Blower Motor Resistor: Description and Operation
THERMAL LIMITER RESISTOR ASSEMBLYBlower Motor Thermal Limiter Resistor Assembly:
The blower motor thermal limiter resistor assembly has three resistance elements mounted on the resistor board to provide four blower speeds. Depending on the blower switch position, series resistance is added or bypassed in the blower motor circuit to decrease or increase the blower motor speed.
The thermal limiter resistor assembly is similar to a standard resistor assembly, except an overheating protective device (thermal limiter) has been added to prevent heat damage to the evaporator case assembly. Overheating of the resistor coil(s) will occur when the system airflow is stopped as a result of the blower wheel being locked.
NOTE: When the thermal limiter resistor circuit has opened as a result of excessive heat, it should be replaced only with an identical replacement thermal limiter resistor assembly. It must not be substituted with a standard resistor assembly which does not include a thermal limiter device.
THERMAL LIMITER
The thermal limiter, used in the thermal limiter resistor assembly, serves as a temperature protecting fuse. Located a predetermined distance from the resistor coils and in series with the coil circuit, it will open the resistor coil circuit when the temperature of the thermal limiter reaches 121° C (250° F) interrupting blower operation in all speeds except high blower. Internal spring-loaded contacts are held closed with wax material which has a melting point of 121° C (250° F). When the wax softens, the spring contacts separate, opening the resistor circuit.
NOTE: The spring contacts cannot be closed again. It will be necessary to replace the entire thermal limiter resistor assembly