HVAC Blower Controls Circuit Description

The blower motor is a variable speed motor. The higher the voltage applied to the motor, the faster the speed.
Voltage is supplied to the blower motor by the heater-A/C control through the blower motor resistor. At low and medium speeds (M1, M2 and M3), the voltage is stepped down by the internal blower motor resistors. Each internal resistor also has a thermal limiter in series with it. At high speed, the internal blower motor relay is energized, removing the internal blower motor resistors from the circuit. Voltage is then applied directly to the blower motor through the relay, thus the blower motor runs at maximum speed.