August 4, 2023

Electric Fan Control

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Hemmings U.S. Radiator

Electric Fan Control

By Jefferson Bryant

Many hobbyists are ditching the power-killing mechanical cooling fan in favor of an electric fan(s), and for good reason. Electric fans are more efficient, more compact, and most importantly, they don’t kill your engine’s horsepower. Installing an electric fan is typically done with a thin shroud that encases the engine-side of the radiator, and then wired up. The wiring side of electric fans is full of peril, however, and can leave you on the side of road if done wrong. In the worst-case scenario, an improper wiring job can actually burn down your entire car.

Wiring is often a gearhead’s worst fear, but it doesn’t need to be. If you pay attention to the details, anyone can wire up a set of fans and get the air flowing with ease. There are multiple ways to get the job done, it just depends on your preferences and engine management options.

Static wiring

The simplest option is to wire your electric fan to run when the key comes on. This works, but it does a few things that are less than desirable. First, you are increasing your current draw on the electrical system. While you may not thing that 15-35 amps is much, if you have an older engine with a 78-amp alternator, it is big chunk. Even the newest engines use 120-150-amp alternators, so you are putting a consistent draw on the system when it isn’t necessary.

The other issue is that the fans run whether the engine is at the optimal temperature or not. This means that the coolant can drop below the optimal temperature range and it can take longer for the engine to get up to optimal temperature, which is a real issue in the winter for running the heater. The correct wiring for an ignition switch fan trigger isn’t much different from that of a basic stand-alone temperature-controlled trigger, you are far better off with the latter.


If you have swapped a modern engine into your older vehicle, you may want to take advantage of the ECM cooling fan circuit. For engine such as the GM LS-series, the ECM puts out a 12v+ signal to the cooling fans, which you can easily utilize to power up the fan(s) when the ECM tells them to. This can also trigger a secondary fan when the AC system kicks on to boost the air flow for the AC condenser. OEM and aftermarket ECMs have the ability to trigger the fans, you just need to know what style of fan output the ECM has.

There are a couple of major caveats to this, however. Some newer engines, such as the GM 2014-up LT-series engines use PWM (Pulse Width Modulation) fans, which instead of a simple off-on 12V signal, the fans variable voltage to control the fan speed. These engine ECMs can be configured to trigger a relay, but that requires specialized tuning software. You need to be aware of the fan control style when you use one of these new engines.

The other potential pitfall is that the factory trigger wires are often just that- a simple trigger for a relay. The wires in the harness are not big enough to flow the required current, and can burn up if you use them to directly power the fan(s). Instead, you need a relay.

Stand-alone Temp controlled

For carbureted and stand-alone EFI engines, there is usually not a fan control option like a full-blown factory-style EFI system. This is easily remedied with a stand-alone fan controller. These are available for standard 12v and PWM fans. U.S. Radiator offers a few different fan control options. With a stand-alone controller, you get control of the start-up temperature and temp probe location, to best fit your cooling system. The solid-state SS-1 and SS-2 fan controllers use a temp sensor that mounts under the thermostat housing bolt. Other controllers may use a probe that installs into the radiator fins, or a typical thread-in sensor. Some controllers have an option for an AC compressor input, which triggers a second fan when the AC is turned on, or a dual-stage temperature control, to turn on a second fan when the engine reaches a higher temperature.

Relays and wiring

Having the right wiring is paramount to a successful electric fan installation. U.S. Radiator offers fan relay wiring kits, which have a simple thread-in temp sender, relay, fusing, and wiring harness to simplify the installation process. Most electric fan installations use relays to power the fan, so the controller is just a trigger wire. For most single fan installs, you need a 30-amp fuse at the battery for the main power line. The fan and relay can be grounded at the same point, and then the trigger wire connects to the relay to trigger the fan.

A minimum of 10-ga wire is required for nearly all electric fans. 10-ga wire can carry 30 amps across a distance of up to 10 feet. If you need a longer run of wire, then 8-ga wire would be more suitable. Verify the amp draw of your fan(s) before installing the wiring, as you may need to go to larger gauge wire. Most kits come with 10-gauge wire for electric fans.

Thermostat temp matching

Some controllers use a basic temperature probe which simply monitors the coolant tamp, the electronics in the controller turn the fans on and off. Some senders are actually electric thermostats that have 20-degree range of operation, for example a 185-degree fan control temp sender will turn on at 185 and above, and then turn off once the temperature drops below 165 degrees. You can match the temp sensor for these to the thermostat in the engine, within about 5 degrees, for optimum cooling system efficiency.

Electric fans are more efficient than mechanical fans, but they absolutely must be wired correctly and safely to protect you and your vehicle. It is not difficult to do, you just need a few hand tools and some patience. As a side note, don’t forget to clean up the wiring with wire loom and use zip-ties or wire clamps to secure the wires from any damage, especially around the fans themselves. Reach out to U.S. Radiator to at 1-800-421-5975 for help selecting the correct thermostat and fan trigger temp for your specific application.