I just purchased a 1990 300TE to drive during my stay in England. It has the climate control system with the dials (blue/white/red in color). The heat is constantly on, and won't turn off, regardless of the settings. On long trips, it sometimes will stop, but for local driving there is only hot air. Is this a vacuum problem? Please advise.

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1998 W210 E300TD
1997 W202 C230
1990 W124 300TE
1985 W123 280TE (sold)
1983 W123 300D (sold)
1981 W123 240D (sold)
1980 W116 280SE (sold)
1980 R129 450 SLC 5.0 (sold)
1975 W115 240D (sold)