This is a very common issue.
Here is the fix:

Fixing that Damn Climate Control!
http://www.diymbrepair.com/Climate/

__________________
ASE Master Mechanic

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D

https://www.boldegoist.com/