Symptoms of a Bad or Failing Heater Blower Motor Resistor can be found on Ricks Free Auto.
Three or four blower motor speeds can be provided with a multiple speed switch.A pack of resistors is wired into the blower motor.The image is below.The power runs through the three resistors.The higher the speed, the less power there is.Some car makers run blower motor power through the fan speed switch first, followed by the blower Motor Resistor and then to the ground.Other car makers reverse that and run power to the blower motor first.
Resistors are similar to a flow restrictor in a pipe.Reducing current flow causes resistors to generate heat.The heat can cause the resistor to fail.To reduce the likelihood of failure, car makers locate the blower motor resistor in the heater duct.
Other factors can cause the blower motor to fail.For example.As a blower motor ages and wears, it can draw more current than the resistor was designed to handle.The blower motor Resistor will fail multiple times.
If the replacement for the blower motor has failed, it is time to conduct a test on the motor.The blower motor should be replaced if the draw exceeds manufacturer's specifications.
It can cause an overheating condition.In some cases, the heat is too high.It is common on late model GM vehicles.Do not attempt repair if you discover a melted connector.Purchase an upgraded pigtail and connect it to the harness.To rule out excessive current draw from the blower motor, conduct an amperage draw test at the same time.