1985 300D Climate Control Oddity

[xke190sl]

I have a problem with my 85 300D I have never experienced in my 30+years of driving MB diesels! When I push the button to turn on the AC compressor, the vents switch from center to side vents and I have heat. When I push the EC button, I get fresh (non-heat) out of all the vents. This occurs even though the thermostat setting is on cool. Again, cool fresh air comes out nicely until I turn on the AC & I have heat? If I go from AC button back to EC, fresh air returns to all vents (with no heat). The only way I can get the AC on is to move the thermostat wheel to minimum. Is this a problem with push-button main unit? I don't think it is with the mono valve as I have fresh non-heated air with the EC button on? Any ideas why it would switch to heat with the AC buttons?

Richard