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.
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.
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.
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