The most likely cause of fan speed control failure is a resistor failure at the fan’s motor, although it can also be a defective control switch or burned-out fan motor.
Common reasons for this to happen:
- Blower resistor malfunction
- AC control panel is bad
- Blower motor is faulty
How it’s done:
The mechanic will perform a thorough inspection of the AC system. The mechanic will check for motor operation by removing the connector and checking the condition of both male and female sides. They will test motor operation by connecting a battery to the motor’s plug pins and see if it runs direct from that source.
If the motor works fine, the next step is checking the resistor. The mechanic will unplug the resistor from its connector and check for burnt terminals. That often signifies 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 reveals a defective circuit.