For 1990-2009 cars

The HEATER fuse applies the battery voltage to to the blower speed selector switch if 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, the current travels completely through the blower-motor resistor then to ground through the blower motor. The reduced voltage potential that is caused by the blower-motor resistor maintains the blower motor at the LOW speed.

When the blower speed selector switch is in the MEDIUM 1 position, voltage passes 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 the same as the MEDIUM 1 operation with the exception that even less of the blower-motor resistor is in the circuit. The blower motor operation in the MEDIUM 2 mode is faster than the operation is in the MEDIUM 1 mode.

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 equals almost 0, and the blower motor operates on full system voltage at full speed.