A/C air shifts from center vents to defrost when manifold vacuum is low (WOT). Just started doing it, is there a check valve or reservoir that is common to fail? It just happened and I have not had the chance to look in to it. Thought I would ask this forum for help. Any ideas would be appreciated. Thanks

Get a MitiVac hand pump and attach to the supply line in the engine compartment. If the system won't hold vac, you need to fix the leak.

If it does, then you have low vac for another reason, and will have to find out why. Old hard rubber connectors are a typical problem -- they leak, and you have low vac in that line.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

The locking pump in the 140 shows a connection to the HVAVC, I dont how how it does it or what triggers it, I would check hoses or check valves

I had this exact problem on another car (not MB), and it was caused by a faulty vacuum check valve.

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1986 300E 5-Speed 240k mi.