Although the fan motor sounds like it working hard, the airflow coming out of the vents is weak. If the selector is set to the dash vents, the fan does not run at all and if the selector is set to dash and floor vents, air only comes out of the floor vents.

I pulled the heater control unit and measured voltages on the selector switch. When the dash vent was selected, the voltages on about 3 different contacts were 0.24 - to 0.3 volts. Selecting anything else gives readings of over 5 volts at various contacts depending on what is selected. The low voltage readings occurring and the the fan not running when the dash vents are selected leads me to believe that this is an electrical problem rather than a vacuum motor problem.

Any suggestions?