Ok, so just to bring you up to speed. I have had a climate control problem with this car ever since i started driving it (2yrs) . Over that time I have fixed several heat/ac issues and the system is getting better every time I work on it. I have re soldered the push button assembly, replaced leaking vacuum pods, replaced some cracked vacuum lines, etc...
So now the A/c works almost perfect, but it is no longer summer . Now I want the heat to work. So After trying to nail down some exact symptoms I came up with this.
With the car warmed up, running in park, heat works perfect, fan changes speeds temp output changes locations full range from cold to warm. But when I drive the car on the road, the heat comes up to temp, then starts to blow cold air and the fan never changes speeds, it keeps blowing on 1 step down from max. So then to get the vent temp back up from cold I have to turn the temp all the way down past the detent for cold and back up to warm. The fan never changes but it starts blowing warm again until the process starts all over again.
Looking for Ideas..

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1983 300SD