w123 HVAC recirculation modification
 

One issue that I come to hate with the w123 fully automatic climate
control system is the inability to manually recirculate the cabin air
when you need to. The ACC-III unit is supposed to recirculate the air 80% of the time with 20% outside fresh air. Unfortunately that fresh air comes on when I don’t want it, like driving by dairy farms or stuck behind some smelly vehicle.

Mercedes gave 2nd gen w126’s, w124’s, and w201’s of this era the
ability to manually engage the recirculation. It will only be on for 30 minutes when the outside temperatures exceed 7 degrees C for the w124 and w126 or 15 degrees C for the w201.

Reading some past arguments, many folks in hotter climate
areas would love have 100% recirculation to help better the cooling.
Many have warned against this as the cabin air would get
stale and the oxygen depleted. Possible good warning, but for myself, I would keep it on “auto recirculation” most of the time and manually recirculate only when needed.
The other drawback on leaving it on recirculation is the possibility of mildew growing in the evaporator. Having
fresh air would help dry the evap and prevent this issue.

The w123 recirculation vent is operated by a switchover valve allowing vacuum to actuate an element or pod for the flap. In this diagram, no. 41 is the recirculation element and no. 18 is the vacuum switchover valve that controls the recirculation element.
http://www.peachparts.com/shopforum/...1&d=1427223936


When power is applied to the switchover valve (no. 18 on the diagram again), vacuum is allowed to activate the flap for fresh air. When no power is applied, there is no vacuum and the flap moves to the recirculation position.
http://www.peachparts.com/shopforum/...1&d=1427224399
http://www.peachparts.com/shopforum/...1&d=1427224399

I will be adding basically a toggle switch to the power lead so I can cut power to the switchover valve. This will force the flap in the recirculating position.

In the first picture is a shot of the switch over valves located behind the center switch panel and climate control. I will be tying into the plug marked “no. 4” with a plug and play wire harness that I will be making.
http://www.peachparts.com/shopforum/...1&d=1427224399

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These next two are pictures of the recirculation switch. The w124 and w126 switches are a bit more complicated for what I am trying to achieve. They basically activate a relay and timer in the climate control panel for recirculation mode.
My switch is made from a rear interior light switch modified to switch off power. Unfortunately the w126 or w124 button does not fit, so I gingerly added the recirculation icon to the light switch button.
http://www.peachparts.com/shopforum/...1&d=1427224325
http://www.peachparts.com/shopforum/...1&d=1427225088

Here are pictures of the new “plug and play” harness using salvaged pig tails or plugs. After shrink wrapping the harness, I transferred salvaged “no. 4” stickers to it to prevent confusion on servicing the wires later on.
http://www.peachparts.com/shopforum/...1&d=1427225026
http://www.peachparts.com/shopforum/...1&d=1427224826
http://www.peachparts.com/shopforum/...1&d=1427224826


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Next set of pics was to plug it all in. I connected the fiber optic light cord to the open port on the junction located at the bottom front of the HVAC heater core cover. Plugged the harness to the existing plugs and then tested the new setup…. Works like a charm!
http://www.peachparts.com/shopforum/...1&d=1427225237
http://www.peachparts.com/shopforum/...1&d=1427225703


Finally, the trim piece. I broke out my trusty RotoZip and cut the new switch hole. I think it turned out really well.
http://www.peachparts.com/shopforum/...1&d=1427225703
http://www.peachparts.com/shopforum/...1&d=1427225703


Overall, I am very happy with this modification.
http://www.peachparts.com/shopforum/...1&d=1427225703


I love the ability to switch over to recirculation when I want. At some point, though not a priority for me, I like to research how to add the 30min. timer to bring it inline with later factory models.

Thanks for checking this mod out.


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As normal, outstanding work !

Here again is a modification I didn't even know I needed until someone here showed me how to do it!! Thanks for the great How-To with tremendous pictures. It's on my "to-do" list now.

Thanks, Mark!

I do believe the wiring for the "Evil Servo" climate switchover valve controls are the same. So this mod should work with it.

I agree; didn't know I needed this either... However, if and when I install a Sanden compressor, it may be unnecessary, unless I wander through LA in August or something.

Recirculation creates fogging of the windows due to humidity in my colder climate. I like your mod and diy instructions though.

My main motivation is to close off smelly outside air. But would certainly help with the Sanden in cooling if needed.


Good point to bring up and I will add it is important with the defrost also. You always need outside air to defog your windows and the climate control will engage to do so. So with that said, I will always manually recirculate when needed and leave the switch for the most part in "auto" position for the climate control to choose.

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Mercedes fixed the smelly outside issues and smelly inside conditions for that matter by adding an active charcoal filter. The filter can be actively controlled in some models dating back to around 2000.

no matter what you do, i think it should still be pulling in some air from outside. theres a vent that feeds into the ductwork above the blower motor but below the normal fresh air flap. it leads to a tube that attaches to an opening in the body forward of the passenger door.

The only problem I can see is that there doesn't appear to be a cabin vent...

All modern cars have flaps that allow positive pressure from inside the cabin to escape, even my Mk1 Golf had seals in the door panel; W123s may have this too, but if they do I am unaware of where they are. Without them, I could see a possibility of a problem.

aren't the vents on a W123 under the aluminum trim piece behind and over the passenger doors?

It's a relatively small tube that brings in a very small amount of outside air from the fender area.

Here are pics of the vents exposed courtesy of charmulu.
http://www.peachparts.com/shopforum/...01-medium-.jpg

http://www.peachparts.com/shopforum/...02-medium-.jpg

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Ah cool, thanks for that...

I guess it is the same in mine, just further back.