Make sure the monovalve isn't leaking, causing the heater core to run at full temp -- if the heater hoses are all hot, this is the problem.

If the valve is OK, it's the climate control pushbutton control unit, usually, although the coil can go bad in the monovalve, too.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!