I have a 93 500SEL that has a problem with the heat. I get a slight blowing of air (with fan on MAX) from the front vents, both center and sides. However, with the temp selectors both on HI I still get no heat. I've found that if I depress the red "booster" in the center, I can get some heat. It's not the kind you could cook a hotdog over, but it would do. In the rear, I get nothing but cold air no matter I do up front.
I found this old thread No Heat - W140
, but there is never really an answer. I've just flushed and filled the radiator with MB coolant, and replaced the tstat. I also tried the diags test JimF mentions on his site, but I too could only get the sensor values to come up. It's warm here now, so it's no big deal. You'll get a kick out of how I found out I had no heat. I was driving home one particularly hot afternoon when my engin temp guage started to read around 105, so I thought I'd turn the heat on to vent it a bit. Ha! Foiled again!
I'd rather not dump yet another $500.00 into this thing right now, if you know what I mean. If I show up at the inde shop again they are going to give me my own locker!