HomeHowCooling Fan Relay: How Does It Work?

Cooling Fan Relay: How Does It Work?

Generally, the combustion engine in gasoline cars produces heat, cooling fan relay is very important. It’s normal then if your car becomes hot after hours of driving.

Too much heat (overheating) is bad for the engine. To prevent it, automobiles have cooling fan relays to cool the engine.

But there’s more to how they work; we’ll discuss that in this post. First, let’s find out what it is and its function in a car.

Table of Contents

What Is a Cooling Fan Relay?

A cooling fan relay is an electrical switch that controls the cooling fan. It works hand-in-hand with the radiator since the radiator contains the cooling fan.

When you activate the relay, it sends air into the engine’s compartment and modulates the temperature. If your engine is hot, this airflow will cool it down.

This tool plays a critical role in the performance of your car. If it fails, many problems will arise.

You’ll find the cooling fan relay switch behind the relay on the electric fan assembly. However, in some vehicles, it stays in the relay center or underhood fuse.

Also, there are different types. The most common is Normally Open relays, while the others include Normally Closed and Dual relays.

Car cooling fan relay switch

(Car cooling fan relay switch)

How Does a Relay Work?

The cooling fan relay switch works based on signals from your car system. These signals usually come from a thermostatically controlled sensor in the engine control unit.

In some older cars, there’s no sensor; the signals come directly from the electronic control module (ECM).

The sensor or ECM will send signals if they dictate any unusual temperature in the engine. These signals are typically low voltage, so they don’t affect other car system parts.

When it receives the signal, the relay returns an electric charge to the radiator to switch on the cooling fan. The cooling fan kicks in and sends air flows to the engine.

The air flows will continue until the sensor or ECM informs the relay that the temperature is normal again. Then, the relay returns another electric charge to the radiator to switch off the cooling fan.

With this, the relay is an intermediary between the sensor/ECM and the radiator fan. Suppose the signal doesn’t get to the relay switch. In that case, the radiator cooling fan cannot turn on. 

That’s risky, yet it happens to many vehicles. And it’s usually due to a bad or failing one. It helps, then, to know when you need a new kit for your car. 

Turning on the cooling fan in a car

(Cooling fan under the hood)

Symptoms of a Bad Cooling Fan Relay

Your car’s relay will go bad with use over time; it’s normal. When that happens, Getting a replacement cooling fan relay kit is critical. Else, the cooling fan will remain in an open or closed position.

Here are some symptoms of a malfunctioning relay, so you know when you must replace it: 

Engine Overheating

As explained, the primary duty of a cooling fan is to cool the engine. If your engine stays hot most of the time or overheats, it most likely means the relay isn’t working. It would activate the cooling fan to prevent overheating if it worked.

Nevertheless, note that other factors contribute to overheating in cars. Like a low oil level, broken water pump, and belt and hose issues.

The point here is that you can’t conclude on your own. Take your vehicle to an expert mechanic to diagnose the exact cause of the overheating.

Turning on the cooling fan in a car

(An electrical engineer inspects a car engine)

Fans Not Coming On or Not Going Off

Here’s a textbook symptom of a bad cooling fan relay switch. The relay is what turns on the cooling fan in the radiator.

Hence, if it goes bad when the cooling fans are off, they will not come on.

It’s similar if the relay goes bad when the cooling fans are on. In such a situation, the fans will not go off. If your car has these symptoms, consider adding getting a new cooling fan relay kit to your bucket list.

Turning on the cooling fan in a car

(Turning on the cooling fan in a car)

Checking for Problems: How To Test Cooling Fan Relay

Testing your vehicle’s cooling fan relay is simple. Here’s what you should do:

  • Turn on the car engine
  • Allow it to steam until it reaches standard operating pressure
  • Switch on the air conditioning system

The cooling fan will come on if the relay is working properly. Otherwise, nothing will happen.

Your cooling fan relay can also go bad if any part inside it breaks. Hence, you can test it somewhat traditionally. Remove the relay component from the car and shake it.

Checking if the air conditioning system is working

(Checking if the air conditioning system is working)

Cooling Fan Circuit Problems

While the cooling fan relay plays an important role, it’s not the only component in the cooling fan circuit.

Others include the fan motor, temperature sensor, and control module.

Among these components, the one with the highest tendency to fail is the relay switch; that’s true.

Nevertheless, if your cooling fan isn’t working, it’s also ideal for checking the other components. To eliminate all doubts, at least.

Sometimes, the cooling fan relay switch suffers high electrical loads within the circuit even when in perfect condition. That, too, can affect its performance. In particular, it affects the performance of the relay coil.

Earlier, we learned that the signals from the sensor or ECM to the relay switch are typically low voltage. 

How about the resistance to the current in the cooling fan circuit? A perfect working cooling fan relay coil will resist 40 to 80 ohms. Anything higher than that may break the coil. And if the coil can no longer hold resistance, the relay switch cannot work. 

Car relay fuse box

(Car relay fuse box)

Steps To Install a Replacement Cooling Fan Relay Kit

To change a relay kit, you can take it to an expert mechanic. But if you want to save the dollars, you can do it yourself.

It’s not overly complex. You only need a small screwdriver and a set of pliers. Before going about it, ensure your car is off.

Here’s what you should do:

  • You’ll find it in the relay box or under the fuse. And removing simply involves unfastening fasteners and clips. Note that you’ll find many relays, especially if it’s in the relay box. Be sure the one you remove is for the cooling fan. Usually, there should be a print on each relay box for easy identification. If there’s none, consult your car’s manual.
  • Before installing the replacement kit, check that it matches the old one. Both relays must be the exact dimensions in particular. Also, review the orientation and terminal number. If there’s a variance, you most likely bought the wrong one. But if there’s none, continue.
  • Place the new relay in the position where the old one was. Press it into place; gently so it doesn’t break. Then refasten the fasteners and clips.
  • Your eyes may tell you the new relay switch was installed successfully. However, you can only be sure it works when tested. The best way to do this is by turning on the air conditioning system. If you hear the whirls and swirls of the cooling fan, you’re good to go.
Cooling fan under the hood


The cooling fan relay switch keeps the car engine temperature in a favorable condition. Without it, your car will suffer overheating and other serious problems.

If your switch goes bad, you must immediately replace it by following the listed steps.

Our guide has listed the easy steps to get this done fast and efficiently without breaking the bank.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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