Defrost is a system override like using high or low fan settings instead of automatic.
Mine does the same thing. I'm glad it does that so that I do not get an icy blast. BTW, I have to do vacuum work on the HVAC system. Air is split between all the dash vents and the windshield vents.
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RRGrassi
70's Southern Pacific #5608 Fairmont A-4 MOW car
13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.
91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K
90 Dodge D250 5.9 Cummins/5 speed. 400K
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