A/C fresh air - PeachParts Mercedes-Benz Forum

Mismost · #1 06-12-2006, 01:41 AM

Ok, this is not my idea, I read it somewhere, wish I could find it again. I take no credit for this, but I would like some of you Guru's with more experience to verify this modification. The mod stops the intrduction of fresh air while in the A/C mode, and gives you 100% fresh OR 100% recirc air.

Remove passenger kick panel, glove box, ashtray, CC unit, and radio.

Looking in from the glove box on the left is the main air door (MAD) with a red vac line, it opens/closes outside air from in front of the windshield. It works when vacuum IS applied. In fact, you can pull and plug the red line and stop pulling in outside air...which makes your cold air colder, try it, it works.

In the right corner is the fresh/recirc door (F/RD) with a green vac line. This door is only open when vacuum is NOT applied...so the two doors are opposite in operation. SO CONNECTING BOTH TO THE SAME SOURCE MAKES ONE OR THE OTHER WORK, depending on what the CCU tells the source to do...vacuum on or off.

Now behind the CCU is a row of vacuum valves.
First on the right is for the MAD, red line
Second on the right is the F/RD, green line....THIS IS THE NEW CONTROLLING VALVE

Cut and splice the red and green vac lines going to the pods together with a TEE vac fitting. Plug the green vac line from the vacuum valve into the Tee. Now pull the electrical plug off the MAD vacuum valve. That's it, you need one TEE fitting and a pair of clippers.

I have not yet made this modification. I spent about three hours playing with the system, watching, and trying to learn how the system works. I think it will work! It is easy enough to do OR undo and cheap too. Me like cheap and cold air.

Any thoughts, observations, dire warnings??

AND AFTER ALL THAT, MY CENTER VENTS STILL DON"T FREAKING WORK!

Mismost · #2 06-12-2006, 01:52 PM

BUMP

Where are you a/c vacuum Guru's??

BTW, found a vacuum system diagram that is pretty clear at this link

http://home.comcast.net/~phantoms/vacuum/heating_ac.jpg

Mismost · #3 06-14-2006, 12:29 PM

Well, I went ahead did the modification, I figure it takes about 2 minutes after the glove box is removed and would take about the same amout of time to undo if you wanted to.

It works very well. The cold is colder since the 20% hot fresh air is cut off. The car cools down faster. All good stuff.

Also fixed the center vents by manually opening the door using the white plastic pivot point and then wedging a 1" piece of vacuum hose, stuck on the end of a screwdriver poked through the vent, between the door and the duct. Crude, but effective. Of course if you don't want air out of the center vents, you will have to labor hard and reach over and twist them closed instead of pushing a button...but life is tough, get hard!

In the Texas heat, this is a great cheap modification for the W123 cars. Downside, if you want fresh air you gotta open a window or the sunroof.

my123ca · #4 06-14-2006, 08:50 PM

Ive read that article too. I wanted to try that modification when I removed the dash on my car. After careful consideration, I decided not to do it. The reason being that I dont know too much about the CCU. AFAIK, the CCU only switches the valve open or close. It doesnt meter the valve to close(or open) 80%. If you look at the recirculating door, there is an opening, which is about 20% of the surface of the door, for the lever. Maybe thats the 20%? Maybe if you unplug the electric connector from the main air flap switch, that would keep the outside air from coming in?

sixto · #5 06-14-2006, 09:17 PM

The recirculation actuators, IIRC there are two of them, each have two chambers. Applying vacuum, so to speak, to one chamber moves the rod only so much to effect the mix position of the recirculation flap. If neither chamber sees vacuum, the flap is open to outside air, if both chambers see vacuum, the flap is closed to outside air.

I didn't read the original post completely but I think the gist is that both recirculation lines are plumbed into the center vent solenoid so whenever air blows through the center dash vents, presumably only for cool air, the recirculation flap shuts out outside air.

Sixto

Mismost · #6 06-14-2006, 10:20 PM

Nope...both lines are plugged together and then into the fresh/recirc valve...reread the first post...really simple.

sixto · #7 06-15-2006, 04:13 PM

Got it.

Sixto

my123ca · #8 06-16-2006, 07:18 PM

Quote:

Originally Posted by sixto

The recirculation actuators, IIRC there are two of them, each have two chambers. Applying vacuum, so to speak, to one chamber moves the rod only so much to effect the mix position of the recirculation flap. If neither chamber sees vacuum, the flap is open to outside air, if both chambers see vacuum, the flap is closed to outside air.

Sixto

On my 300D at least, there is only one chamber for each pod except for the defroster which has two. http://home.comcast.net/~phantoms/vacuum/heating_ac.jpg

Jassper · #9 06-20-2006, 02:42 AM

Hello there gentlemen!

This thread comes the closest to what I think is problem with my recirc flap. Or, is there no problem after all?

When it is hot, after a stay on a parking lot with lots of sun, or even moving out from the garage where it is warm (but not hot), with A/C on, the system starts to cool and closes the air recirc flap.

However the problem is that even after half an hour or so the flap does not open and let any fresh air in. The car is long cooled down inside. Mind you, after a few minutes one gets to feel that the air is not fresh and after half an hour the air is not at all so good. Flipping the air recirc switch does not do anything, the flap remains closed no matter what I do.

Of course when I turn the A/C off the flap opens, it only behaves as above-described with A/C on. The daily temp are up to 86 degrees Fahrenheit and less. When the temps drop down in the evening the flap opens. Surely though, the flap just can't stay closed for good and not let any air in for a long time, even with high outside temperatures?

I have a new CCU, new inside and outside air temp sensors. There isn't a vac leak anywhere in the system.

So am I right to expect I am not supposed to faint after driving for an hour in the sun and there is something wrong with the flap behaviour or is the flap supposed to remain closed for eternity under hot conditions?

Thank you all for your answers.

Mismost · #10 06-20-2006, 10:45 AM

What kind of car? The diagrams I've seen show the 123 to be a bit different from the 126 models.

No fresh air is kinda of good thing because it means your AC isn't trying to cool the inside and the outside at the same time! If I understand your post, you HAVE what I WANTED. Fresh air is also easy to aquire, there are some more little buttons that work the WINDOWS, bump switch and crack the window.

Jassper · #11 06-20-2006, 10:58 AM

Quote:

Originally Posted by Mismost

What kind of car? The diagrams I've seen show the 123 to be a bit different from the 126 models.

No fresh air is kinda of good thing because it means your AC isn't trying to cool the inside and the outside at the same time! If I understand your post, you HAVE what I WANTED. Fresh air is also easy to aquire, there are some more little buttons that work the WINDOWS, bump switch and crack the window.

It is a 124 250 TD. Or 300D 2.5 Turbo. No fresh air at all is, I think, a bad thing because then you are getting to breathe used air and you do feel that after a short while. Besides, even with the flap open the system cools excellently. Of course I could just wind down the windows but I am trying to figure out a more scientific way to that

sixto · #12 06-20-2006, 12:57 PM

AFAIK MB climate control systems will only hold the recirc flap closed for 30 min with AC compressor on and 5 min with AC compressor off. Maybe there's something wrong with the timer. Will the recirc flap open if you disengage the compressor (e.g., select econ mode) momentarily?

Sixto

Jassper · #13 06-20-2006, 04:30 PM

Quote:

Originally Posted by sixto

AFAIK MB climate control systems will only hold the recirc flap closed for 30 min with AC compressor on and 5 min with AC compressor off. Maybe there's something wrong with the timer. Will the recirc flap open if you disengage the compressor (e.g., select econ mode) momentarily?

Sixto

Yes the flap will open when non-compressor mode is selected. Is the timer built in the rocker switch? The system definitely holds the flap closed for more than 30 minutes with A/C on. When hot outside. The rather interesting thing is the flap will open in the evening just perfectly when the temps go down to say 75 Fahrenheit.

jcyuhn · #14 06-20-2006, 05:39 PM

I very seriously doubt that your car is so incredibly well sealed that lack of fresh air is a problem. Doubly so on a car which is at least 11 years of age. None of my many Mercedes have been that well assembled.

The 124 decides whether to open or close the recirculation flap based on a combination of the outside air temperature, the temperature inside the car, and the setting on the climate control wheel. Basically, the hotter the outside temperature and the greater difference between the interior temperature and the setpoint of the CCU, the more likely you are to be in recirculation mode. Try turning up the interior temperature wheel.

The 30 minute thing only has to do with the manual recirculation switch. The unit itself holds the system in recirculate until it's happy with the interior temperature of the car.

BTW, the 124 has a more sophisticated recirc system than the 123. It has three positions - fresh air, 80% recirculated air, and 100% recirculated air. The system selects based on how much cooling is needed. The switch on the dash forces it to 100% reciculated.

I'm going back to my day job now,

- JimY

Mismost · #15 06-20-2006, 07:07 PM

Hey! JCYUHN is the guy that deserves credit for this modification! It was a DIY sheet he posted in 1999 that pointed me in this direction.

Perhaps he would be so kind as to re-post it here...his is much better, easier to read.