When you see one of these, you'll wonder. This one is in excellent used condition. This is the air recirculation valve (ARV). Mercedes’ factory manual calls it the “circulating air safety valve.” Its purpose is to help the EGR system operate at partial engine load.
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What it does: Vacuum pulls the diaphragm open, opening an internal bypass that sends some compressed air back to the inlet side of the compressor. This partially reduces boost pressure, allowing more exhaust gas to flow into the intake through the EGR valve. The factory description explicitly identifies increased EGR flow as its purpose.
Where the vacuum comes from: The engine-driven vacuum pump supplies it through an electrically operated switchover valve. The emissions control unit commands that valve according to engine operating conditions. In the factory’s 1984 California diagram, the components are:
Thus, the path is vacuum pump → electric switchover valve → turbo diaphragm. The control system uses engine speed, injection-pump rack position/load, and coolant temperature information. Exact routing should be checked against the year and emissions version.
This is separate from the turbo’s exhaust-side wastegate, which controls maximum boost. The ARV bypasses intake air inside the compressor housing.
