How To Troubleshoot A Radiator Fan Not Turning On
Radiator Fan Fuse
The very first thing you're going to want to check is your radiator fan fuse. This can be located inside your fuse and relay box underneath the hood. The vehicle demonstrated here is a 1998 Honda Civic. The fuse box is located by the passenger side fire wall. Every fuse box comes with a fuse diagram layout, reading this will give you the location of the radiator fan fuse you are looking for. Diagnosing a blown fuse is very simple, you pull the fuse, shine some light through it and see if the fuse element inside has been blown apart.
Radiator Fan Relay
The radiator fan relay is also located in the fuse box under the hood. Have a look at your fuse diagram on the fuse box lid or underneath the fuse box lid and locate the radiator fan relay. Once you've located the relay, you're going to want to jump the connector that plugs into the thermostat fan switch sensor. The thermostat fan sensor location for this 98 Civic is located on the thermostat housing. Now that you have jumped the thermostat fan sensor plug connector you are going to turn the ignition key to the position before it starts the vehicle, in this case it was 2 clicks. Now your Ignition key is turn to the correct position and you have your jumper wire pushed into the thermostat sensor connector, now you are going to gently tap on the radiator fan relay and see if the radiator fan kicks on for a second. If the radiator fan jolts on for a split second this tells you that you have a bad radiator fan relay and it needs to be replaced. If it does not kick on at all, then more than likely the radiator relay is not the problem.
Radiator Fan Switch
A bad radiator fan switch will most definitely cause the radiator fan to never turn on, or turn on at the wrong temperature. Testing this theory is simple, unplug the radiator fan switch connector plug from the thermostat sensor. Take a jumper wire and push it into both female pins inside the connector. Make sure you have a good charge on your battery and the Ignition key is turned to the correct position. If the radiator fan turns on then it is most definitely a bad radiator fan switch sensor and replacing the sensor will resolve the problem. If you are quick enough you can replace the radiator fan switch sensor without draining the radiator fluid, make sure you get a small pan underneath it to collect the anti freeze. If you are going to drain the anti freeze you will need to locate the radiator drain plug. Some radiators do not have drain plugs and you will have to remove the lower coolant hose from the radiator to drain it.