Control valves equipped with position adjustment are used for temperature control and regulation in motor vehicle heating and air conditioning systems. The most important known valve types are upper valves, cam valves and friction valves. Known position adjustment means include electric drive motors and pneumatic vacuum chambers utilizing a mechanical crank gear and the like. Relatively good results have been obtained in motor vehicle air conditioning systems especially with cam valves and directly coupled, electric-motor operated stepdown gearing.
A disadvantageous feature of these known devices is that they are characterized by close tolerances in the valve seats and drives. Consequently, manufacturing costs for these parts are relatively high.
Furthermore, the life of valve parts is largely dependent upon contamination in the water circuit. In addition, hostile environmental conditions under the hood tend to shorten service life.