Blower Motor Resistor: Description and Operation

BLOWER MOTOR SWITCH RESISTOR
The heater blower motor switch resistor assembly operates in the following manner:
^ Three resistance elements are mounted on the resistor board to provide four A/C blower motor speeds.
^ Depending on the heater blower motor switch position, series resistance is added or bypassed in the A/C blower motor circuit to decrease or increase the A/C blower motor speed.
^ An overheating protective device (thermal limiter) prevents heat damage to the A/C evaporator housing.
^ Overheating of the resistor coils can occur when the system airflow is stopped or reduced as a result of the A/C blower wheel becoming locked and/or unable to turn freely.

BLOWER MOTOR THERMAL LIMITER
The thermal limiter, used in the heater blower motor switch resistor, performs the following functions:
^ It serves as a temperature protecting fuse.
^ It is located a predetermined distance away from the resistor coils and is in series with the coil circuit.
^ It will open the resistor coil circuit when the temperature of the thermal limiter reaches 121° C (25O° F), interrupting the blower motor operation in all speeds except HI.
^ The spring contacts separate when the wax softens, causing an open circuit in the heater blower motor switch resistor