For 1990-2009 cars

Battery voltage is applied at all times through the HTR fuse to the switch contact of the heater relay. Ignition voltage is applied to the coil of the heater relay when the ignition switch is in the ON and START positions. When the blower speed selector switch is moved to any position other than OFF, the coil of the heater relay is provided with a ground through the blower speed selector switch to G204. With power and ground provided, the relay energizes; the contacts close and voltage is applied to the blower motor. The blower motor speed is maintained by controlling the voltage potential to the blower motor. This is achieved by increasing or decreasing the resistance within the blower motor's ground path.

With the blower speed selector switch in LO, current must travel through the entire resistive element in the blower motor resistor before the blower motor's ground path is completed through the blower motor resistor at G203. This reduces the voltage potential to the blower motor greatly and in turn causes the blower motor to run at low speed.

When the blower speed selector switch is in the M1 position, the blower motor ground path travels only partially through the resistive element in the blower motor resistor before completing the ground path through the blower speed selector switch at G204.

Since less of the blower motor resistor is in the circuit, the resistance in the circuit is lower, thus making the voltage potential of the circuit higher. Therefore, the blower motor operates at a higher speed than it does with the blower speed selector switch in the LO position.

M2 operation is identical to M1 operation with the exception that even less of the blower motor resistor is in the circuit. Therefore, the blower motor operation in M2 is faster than it is in both M1 and LO.

With the blower speed selector switch in HI, the ground path of the blower motor bypasses the blower motor resistor entirely and passes directly through the blower speed selector switch to G204. Therefore, the resistance in the ground path is nearly zero and the blower motor operates on full system voltage. Under these conditions, the blower motor operates at full speed.