Check your vacuum coming into the cabin. If it is less than 15", your vents won't work properly. My line was green and yellow, if I recall.

You also need to check the signal at the switch. Hook up a test meter to the contacts on the plug, push your center three buttons with the system on, and see whether you have 12v.

ALternately, you could test the vacuum being pulled at that switch when the system is on and you push the center three buttons. If you get a vacuum, your switch is fine. If not, you either have no signal coming from the control unit, or bad vacuum somewhere else.

IF you have good vacuum coming into the switch, AND if you have 12v at the plug, the switch is bad.

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Peter
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K