Cooling fans coming on as soon as I turn the key in the ignition?

PAULPOLO

MEMBER
2005 MITSUBISHI LANCER
2.0L
4 CYL
2WD
AUTOMATIC
117,500 MILES

The problem I am having has to do with the cooling fans coming on as soon as I turn the key in the ignition. The engine is cool. I do not have a problem with the overheating now. The fans run erratically at times, and sometimes they run at a normal speed, but they are always on when the car is running. They dont run once I turn off the car. What elements do I need to look into: control unit, temp sensor, relay, thermostat? Please help.

1 Reply

KASEKENNY

MECHANIC
18,907 POSTS

More than likely this is a relay issue but we need to verify this first.

Here is a guide that will help with checking a relay:

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

What I would do is remove the fan control relay and make sure you have 12 volts on pin 1 and 4. Then check the control side by putting your meter lead on battery positive and the black lead on pin 2. Then start the engine and see if the fan is being commanded on.

If it is not then your relay is shorted and needs to be replaced. If it is then we could have a temperature sensor issue. Basically if the temperature sensor is telling the PCM that the engine is hot when it is not then it will turn the fan on.

https://www.2carpros.com/articles/symptoms-of-a-bad-coolant-temperature-sensor

Basically to test this, you need to monitor the resistance across the sensor when you are heating the sensor in water. It should move steady and not jump around.

Please find the wiring diagram below that will help with all this. Please let us know if you have questions and what you find. Thanks

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