Climate Control issue. 79 300SD

[bmonk]

I am guessing this issue is fairly common, but I have not found my exact problem so far on any forums. My heat works just fine, too well actually. It just wont quit. Well it turns off, but thats the only way to make the car non-oven like. The temperature dial does nothing. Wanted to know if anyone has suggestions for where to start troubleshooting. Seems pretty obvious I suppose, but its nice to get some ideas before taking things apart. Thanks in advance.