My 91 300E only blows full heat out the defrost vents when the defrost button is pushed in. All other parts of A/C control work well and correct. A/C blows cool when required. Compressor works as it should.

What is causing the full hot heat out defrost???? I've reviewed other post without answers. Is there a valve not pulling a flap correctly? Defrost does work, but in the summer during rain, the full heat coming out the defrost vents is terrible.