always been the same.

Maybe my entire issue is this flap not closing as you say jcyuhn. Its supposed to arrive Saturday, so I will report back.

The used vacuum bypass unit arrived today. Problem solved!
As soon as I hooked everything back up, I saw the pod start to move and the defrost vents shut.

I don't think that the sole culprit, but its definitely contributing!

Will report back with cabin comfort differences once I get her on a good drive.

The pic is parallel. .. The item may not be...

I wanted to report back with a long 20mi drive today it cooled down much faster.

The additional airflow bow being directed at the cabin not the windshield I would say got me an extra 30% more air.

Vent temperatures still aren't perfect, but this will get me through the hottest part of the summer. In a few months when it cools down I plan to fix the small leak it has.

I put dye in the system and using a UV light I checked every connection and couldn't find anything so I assume it's the evaporator which is original.

I will replace the evaporator heater core and compressor expansion valve drier, flush the system and then convert back to R12. That should get me back to what should be considered the best the car can do.
With all those replaced I will have nothing but the hoses that aren't new.

Will post back with the fun photos of a ripped apart interior. I did the job on my old 260e so familiar but it's been a number of years. It wasn't fun I remember that part!!

Well it was short lived. Defrost vents stuck open today.

Could it be the climate control unit failing to give the signal to keep shut?

100+ summers
 

In my '90 300E (original a/c components) when it's over 100 outside (my outside temp display still works), my infra-red instant temp reader says that I'm getting 44 deg at the center vents and 51 at the side vents with the Re-circulation feature on. It does get so cold that I have to turn the temp wheel so that no Blue band is visible. Then again, it's a sedan and I'm still on R-12.

When I had my '90 300TE, original components and also still on R-12, the vent temps would be roughly the same, but because of the size of cabin, it would take much longer get the same feeling of coldness, even after venting the cabin (windows partially down and sun roof tilted for a few minutes) at speed initially to flush all the hot air. Tinting the windows made a huge difference in reducing the heat when parked and increased the effectiveness a/c.

AL

Drove home today and started working again...

Maybe that used one I bought is on its way out for the same reason too!

It seems strange that its NEVER been shut, then I swap out the thing and now all of a sudden it intermittently starts. Guess I'll cough up the dough for a brand new one...
I'll verify its not something weird with the vacuum hoses first too.

Why don't you just manually close the flap, disconnect the hose to the switchover valve and plug that end with a small bearing so the flap stays closed? I have no reason to use my defrost so mine is permanently closed.

In the winter I go up north for snowboarding and so no bandaid fixes.