I'll do my best to explain the symptoms, and then look for a recommendation on which sensor is the likely culprit. Thx!

1999 E300 TurboDiesel climate control (AUTO mode)
Summer months - No lack of air conditioning. Car gets plenty cold. During daylight hours, car pumps out A/C seemingly regardless of temp setting. Set to 76 and car will cool much below that. Soon as the sun goes down, or if it's a dark overcast day (or outside temp drops??), car seems to respond "properly" to temp setting, actually pumping heat.
Winter months - Lack of heat. Cabin does not seem to warm properly, regardless of temp setting. Setting to DEFROST mode (HI) though, and it gets hot enough to blowdry your hair. Lack of heat more noticeable on extended highway trips.
I know how to access the diagnostic mode of the climate control, can anybody make any recommendations on troubleshooting this?

Try the simplest thing first. Take a 2" square thin tissue paper, and while the blower is on high, cover the intake grill of the cabin temperature sensor. The tissue should stay put. If it falls down then, using a vacuum cleaner, clean the inlet grill and try the tissue test again. The intake grill is about 1" square and is located in the overhead console.

That's an easy, and quick test to try before you go on to more complicated diagnostics.

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]

I tried the tissue thread - a 2" square - and it stayed in place. I'll vaccuum it anyway just to make sure it's clear. What should I try next? I saw a comment on another thread from M.B. Doc that this might be a known problem on the W210?