For 1990-2009 cars

Battery voltage is applied through the HEATER fuse to the blower speed selector switch when the ignition switch is in the ON position. When the blower speed selector switch is moved to the LOW position, system voltage is applied through the closed contacts of the blower speed selector switch. With the blower speed selector switch in the LOW position, current must travel all the way through the resistive element in the blower motor resistor before traveling to ground through the blower motor. The reduced voltage potential caused by the blower motor resistor maintains the blower motor at LOW speed.

When the blower speed selector switch is in the MEDIUM 1 position, voltage passes only partially through the blower motor resistor before reaching ground through the blower motor. Circuit resistance is lower because less of the blower motor resistor is part of the circuit. Therefore, the voltage potential at the blower motor is higher and the blower motor operates at a higher speed than in the LOW position.

The MEDIUM 2 operation is identical to the MEDIUM 1 operation with the exception that even less of the blower motor resistor is in the circuit. The blower motor operation in MEDIUM 2 is faster than the operation is in MEDIUM 1.

With the blower speed selector switch in the HIGH position voltage bypasses the blower motor resistor entirely. The voltage passes directly through the blower motor to ground. The resistance in the ground path is nearly 0 and the blower motor operates on full system voltage at full speed.