I can not find my monovalve. I have looked where everyone says. I have searched exactly where my Haynes manual says it should be. NADA. not there. I have read that not all of them have a monovalve. Is this my case? Its a 1980 300D. When i bought it a month ago, the heat was good. Then it got weaker, and now its just blowing cool air. My unit has the temp. wheel to the left (oriented up and down) and the buttons to the right (also up and down). I have only seen one other like this. Does that mean no monovalve? If there's no monovalve, then where do i go from here to fix my heat?

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1980 300D