Fan Relay Wiring: Check it first before replacing your cooling system


As the system gets heated, you need a mechanism to cool it. Most equipment uses cooling fans that take the heat out of the system. These cooling fans are based on a few components, and the fan amp relay is an important part. Let’s see how the cooling fan relay wiring works.

Table of Contents

How a Cooling System Works

Liquid cooling is a well-known fact in an automotive engine. As the circulating coolant draws heat from the block and cylinder head by conduction, the radiator releases it through specially engineered fins. Here, convection carries the heat to the radiators. Because of what it does, the other name for the radiator is a “heat exchanger.”

As part of the cooling system, the vehicle air conditioning system has a condenser – another heat exchanger in front of the radiator that needs airflow. Also, two fans are available, running at a slower speed, depending on the vehicle’s configuration.

There are two ways to control the fan’s speed in a machine: a module or two-three relays working together to run the fans. These electric fan relay wiring can be in series to split the voltage or in parallel so that both receive full voltage simultaneously. Moreover, you can energize one of the two fans at once to control the speed.

While driving, air naturally moves through the radiator. However, when it is idle, the fan pulls air through it to lower the coolant’s temperature, returning to the system and absorbing excessive heat. 

What Does a Cooling Fan Relay Do?

The cooling fan relay is a magnetic switch that handles the power going to the cooling fans in the engine. Moreover, it ensures that machines that use a lot of energy only run when necessary, usually when you stop the car.

As the engine’s temperature gets too high, the computer controlling the switch in the car turns on the condenser fan relay. When you turn on the relay, it completes a circuit, which lets electricity flow through it to the cooling fans. 

Radiator cooling fan

Caption: Radiator cooling fan

How Does a Relay Work?

A relay uses a small amount of electricity to turn on or off a much more significant amount of electricity. An electromagnet is what a relay mainly consists of, as it is a coil of wire which acts like a temporary magnet as the current flows through it. You can think of a relay as an electric lever. When you turn it on with a small current, it turns on another thing with more considerable wind.

Many sensors are sensitive parts of electronic equipment. They make small amounts of electricity. We need them to power more giant machines that need stronger currents. Relays fill the gap so small winds can turn on more significant flows.

working principle of fan relays

Caption: working principle of fan relays


Different Types of Cooling Fan Relays

There are different kinds of relays for cooling fans. The main difference is how they finish an electrical circuit by winding the armature.

Open Relay

An open armature is used in the most popular type of relay. The armature closes only when the relay coil is turned on. Now that the circuit is complete, electricity can flow to the cooling fan.

Closed Relay

This type of relay has a closed armature, the opposite of a normally open relay. The armature opens by giving the relay coil power, and the circuit is complete.

Dual relays

This type of relay has two circuits. One has an open armature, and the other has a closed one. Because both armatures are used, a dual relay can carry an electric current when available and close. 

Fan Relay Wiring

Here are the two popular setups of the relay.

Single fan relay wiring diagram

Caption: Single fan relay wiring diagram


Dual-fan relay wiring diagram

Caption: Dual-fan relay wiring diagram


Bad Cooling Fan Relay Symptoms

Keep an eye out for the following signs of a broken cooling fan relay:

The engine gets hot or gets too hot.

Keep an eye on the engine if it is getting too hot more quickly and without warning. A broken fan relay, circuits, or the fan itself could cause the problem. If the AC Condenser fan isn’t working, the condenser may stop cooling when the car is stuck in traffic, and the “pop-off” piston on the cooling system may discharge the high pressure due to high temperatures and make a loud noise with spraying mist from under the hood.

The fans don’t work.

Instead of unusually high engine temperature, the driver may observe that the fan has stopped turning. If a relay is broken or doesn’t work right, it can’t send power to the cooling systems. In turn, the fans that don’t have the ability won’t run, putting the radiator on its own.

Since the cooling fans aren’t moving air through the radiator, it can’t eliminate as much hot air as it generally does. It transmits still-hot coolant to the engine, which raises the temperature because the coolant isn’t doing its job of cooling down the motor.

Cooling fans keep running.

On the other hand, the cooling fans may continue to run even when the engine is off. If a relay is stuck in the closed position, the situation happens. But some cars are made to run the fans even when the engine is off. This strange behavior shows that there is something else wrong with the relay that controls the fans. The important thing is whether this just started happening or if you’ve noticed a new pattern.

If there is a break in the wiring in the relay coil, the relay contacts can stay open and never close. Power can keep going to the fans even if the control circuit is shorted or the relay contacts are stuck. Fans that run all the time will drain the battery.


If the cooling fan relay fails, you may see warning lights like the “Check Engine” and “Temperature” lights on the dashboard. However, you may first notice that the air conditioner stops cooling.

Bad performance from the air conditioner

A broken cooling fan relay can also hurt the performance of an air conditioner because, in most cars, the cooling fan also works as a condenser fan. The fan moves air over the condenser when the air conditioner is turned on, removing coolant, which changes from a gas to a liquid as it gives off the heat from the evaporator. It is why that part is called a “condenser.”

 Try the following methods to see if the cooling fan is the culprit to blame for overheating the engine.

  • As the engine is not working, try swapping the current relays from the other systems. If the engine starts to work after changing the relay, it will confirm your doubts.
  • Measure the coil resistance. The coil fails if it exceeds 80 Ohms, whereas a less than 40 Ohms value means the relay has broken.
  • Listen to the noises as the broken armature of the relay can produce a rattling sound.

Also, you can bump or thump the relay. If it clicks, the relay is most probably working fine.

Mechanic checking cooling fan

Caption: Mechanic checking cooling fan

Possible Causes of a Bad Cooling Fan Relay

Many things could cause a cooling fan relay to stop working. Some common reasons why relays don’t work are:

  • Failure of the workstation, the thermal switch, or the module controls the relay and turns it on or off.
  • When you switch between circuits with varying electrical loads and speeds, you can cause damage to the relays. The situation happens more often than you might think.
  • Failure of the relay because of a high current pull from a partially short fan motor or a fan circuit that shorts out often.
Vehicle engine system

Caption: Vehicle engine system

How to Replace a Cooling Fan Relay on Most Vehicles

To replace the relay, you can try the following steps.

  • Turn off the power of the machine. Locate the fuse relay box under the hood and remove its cover. Now, identify the cooling relay inside it.
  • Gently pinch the relay fan and pull it to disconnect. Then, take an identical relay to replace the original one.
  • Position the new cooling relay where you have removed the original. 
  • Verify that the new relay fan is correctly adjusted and working. 

For further maintenance, you should schedule a monthly examination of the relay panel via any professional relay experts. Also, you should use an amp relay or an electric fan relay kit for self-maintenance.


Cooling relays are essential to maintain the temperature inside the engine. However, they can overheat your car while producing rattling noises when they turn bad. So, it is better to keep an eye on the malfunction and replace it as soon as possible. Here at Cloom, we offer wiring harnesses to make your connection with attention to detail.