Do a search and try running the diagnostics. I had sort of the same problem, I blew a thermostat last summer on my 93 400SEL and replaced it but noticed that I wasnt getting barely any heat when winter came around. This spring I drained and changed the coolant and when I was done I had awesome heat but now I had lost my A/C! After I ran the diagnostic tests the whole system seemed to reset itself and I now have great heat and great A/C again.
You have to get both sets of readings to come up, one of the posts here will tell you the trick to it.