First things to check would be the FAN fuse under the dash. If it is blown, the fan relays won't have power and won't turn on the fans.
Next thing you can check is the fan relay. It is located in the engine compartment, on the firewall, in the corner right by the brake booster.
Unplug the relay.
Identify the wires on the connector:
- an orange wire (might be actually red on some old models) that supplies 12V power
- a black/red wire which powers the fan
- one thinner brown wire for relay power
- one thin green/white wire from the ECM
(for simplicity, I will stick to the newer wire colors - please substitute them with your wire colors if you have an older engine model) Wiring diagram
Measure the voltage (or use a test light) on the main orange wire. If you don't see 12V, you have power supply issues. Check the wiring and especially the fusible links.
Using a paperclip, jumper the orange power supply wire to the black/red fan wire. Does the fan turn on? Likely a relay issue or ECM wire problem. If it doesn't turn on, it's probably a bad fan motor.
Measure the voltage on the brown wire. With the key on, you should see 12V. If not, you have fuse or wiring issues.
Test the green/white wire. Plug the relay back in. Ground the green/white wire and turn the key on. Does the fan turn on? No? Bad relay.
If all of this checked out and your fan still doesn't run, there might be other reasons.
- The computer might not be seeing the engine temperature, thus not needing to cool it. Normally, the ECM turns on the primary fan at above 220 degrees F.
- The wiring between the ECM and the relay might be bad. Although, this happens pretty rarely, it does happen.
On vehicles with working A/C system, the fan should come on when the A/C is turned on.
On vehicles with dual fans, the secondary fan comes on when the engine reaches critical high temperatures (238F+). It is actuated by a grounding switch in the passenger side head, between the last two spark plugs.