How To Check a Fan Relay

Different car models can have several cooling fans installed under the hood to keep the engine and the car air conditioner at the right temperature to work efficiently. A defective cooling fan or shorted wirings will cause the engine to overheat and stall. The cooling fan is connected to an electric motor which makes it run automatically when the engine is turned on. The other cooling fans only work when the apparatus connected to it is turned on, like the car air conditioner. The electric motor is connected to the power source through a relay, which acts as the switch that is controlled magnetically. The fan relay looks like a square plug that has four terminals. There can be as many as four fan relays in a car. Here’s what you need to do to check a fan relay.

  • Get out your car’s wiring diagram. Open the hood of your car and turn on the engine. The fuse and relay box is usually located at the flat partition under the hood, below the front windshield on the driver’s side. Depending on the car that you have, you may find a high-speed relay as well as a low-speed one. While the engine is turned on, look at the ground side of the relay coil to find the ground wire. Ground this wire and wait for the relay click. The fan or the fans connected to the relay with turn on if the relay and all the wiring are in good working condition. If the fan did not run, check the wiring. Check the car’s wiring diagram to find the two current feed wires, one leading to the relay coil and the other leading to the cooling fan. Use a test light to determine if there is power in these two wires. If only one of the wires has power it is likely that the circuit from the fuse to the relay is open or has a short circuit.
  • Disconnect the negative battery connector and remove the cooling fan relay. Check the continuity among all four terminals of the fan relay using an ohmmeter. If continuity still exists among the terminals, the next step is to check the operation of the relay by connecting the battery to the specific terminals. Use the ohmmeter to make sure that there is no continuity between the terminals. This means that the fan relay is defective and needs to be replaced.
  • Continue testing all the fan relays in your car and replace the defective ones. Use the two probes of the ohmmeter to test each individual fan relay terminal against the other terminals to determine that there is no continuity in their operation. This means that the terminals are not receiving electrical current. If the fan relay checks out, you have to check all the wiring system connected to the fan relays for locate open circuitry and frayed wires. You should also check the fuses and replace damaged ones.

Bring your car to the mechanic if you are not familiar with the electrical circuitry of your car. For a minimal service fee you can ensure that your car will be trouble free. If you are a person who likes to do things on their own, you can buy a wiring diagram for your car model in most stores selling auto parts. You can also search online for free wiring diagrams.


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