I will assume that the compressor clutch is ENGAGED but not cycling. Most likely the problem is not charge related. The heater valve is controlled by the pushbutton climate control unit. The problem is probably either the pushbutton control unit, or the heater valve itself.
As I recall, the climate control supplies a ground to the heater valve to inhibit coolant flow to the system. If the valve fails and flows coolant, or the climate control unit fails to provide the ground to the valve, hot air is the result.
There are many things that could be causing your problem, but the climate control is a commonly failing unit.
The best bet to properly troubleshoot this system is to get the CD and follow the procedure. The climate control unit is about about $200 exchange, not a part to replace just to see if that's your problem.
Additionally, if you have to replace the unit. Be sure that you check your aux water pump for current draw. If it draws over 1.3 Amps, replace it or it will blow the new climate control unit. Most suppliers of the CCU will not warranty the unit if the aux water pump was not replaced. I checked my aux. water pump for current draw rather than replacing it, it was okay, and I have had no more problems. That was a year ago.
'84 Euro 240D, 523K miles
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'81 300D Daughter's Car
Over 800,000 miles in