Hi,
I have a 1984 300TD station wagon that belonged to my mother. I got the car four years ago. Overall the car is in great shape, 105,000 miles, practically everything works, etc. The only major problem was no cooling. It took only $100 in parts to make it pass the Maryland vehicle inspection. My wife and I decided, rather than getting an SUV or van for our newish family, to get the AC fixed and use this one.
The climate control controls all seemed to work fine, i.e., the air came from the right vents at the right time, and the heating and cooling (without chilled air, of course) came on at appropriate times. In the winter, there was no need to touch the controls. All this car needed was the means to chill the air. So we took it to a garage and they basically renewed the heat and AC.
They replaced the evaporator, heater core (since they were already in that compartment), AC hoses, expansion valve, filter drier, and belt. The compressor seems fine. In fact, the chilled air is now all you could ask for.
The problem is that now the cold air comes only when the temperature setting knob in the control cluster is set to maximum cooling (or "Min"). Whenever the knob is moved away--however slightly--from "Min" the heat comes on full tilt, the vents change from cooling (all four dash vents) to heating mode (side vents only) and the AC compressor continues to run. It appears that both heating and cooling are going full steam and the heating is winning. From the feel of the temperature control knob as it moves, the "Min" (and "Max") settings seem to trip some sort of override circuit that forces maximum output. Evidently, the "Min" setting forces the heating to shut down. When the control knob is set to the extremes, the fan speed usually increases if it wasn't already running at that speed. I don't know if the "Max" setting increases the fan speed now. It did in the past.
The mechanic has been scratching his head for over a week now. He has replaced the climate control control cluster, the thermostat (whose sensor is positioned between the evaporator and heater core), the auxiliary pump in the heater hose under the hydraulic leveling fluid reservoir under the hood and has verified that the heater control valve attached to the firewall under the hood is opening and closing as it should. Nothing seems to make the slightest difference and he has reinstalled the original equipment. He has accepted that the heating functioned correctly when I brought the car to him and that something must have gone wrong during the repairs so I can't complain and I'm not. In checking the car after getting it back the first time, I discovered that the fuse for the climate control, et al, had been replaced. I suspect that he didn't disconnect the battery at first and shorted something. The Mercedes garages around Baltimore would probably have disconnected the battery but they would have charged me 5 times as much and are clearly too snobby to have been willing to accept any responsibility for a snafu like this if they had caused it.
I keep thinking that the problem is with the thermostat in the plenum housing of the evaporator and heater core--it's not positioned correctly--but I have no practical way to prove anything.
The point is (thanks for staying with the narrative) that there seems to be nothing left to account for this malfunction. I am hoping for someone who will read this and say, "No problem. There's a blue and green control module just behind the glovebox that has gone bad and you just replace it. Happens all the time." or something like that. :-)
Thanks very much in advance for any light you can shed on this problem. If there is any further info you might need or want please don't hesitate to say so. Also, if you respond to the list would you please cc to me personally?
Thanks again,
Ken