Blower Motor Switch: Description and Operation

Blower Motor Control

The blower motor is controlled by a four-position heater blower motor switch located on the left side of the heater control. 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 heater blower motor switch at LOW, current flow in the ground circuit is through three heater blower motor switch resistor coils to obtain the lowest operating speed.

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

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

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

The heater blower motor switch resistor, in addition to controlling speed, can sense heat build-up in the assembly with a thermal limiter which is located a pre-set distance from the coils. If the temperature of the limiter should reach approximately 121 °C (250°F) the limiter contacts will open and interrupt the circuit for all speeds except high speed. Since the thermal limiter will not reset, the heater blower motor switch resistor must be replaced if the limiter opens.