you may have a fault on the blend door system ..

Climate Control Assembly
The climate control assembly has three control systems:
The A/C heater function selector switch combines a vacuum selector valve for direct control of the vacuum control motors and electrical input to the instrument cluster module.
The temperature selection is accomplished with a potentiometer connected to the electric blend door actuator that controls positioning of the temperature blend door. Movement of the control knob (18519) from COOL (blue) to WARM (red) causes a corresponding movement on the air temperature control door and determines the temperature that the system will maintain.
The blower motor switch controls the blower motor speed through a series of resistors and a relay.

Answered by Dave H (expert)
13,682 answers provided