Blower Motor: Description and Operation

Blower Motor Speed Control

The blower motor is controlled by a four-position heater blower motor switch located on the left side of the control assembly. The four positions are LOW, MEDIUM LOW, MEDIUM HIGH and HIGH. The four operating speeds are obtained with a heater blower motor switch resistor connected in series with the heater blower motor switch operating on the ground side of the blower motor.

With the function control knob in the OFF position, the blower motor will not operate.

With the heater blower motor switch at LOW, current flow in the ground circuit is through three coils to obtain the lowest operating speed.

With the heater blower motor switch at MEDIUM LOW, current flow in the ground circuit is through two coils to obtain a medium low operating speed.

With the heater blower motor switch at MEDIUM HIGH, current flow in the ground circuit is through one coil to obtain a medium high operating speed.

With the heater blower motor switch at HIGH, current flow in the ground circuit by-passes the heater blower motor switch resistor to obtain the maximum operating speed.

The heater blower motor switch resistor also contains a thermal limiter which acts as a temperature protecting fuse. The thermal limiter is located a preset distance from the coils. If the temperature of the thermal limiter reaches approximately 121 °C (250°F), the limiter contacts will open, interrupting the circuit for all speeds except high speed. The thermal limiter will not reset and the heater blower motor switch resistor must be replaced if the thermal limiter opens.