To replace a resistor on an air conditioner blower fan, first check the unit for other defects to be sure that the resistor is faulty. Check the heating, ventilating and air conditioning fuse, fuse circuit, and blower motor for defects.
Start by locating the blower motor and resistor. To do this, check around the motor for a 2- by 3-inch plate usually held in place by either two or four screws. To ascertain that the resistor is faulty and needs replacement, test it by turning the fan control to all positions to check the speeds, taking note of the speeds that work and those that fail to work. A faultless resistor enables the fan to work on all the speeds.
If the fan speeds fail to work, check the heating, ventilating and air conditioning system fuse. Remove a faultless fuse, turn on the ignition key, and check if the volts reach the fuse with a voltmeter. If the voltmeter does not indicate any reading, check the fuse circuit for any defects. Check the relay terminals to see if there is power at the fuse, and replace the relay if necessary.
To check the blower motor, turn on the ignition key and fan, and check the relay and fuse for defects by testing the electrical connector voltage. If the two blower motor resistor terminals have power, replace the blower motor. Replace the resistor if there is no power to one of the two terminals.