How to Troubleshoot a Blower Fan Not Workingby Troy Lambert
To determine the cause of a blower fan failure, isolate the motor, and perform some basic electrical tests. Bring broad knowledge of electricity and specific knowledge of the use of electrical testing equipment. A wiring diagram for the specific vehicle is also helpful. Observe safety precautions while working with electricity.
Follow the Power
Check fuses first. Fuse failure often causes component failure. Replace fuse if blown; if not, proceed to Step 2.
Check for power at the motor itself. Using a test light or a multimeter set on volts, unplug the blower motor. Keeping the black lead on a good ground, usually a bolt on the engine block, and the blower switch turned on, use the red lead to probe the plug that usually attaches to the blower motor. The meter should read between 9 and 11 volts or the test light will light up. If the voltage is right or the test light illuminates, go to Step 3. If not, skip to Step 4.
Test the connection to the blower motor. Take two jumper leads, one black and one red. Connect the black lead to a ground and then to the blower motor assembly. Connect the red lead to a power source and then to the leads on the motor side of the disconnected connector. If the motor turns, the connector is bad. Repair the connector. If the motor still does not turn, the motor is bad. Replace the motor.
Test the switch. Plug the connector back in to the blower motor, and then trace the wire harness back to the connector closest to the blower motor switch, usually located under the dash. Disconnect the connector and set the multimeter to ohms. Place one lead on one prong of the connector and the other lead on the power in prong of the connector, taking care that they do not touch each other. If you turn the switch on, you should get a reading between .01 and .03 ohms. A slightly higher reading is acceptable because switches vary in resistance. If you get no reading on any of the switch settings, the switch is bad. Replace it.
Test to see if the switch is getting power from the battery. Leaving the switch disconnected, switch the multimeter to volts again, or use a test light. Grounding the black lead as before, use the red lead to probe the connector. Usually a green or red wire will be the hot, or power, wire. Consult the wiring diagram for the specific vehicle if the wire color is unknown. If the meter shows the correct voltage, or the test light illuminates, the switch is getting power, skip to Step 3. If it does not, proceed to Step 2.
Look for a break in the wiring harness if the test light does not light, or the meter shows no value, there is a break in the wiring harness between the switch and the battery. Follow the wiring harness, checking the hot wire for voltage at every connection, until you find the break. Repair the break.
Look for a break in the wiring between the switch and the fan if the meter shows voltage or the light illuminates and you have tested the switch and motor. Follow the wiring harness as carefully as possible between these two locations, looking for burned out spots or breaks in the wire. When the break is located, repair or replace the wiring harness.
- If you find neither the motor nor the switch faulty, you may want to leave the deeper troubleshooting to a professional.
- Beware of shock hazards inherent in working with electrical systems, and use caution.
Items you will need
- Jumper leads
- Test light
- "Haynes Techbook, Automotive Electrical Manual"; H. Haynes; 1990
- Digital Multimeter image by TekinT from Fotolia.com