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HomeTips & GuidesHow To Wire Dual Electric Fans: Single vs. Dual Relay Setup

How To Wire Dual Electric Fans: Single vs. Dual Relay Setup

Here’s a guide about how to wire dual electric fans in your vehicle error-free.

Electric fans are essential in a vehicle’s cooling system, and you can upgrade to a two-fan setup if a single one is inefficient.

However, wiring this system is not as simple as plugging the extra fan into the existing electrical connections. 

You have to make a few changes, particularly on the relay coil. Read on to learn more about the dual electric fan wiring.

Can You Use a Single Relay To Run a Dual-Fan Setup?

Yes, you can. But it is risky. The electric cooling fan setup has a high amp draw on start-up, which can blow the 30-amp fuse that protects the relay and wiring. Most relays have a 30-amp fuse.

If this fuse blows, the cooling system goes offline, meaning the engine will overheat.

A dual fan setup with a large and small fan

A dual fan setup with a large and small fan

You can install a high-current single relay controller, but this component is costlier than fitting two relays, one for each fan. Plus, this single relay setup does not enable you to control each fan separately.

How To Wire a Single Electric Fan

Before we get into the dual electric cooling fan wiring, let’s analyze the single fan setup for comparison.

This wiring setup is straightforward and contains these components.

  • Temperature sender (thermostatic sending unit)
  • 30-amp fuse
  • Relay
  • Electric fan
  • Override switch (optional)
A car cooling fan

A car cooling fan

The central component in this setup is the relay, which has four terminals. Terminal 30 brings the fused power supply from the battery’s positive wire via the 30-amp fuse.

Pin 86 connects to the ignition switch to control power flow into the primary circuit, which regulates the secondary circuit.

Terminal 87 is the positive wire or power supply line from the secondary circuit, so connect it to the positive terminal on the electric fan. 

The last terminal (85) should link to the ground (black wire) to create a closed-loop circuit in the primary low-power circuit.

Car relay switches

Car relay switches

If you connect terminal 85 and the fan’s negative pin to the chassis, the fan will run non-stop when you switch on the ignition. 

But we don’t want this type of cooling mechanism. The fan should only run when the engine coolant becomes hot.

Therefore, the circuit needs a temperature sender or thermostatic sending unit.

This component has two terminals. One connects to terminal 85 and the other to the chassis ground.

Usually, this temp switch creates a direct ground connection (contacts inside) at a specific temperature (around 185-200°F). When the engine cools to about 165°F, the switch disconnects.

A single cooling fan setup

A single cooling fan setup

How To Install the Thermostatic Sending Unit

Mount this component in one of the intake’s cooling passage ports and fix it using a high-temperature thread sealant to prevent coolant leaks.

After installation, mount one spade connector to the relay terminal and the other to the chassis. 

We recommend using a star washer if the chassis has protective paint above the metal to create a solid ground connection.

Single Relay Fan Wiring Diagram

Here’s the resulting wiring diagram, which you should use as a guide.

When you turn on the ignition, the primary coil remains off as long as the temperature sender senses the heat is below the threshold temperature.

But once the temperature rises past the threshold, the sender’s internal circuit contacts, creating a direct connection to the chassis ground.

This link energizes the primary circuit’s coil, which closes the secondary circuit switch to power the fan.

You can install an override toggle switch in the cabin to bypass the temperature sender should it fail.

For instance, if the fan is not running and the dashboard gauge shows the engine is hot, turn on this switch to lower the temperature. 

You can avoid immediate repairs and get to your destination without damaging the motor.

How To Wire Dual Electric Fans Using a Single Relay

This wiring setup introduces a second fan to the wiring. It also includes a 40-amp fuse and a high-current single relay controller to handle the high amperage draw. But the operating mechanism remains the same.

Wiring Diagram

How To Wire Dual Electric Fans Using Dual Relays

This wiring reverts to the 30-amp fuse and standard relay. But both relays share the electric fan sensor (temperature sender) data and the override switch. 

They also share the ignition switch control, which sends power to the primary circuit.

You can use dual thermostatic switches, but one is enough.

Circuit Diagram

Each fan has a separate relay to control the power supply, and each fan relay has a separate 30-amp fuse to safeguard the circuit.

You can still use the manual switch to ground the relays and power the fans manually should the temperature sender fail.

This dual setup is safer than the shared relay circuit because one fan can remain functional if the other fails.

For instance, fan two will remain online if the first relay wears out or the fuse blows due to short-circuiting.

 Should this fault occur, the cooling system will be half as efficient.

So, if you drive carefully without revving or overworking the engine, you should get home or to the mechanic without cooling issues.

An infographic showing how a car’s cooling system works

An infographic showing how a car’s cooling system works

When installing this additional wiring, crimp the electrical connections firmly and cover them using heat-shrink tubing for protection to maximize reliability.

It is vital to note that upgrading your vehicle’s cooling system from a single to a dual fan increases power consumption. Therefore, you should ensure the alternator can handle the additional load.

Wrap Up

Once you understand the relay terminal functions and how the temperature sender works, wiring dual electric fans to your vehicle will be a walk in the park.

And even if you are new to wiring, the circuit diagrams above should make it easier to understand the controlled power flow that turns on the cooling fans.

Should you need wires for this electric fan wiring, contact Cloom Tech to get custom wires at reasonable prices.

I am Lillian Yang, having been a sales manager for over 10 years.

I have received many positive reviews from customers. They have praised our excellent service, on-time delivery, and high-quality cable assemblies.

For your projects, please provide cable assembly files/images/smples, etc., so that I can send you a quotation within 24 hours.

Contact me now and let’s get started on building your wire harnesses!

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