I just looked through about 500 posts on climate control and heat and did not find anything mentioning this so here is the question:

Currently, AC works fine, got no heat. Fan is blowing normally. When on the max heat setting the air from the vents blows at the indicated fan speed although not hot when you select each vent setting and air is coming out of the correct vents for each setting. BUT when you hit the defrost button the fan goes to max and is blowing out the defrost vents and the center vent.

Any ideas what in the world is going on here?

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89 190E
2.6L
Arctic white
Grey leather
Sunroof
Pirelli P400 rubber
Smoked corners