How to Wire Dual Electric Radiator Fans | Two-Relay Method

Dual electric radiator fans need two 30‑40 amp relays with 10 AWG wire and a thermostatic switch wired to ignition power for safe automated operation.

Wiring two electric cooling fans into your car or truck is a straightforward upgrade that keeps engine temps in check during heavy traffic or off-road crawling. Here’s exactly how to wire dual electric radiator fans without guesswork.

What You’ll Need for a Dual-Fan Install

Gather the right parts before you start. Undersized components are the most common cause of blown fuses and melted harnesses.

• Two 30–40 amp automotive relays — one per fan. A single relay for both fans is only safe if each fan draws under 15 amps.
• 10 AWG main power wire from the battery to the relays. For the load side (relay to fan), 12 AWG is recommended for high-draw fans; 14 AWG is the minimum.
• 16–18 AWG control wire for the relay trigger circuit (pins 85 and 86).
• Inline fuses: one 30‑amp fuse per relay on the battery feed, plus a main fuse sized just above the combined draw of both fans.
• Thermostatic switch — screws into the block or radiator and completes the ground circuit at your target temperature, activating the relays automatically.
• Heat shrink, terminals, and a crimp tool for permanent, vibration-proof connections.

If you’re selecting fans for this setup, our tested roundup of the best electric radiator fans covers models that pair well with dual‑relay wiring.

How to Wire Dual Fans Step by Step

Disconnect the battery negative cable before starting. Follow this sequence in order.

1. Run main battery power. Connect a 10 AWG red wire from the positive battery terminal to an inline fuse holder, then run the fused output to pin 30 on both relays.
2. Ground everything properly. Run a 10 AWG black wire from each fan’s negative terminal to a clean, unpainted chassis ground or the battery negative. Connect a 16–18 AWG black wire from pin 85 of both relays to the same ground point.
3. Connect fan power leads. Run a wire from pin 87 of relay 1 to the positive terminal of fan 1, and a separate wire from pin 87 of relay 2 to fan 2’s positive terminal. Separate lines prevent voltage drop on high-draw fans.
4. Wire ignition-switched power. Connect a 16–18 AWG yellow wire from an ignition-switched 12V source to pin 86 on both relays. This keeps the fans from running when the engine is off.
5. Install the thermostatic switch trigger. Splice the black ground wires from pin 85 of both relays together and connect them to the thermostatic switch. When coolant hits the temperature threshold, the switch grounds the circuit and both relays engage.
6. Test and secure. Crimp and solder every connection, cover with heat shrink, and mount the relays away from exhaust heat. Reconnect the battery and verify the fans cycle on as the engine warms up.

Champion Cooling’s relay wiring guide provides additional detail on terminal identification and common faults.

Common Wiring Mistakes That Cause Problems

Three errors show up repeatedly in failed dual-fan installations. Each is easy to avoid once you know what to watch for.

Single relay overload. Running both fans off one relay is the most frequent mistake. If each fan pulls more than 15 amps, that single relay overheats and fails. Two relays, one per fan, is the only reliable approach for standard aftermarket fans.

Undersized main power wire. Dropping to 14 AWG or thinner for the battery-to-relay run creates voltage drop that slows the fans and heats up the wire. Stick with 10 AWG for that main feed and you’ll never have to re-run it.

Bypassing the ignition switch. Connecting relay coil power directly to the battery instead of an ignition-switched source keeps the fans running anytime the circuit is live, draining the battery overnight. Always use a circuit that’s hot only when the key is in the run position.

FAQs

Can I use one relay for two fans if they’re low-draw models?

Yes, but only if the combined amperage stays under 30 amps and each fan draws 15 amps or less. For most aftermarket 12-inch or larger fans, dual relays are still the safer bet since inrush current at startup can briefly spike well above the rated draw.

What happens if I wire the thermostatic switch backward?

The switch is a simple ground trigger — it has no polarity. If your fans run constantly, the switch is likely getting ground from somewhere else, or you’ve wired the relay trigger circuit to a permanent 12V source instead of an ignition-switched one. Recheck the wiring to pin 86.

Do I need a diode across the fan motor terminals?

Not for a standard street-driven vehicle. Diode protection is mainly for high-performance racing setups where electrical noise from fan shutoff can interfere with sensitive ECU signals. For daily drivers and mild off-road builds, clean connections and heat shrink are sufficient.