Climate Control 1990 200 turbo avant
 

The climate control is only blowing hot air. Seems to be no change when i change the temperature from low to high. Where should i begin in the troubleshooting processl?

Either the climate control module is bad (replace with another from junkyard or ebay) or perhaps the flaps (or the arms that control the flaps) are not moving to blend the hot and cold air sources. Those are all accessed with the dash off, so it's a big job, such as changing the heater core.