If your fan never comes on low speed, some possible reasons - 1. no refrigerant to create the pressure, 2. a bad switch, 3. a bad fan resistor, 4. a bad fan motor. With the AC on, try helping the fan blade with a pencil, if that gets it started, you'll know it's the fan motor, otherwise the resistor does burn out, and the wiring does get hot in that area. I guess a fuse could also be blown.