The monovalve sits on the firewall inboard of the battery, next to the ABS computer. It is not Vacuum controlled, only electronically controlled. When you pressed the defrost button the vents did not change? That is pointing to a CCU ( climate control unit) fault. Something electrical may have happened with your car. Have you checked all of your fuses in the fusebox? Have you checked the 10Amp fuse on top of the OVP? That is the Over voltage protection unit which sits on the driver's side of the firewall next to the KLIMA relay. Do you have any dashboard lights on while you are driving, specifically the ABS light?

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John
1986 300sdl
1985 380se