Radiator Fan Common Problems – When Should it be On?

If your car is having trouble cooling and is running extra hot, it could be from a cooling fan not turning on at the right time. A car can also have difficulty warming up if the fan is running constantly. Both of these things happen when the smallest of the cooling system components goes bad and is very common, so here is how to resolve it!

The cooling system of your car, with so many moving parts all work together to keep the engine running in a small window of the optimal operating temperature. When a temperature sensor or fan relay or other component goes bad it can be difficult to diagnose. I have had some experience troubleshooting these problems and here is what I learned and hopefully will help you get your radiator fan to be on at the right times.

How soon should the car cooling fans come on if you let the car Idle?

After starting your car, the cooling fans will turn on once the engine coolant reaches a certain temperature. Usually this is slightly higher than the temperature set for the thermostat to open. Most thermostats open at around 185F and the fan should come on at about 200F. Generally this takes 10 to 15 minutes of idling.

When the Radiator Fan Should Turn On

The radiator fan is normally controlled by the ECU (electronic control unit) which is the brains of your car. The ECU knows when to turn the fan on by the temperature sensor near the thermostat reading the temperature of the coolant and sends a signal to the fan relay.

The radiator fan is programmed to turn on once the car is above operating temperature to add airflow to the engine drawing air through the radiator. This is especially important when sitting in traffic since there is not much airflow while the car is not moving.

The fan is also most likely programmed to come on if the AC is on. This is because of the extra amount of work that the turning of the AC compressor puts on the engine.

So, the radiator fan should turn on once the engine is fully warmed up, or when the air conditioning is turned on.

If the Fan does not Come On at All

What is the problem if my radiator fan wont come on?

If you notice the fan does not turn on at all, it may be caused by a number of problems but here are the most likely 3 reasons.

  • The temperature sensor is bad. Generally located by the thermostat housing, this should be the first thing to check and replace since it is easy and inexpensive.
  • The fan relay may have failed. If the fan relay is stuck open, the fan will never turn on even if the sensor signals that it should. Replace the fan relay.
  • The fan motor may be bad. This can be tested by unplugging the fan and directly connecting a power source to it.

Before you start replacing components though, try warming up the car really well. Idling can take longer depending on the temps outside so take it for a ten minute drive and then let it idle. Allow the engine to get really warm before you are certain that the fan is not working. If the fan still does not come on you know you have a problem.

If the Fan Stays on Constantly

What if my radiator fan is constantly on?

A radiator fan that is constantly on may be triggered by the air conditioning when it is on. Be sure the air conditioning is off. If the radiator fan is still constantly on, it is most likely caused by the fan relay being stuck closed. Replace the fan relay. If this is not the issue the temperature sensor or ECU may have a problem.

You may have noticed the engine fan always on because your car is taking extra time to warm up. The fan always running will wear out the fan sooner and will prevent your car from warming up but will not harm anything besides this. Still it is a good idea to find the reason for the malfunction.

The first place to check is the fan relay. This is basically a switch, is usually a small black cube and is most often located in the fuse box. Tap on the relay to unstick it or buy a new one for a few bucks and replace it.

If the relay is replaced and the issue does not go away then the next most likely cause of a radiator fan that is constantly on is a temperature sensor that is sending a bad signal to the fan.

Radiator Fan On when the Car is Off

I have mostly worked on cars much older than me and recently was surprised when I heard that I should be careful around cars with electric radiator fans – that they can turn on even when the car is off! After investigating this, I have found it to be true. The radiator fans can indeed turn on even when the car is off. This makes sense but I was just surprised by it and here are the things I learned about the radiator fans turning on.

Should the radiator Fan come On When the Car is Off?

It is normal for the radiator fan to remain on even after the car has shut off to continue cooling the engine. Most cars are programmed to operate at about 200F. The fan should kick on once the coolant gets above about 200F and cars are programmed to allow the fan to cool the engine even if the car is turned off if the temp is above 200F.

No need to worry if you notice the engine fan stay on even after you park the car. This is normal and the fan may stay on for up to 10 minutes after the car has been turned off.

If the fan is on for longer than 15 minutes after the car is off though, you have a problem most likely with the ECU or the fan relay. Inspect both of these and replace them when necessary!

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