Fan Relay Wiring: Your 5-minute Why and How-to Guide

It is midday in summer, your car cooling fan is blowing, and then everything comes to a grinding halt.

What do you think might have happened? 

In most cases, the fault is terrible fan relay wiring.

Reading along, you will learn all about your vehicle’s cooling system.

In particular, you will find how necessary the fan relay wiring is to the system.

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What is the Cooling Fan Relay?

Cooling fan wiring diagram

A cooling fan relay controls when the electric radiator cooling fan turns on and off.

It does so based on the temperature information from your car’s electronic control module or a thermostatically controlled sensor. 

For example, if the thermostatically controlled sensor measures an abnormal engine temperature, it signals the cooling fan relay to send a 12-Volt electric charge to power the fan.

Consequently, your radiator receives a constant gust of cool air bringing its temperature down.

Once the temperature drops below the threshold, your car’s electronic control module sends a new signal to the cooling fan relay instructing it to switch off the charge. 

However, you will find cars with electric cooling fans running after the engine shuts off. 

What common symptoms indicate you may need to replace the Cooling Fan Relay?

Old relays

Here are the typical signs that you need to replace your cooling fan relay:

  • Your engine temperature is higher than usual. It means the fan relay does not respond to the thermostatically controlled sensor or electric control module signals.
  • Your electric fan continues to run even after you turn off the ignition. Here, you may have a blockage within your fan relay system, causing the switch not to toggle between on and off.
  • Air-conditioning does not function. Some vehicles’ air conditioning compressor fans also use the fan relay to turn on and off. Consequently, they fail to turn on whenever there is a fault with the cooling fan relay system.

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Where Do the Four Wires Of an Electric Fan Relay Go?

A relay without housing

Use our fan wiring diagram and guide to ensure you correctly wire your fans to your relay.

Before that, let’s have a clear picture of where the four wires of an electric fan relay go.

  • First, wire 85 connects the relay to the thermostatic switch, determining when the fan is operational. 
  • Secondly, wire 86 is the connection to your car’s ignition switch.
  • Third, Wire 87 connects to the electric fan’s positive wire.
  • Finally, wire 30 links to your car battery, requiring a constant 12-Volt current.

The wire insulation colors may vary depending on your electric fan relay kit supplier.

How to Wire an Electric Fan with A Relay?

The type of electric fan cooling system you use depends on your required cooling.

As such, you can opt for either single or dual cooling fans.

Additionally, the installation procedure may vary depending on your electric fan relay kit.  

Wiring a Single Cooling Fan.

  single electric fan

  single electric fan

  1.  First, remove your car’s stock mechanical fan and mount your electric fan onto your radiator.
  2.  Then, install the single relay where it is safe from excessive heat and water. Exposure to these elements can compromise the functionality of your new cooling system.
  3.  Next, connect each of the wires as per the wiring diagram. For example, wire 30 only connects to the fan’s positive terminal.
  4.  Finally, if all connections are correct, your engine should receive sufficient airflow to cool it.

Wiring Dual Cooling Fans.

Dual electric fans

Dual electric fans

The process of wiring double electric fan relay systems is similar.

However, the space available may cause you to make some adjustments.

For example, you can use a single activation fan relay for both fans.

However, doing so will require you to use a higher gauge connector wire and fuse a slightly higher amperage.

Below is the procedure for installing a dual-activation Fan Relay.

  1.  First, remove your car’s mechanical fan and mount the dual electric fans onto your radiator.
  2.  Secondly, install the activation fan relay kit in a heat- and moisture-free location. Depending on their required power, you can opt for a single activation fan relay for both fans. However, you will need a dual-activation fan relay if they draw more than 15 amps each. Cloom recommends the AR-79 with an amperage rating of 60 amps.
  3.  Next, install a 30-Amp fuse on both wire 30s of the dual activation fan relay and splice them to a single wire before reaching the battery. You can also add a circuit breaker to the setup to prevent short circuits.
  4.  Then, splice both wires 85 from the dual activation relay and join them to a single conductor that connects to the thermostatic switch. Additionally, make sure that you have a ground connection from both fans as well. Finally, splice the wire 86 of each relay and conjoin them to a single conductor that connects to your ignition switch or your car’s fuse box.
  5.  With all connections in the right place, your engine cools even when idle.


The information is the standard installation procedure for Cloom electric fan relay kits.

Also, you can refer to the wiring diagrams for each type to ensure that you have the correct connections to complete the circuits. 


Why is an Electric Fan Better Than a Mechanical Fan?

If you take into account a number of things, then you can effectively use a mechanical fan for high HP (horsepower) applications.

A shroud, bigger radiator, and bigger clutch fan would most likely keep the engine cool; a mechanical fan could, in some cases, function better than an electrical fan.

With some, not only are the electrical fans keeping the engine cool in traffic but also at stop lights. A mechanical fan can only keep things as cool as possible according to the engine speed.

However, an electrical fan draws the max cfm when idle since it works independently of engine rpm.

The only setback to installing additional electrical components to your car is that, in most cases, the original alternator wasn’t designed to facilitate current draws of over 45 to 60 amps.

Therefore, if you add a dual or single electrical fan, you’ll need to use a beefier alternator in order to increase your charging system’s output.

How to Wire the Fan Relay?

Relays are relatively simple, and there’s a standard 5 or 4-prong relay that almost everyone uses. The 5th prong (87b or 87a) is not usually used unless for particular circumstances. 

On a standard relay, there’s a switch system working at a lower amperage than what the component uses.

A relay protects a switch circuit since most automotive sockets aren’t designed to convey high currents. The relays consume the current turned on by a triggered connection either by a temperature transmitter or switch.

Hence, why are two wires of a bigger gauge size compared to the other two?

To turn on the relay, the 2 smaller wires create a connection from ground to hot; over 12 V are received from the ignition source, and a negative connection is received from the temperature transmitter designed to turn on at 1850C.

Once the connection is created, a magnetic pull completes the connection between 30 and 87 terminals, totally isolated from the switch connection.

Despite it not being crucial to connect the 30 to your battery (12 V power source), it’s the standard. That connection provides the 12 V to the component (fan) connected to terminal 87. 

Relays are capable of handling loads of higher current compared to switches. However, even relays have current rating capacity they can’t exceed.

If exposed to an excessively high current, the circuit will open, and the relay won’t work. Therefore, it’s essential to use a relay with the current load rating that your fans need.

Fan Relay

Caption: Fan Relay

When Do You Need a Second Fan Relay Kit?

With a smaller fan, you can connect both fans to one relay and install a fuse to protect your circuit while still providing sufficient current flow without going over the fuse rating. If your fuse blows, this is a sign that you’re drawing too much power on your system and need to add a second relay kit.

If you have a bigger fan that draws over 15 amps, it’s best to install a second relay kit.

Will a Custom Relay Wiring Harness Make Installation Easy?

If you buy a relay kit, it contains all the necessary connectors to splice wires together. It’s highly recommended to use a good crimp connector and follow the instructions provided in order to ensure you have a hassle-free installation.

However, you can make the installation way easier by customizing your own relay wiring harness.

For instance, an immaculately configured relay wiring harness has fuses in the power supply side of the headlamp power circuit, very close to the power pickup point, inches away from the alternator B+ terminal or battery positive terminal. 


As you can see, an electric fan relay is necessary to ensure the painless performance of your engine cooling system.

Additionally, you can also add an air conditioning relay.

It allows extra automation from the ac pressure switch for additional temperature readings. 

Consult Cloom for all your automobile wiring harness solutions and installation guides.

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Hey, I am John, General manager of Cloom and OurPCB.

I am a responsible, intelligent and experienced business professional with an extensive background in the electronics industry.

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