Can't regulate heater temp...

[taimyshukah]

I get hot air when i set the temperature to HIGH. But if i set it any lower than that it blows cold air. Even if I set it to 91 degrees, it begins to blow out cold air. Center vents also dont work, if this helps anyone with a diagnosis for my car.

[BillKerler]

I have a '94 C280 with the same problem. No matter if I'm in the economy or automatic mode, hot air only comes out at the HI setting...warm air from the heater core only comes out the side console vents, not the center (cool air from the evaporator comes through the center vent).
One special thing with my car, after sitting overnight, I can get warm air at a normal setting for the first cycle. Once the cabin thermocouples tell the controller that the setpoint temperature has been reached, the system shuts off the dual valves and hot engine water is no longer circulated to the heater core. When the temp drops in the cabin and it's time to reopen the dual valves, that's when I no longer get hot air unless I dial in the HI setting. It sounds complicated, but please check and see if your controller acts the same way.
People on this website will tell you the dual valves are the number one problem area with climate heat control. But I can hear my dual valves clicking open and closed with a screwdriver against the metal valve base as my wife selects between 92 and HI temp settings.
My dealer tells me he thinks its the controller which is located behind the wood grain fascia panel...all the push buttons are part of the controller assembly located behind the fascia panel.
You have to take out the radio to get to some screws holding the fascia panel in place. With the screws removed, you lift the bottom of the fascia panel out and then slide down to disengage the tabs at the top of the fascia panel so it can be removed exposing the climate controller assembly.
In fact the reason I'm on the club 202 website right now is to find the 2 key tools used to pop the radio out...I have a slot in each bottom corner of the radio in which a key is installed to disengage the radio and allow it to pop out. I no longer have those keys and need to get new ones...it might require going to the dealer.
You and I might be the only ones on this website having this problem with the climate control. Please answer my question if your's works on the first cycle.
Also, in the summer time, my A/C works just fine holding the cabin temperature at a set level...in other words, I don't have to select to LO in order to get the A/C system to work (cool air comes through the center vent from the evaporator to maintain whatever temperature setting I want).
It's only the heat cycle that causes me problems. I suspect a nickel and dime diode or transistor in the controller circuitry as the culprit...buying a remanufactured controller is about $500.

[taimyshukah]

I'll check later on. Sounds really complicated..