Driver Controls and Instruments Fresh Air 1. Move the air source lever (if equipped) to the fresh air position. See Fig. 3.37 . Move the air selection switch to the desired position. NOTE: If not equipped with an air source lever, the air source is fresh air. 2. Move the temperature control switch counterclock- wise for no heating, or move it clockwise for heat. 3. Set the fan switch at the desired speed. IMPORTANT: If equipped with an air source lever, do not operate the heater and air conditioning sys- tem with the air source lever in the recirculation mode for more than 20 minutes. This will prevent the buildup of fumes or odors inside of the cab (for example, from smoking). 3.50