I am a newbe to this forum. I have a 1991 300D 2.5 Turbo. I suddenly have cold air (AC?) from the side vents and lower vents while the setting is on heat. It's February in Buffalo NY. Checked fuses and all the vacuum lines I can see under the hood. Does this point to the monovalve? Any help would be appreciated.

Feel the hoses going in/out of the monovalve and see if they're hot. If not, disconnect the plug to the monovalve and see if it gets hot (if so it's not the monovalve, should be open with no power and closed with power).

You can also try unplugging the vacuum to the HVAC, you should then get the no-vacuum default of heat to the defrost (and side?) vent only (no floor heat).

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Gone to the dark side

- Jeff

and one additional question: do you have heat at any vent and at any setting? As Jeff wrote, "Defrost" should give you maximum heat and maximum fan speed with air to the windshield and side vents. If you are getting no heat at all under any circumstances then I would first check the monovalve.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Just got back in town - thank you Jeff and Jeremy for the advice. I'll check further this week.
Ron