Seems to be a common problem with w210 and their dual zone climate control. Mine started acting up on the way back from Baton Rouge -- full heat started blowing out of the vent to my left. I shut the ccu off and back on and that seemed to cure it.

'Till yesterday.

Full heat out of the driver's dash vent. No combo of buttons, on/off nor banging on the dual valve fixed it.

Trusty PeachParts Forum to the rescue. Thanks to lots of threads about this problem, I knew what areas to address before I dove in.

I haven't done the ac panel code diagnostics yet. Based on what I found though, I believe my evap. sensor to be bad.

I wanted a quick fix or bypass to get the heat out of my face since it's still near 80 degrees down here.

Pulled the coils cover off my dual valve but both plungers operated quite freely. Ohmed both coils out and they were good. Tested the plug to the dual valve next. Key on with ac on full cold -- 12v from ground to the passenger side pin; 0v to the driver's side pin.

Ah ha!

Pulled the driver's side pin out of the plug housing, taped it up out of the way, and jumpered across from the passenger side pin. For now, and until I fix the true culprit, I have passenger side control operating both coils of the dual valve and I have icy cold air!

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1998 E300 +310,000K
1996 E300 +460,000K