[bmr12]

I have an '08 328xi. The climate control system seems to have some sort of defect but I'm looking for help on how to diagnose it. When in "Auto" mode, the system arbitrarily decides to throw a significant portion of the output into the defroster for a while, then go back to where it ought to logically go. It cycles through this process every few minutes. Typically, the fan speeds up, it enters this defrost cycle, and then a few minutes later returns to normal.

The damper control itself seems to work correctly--if I override it to switch to defrost, vent, or heat output, everything is fine. Doesn't matter if the car is close to the requested temperature or far apart either as it happens when it is first started and after it has been going a while. It's also not temperature dependent as it has been going on since last winter (where the defroster might be useful) and through this summer (no, I don't need any defrosting on a 90+ degree day).

It's become especially annoying in the heat as I want the A/C coming out the vents and not hitting the hot glass.

Thoughts? Searching found other various HVAC issues but nothing quite like this one.

Thanks.

[emt2010405]

Im having the same issue in my 335xi right now. Did you ever find a solution?

[robthewrench]

I read on another post that the rain sensor on the windshield also contains a humidity sensor. When interior humidity gets to high and the HVAC system is in auto, it supposedly switches automatically to defrost.