The most regular cause of fan speed control failure is a resistor failure at the fan’s motor, although it can equally be a defective control switch or burned-out fan motor.
Common reasons for this problem:
- Blower resistor malfunction
- HVAC control panel is bad
- Blower motor is faulty
How the repair is done:
The mechanic will carry out a thorough inspection of the HVAC system. The mechanic will check for motor operation by taking out the connector and checking the condition of both male and female sides. They will probably test motor operation by connecting a battery via alligator clips to the motor’s plug pins and see if it runs direct from that source.
If the motor works fine, the next place is checking the resistor. The mechanic will unplug the resistor from its connector and inspect for burnt terminals. That often shows a bad resistor. To check resistor functionality, the mechanic will use a multimeter set to the resistance function, then measure the resistances from a common point on the connector to the various pins to make sure the required Ohm values are up to spec.
The mechanic will be looking for an “open” condition (infinite resistance), which shows a defective circuit.