No mono valve on a W201. They have what is called a heater valve, it is either open or closed, vacuum controlled and defaults to full open with no vacuum. The Aux pump will cause no heat at idle and luke warm heat at speed. I thought my aux pump was shot so I tested it and it still worked. I jumpered it on the ground side to get it to work when the ignition was on. This solved my low heat issues. IN the end I replaced the Climate Control Unit and my pump worked as it was supposed to, as well as my heater valve that was never closing when it was supposed to. The W201 HVAC system is designed to default to full heat out the dash vents in case of vacuum loss or failure of any components. So most likely not a vacuum leak. I am betting on the Aux Pump and the CC Unit in the dash.

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....