The quick answer: in cars, the cooling fan relay is usually located in the under-hood fuse/relay box and is labeled FAN or FAN RELAY. In most PC builds there isn’t a separate relay—the fans are driven by the motherboard or a dedicated controller via PWM. This article explains both contexts, how to locate the relay, and what to do if it fails.
Automotive cooling fan relays
Common locations by vehicle type
In most modern vehicles, the radiator cooling fan relay lives in the engine bay fuse/relay box. Some models place it in a separate relay pack near the radiator, the battery, or the front of the engine compartment. If you can’t find it there, the next most common spots are a fuse box under the hood or a secondary fuse/relay box inside the cabin. Note that a few cars use a coolant-temperature switch to energize the fan directly or through a controller rather than a standalone relay.
To locate the exact relay, follow these steps:
- Check the owner’s manual or the diagram printed on the inside of the fuse/relay box cover for a label such as "FAN," "FAN RELAY," "ENGINE COOLING FAN," or a similar designation.
- Open the engine bay fuse/relay box and visually inspect for a relay with the label and shape that matches the diagram; there are usually multiple relays, so confirm the correct socket.
- If there are two fans (engine and A/C), there may be two relays or a dual-relay pack; some cars share a single relay for both fans but drive them via a control module.
- If the relay isn’t obvious, check under-dash or behind the glove box fuse panels; some manufacturers relocate the fan relay there.
- For vehicles with a dedicated controller, the relay may be inside the controller unit or a nearby relay board rather than a standard automotive relay block.
- Inspect fuses first: a blown fuse can mimic a bad relay, and a visibly damaged relay (burn marks, melted plastic) should be replaced immediately.
- When testing, you can swap the suspected relay with another identical relay in the same box (e.g., an A/C or horn relay) to see if the fan behavior changes.
Concluding the automotive section: locating and testing the fan relay hinges on referencing the vehicle’s diagrams and confirming with a swap test or voltage check. If you’re unsure, consult a service manual for your exact year, make, and model or seek a qualified technician, as improper handling can lead to overheating or electrical damage.
PC and server cooling fan relays
How fan control differs in computers
Personal computers and most consumer servers typically do not use a separate relay to switch fans on and off. Instead, motherboard fan headers deliver a PWM signal or a voltage reference that the fan controller uses to regulate speed. High-performance or rack-mounted systems may employ a dedicated fan controller or a small relay/contactor box to switch larger fan banks, but that relay is part of the controller assembly rather than a single standalone component like in cars.
Where to look if your system does use a relay in the cooling path:
- Check the fan controller hub or PCI/ motherboard accessory box for a component labeled "relay" or "contactor." This is common in high-density server chassis or elaborate custom builds.
- Inspect the power supply or a dedicated fan controller card for a small relay board connected to fan power lines; the relay will switch 12V to the fans as commanded by the controller.
- Look for a separate relay module mounted near the chassis or inside a controller enclosure; it may be a small PCB with a coil and a few terminals (common labels: COM, NO, NC).
- If the system uses a relay, diagnosing typically involves testing the controller’s output signal and ensuring the relay coil energizes when the controller requests fan power.
- As a practical test, you can temporarily connect a fan directly to a 12V supply with proper precautions to verify the fan motor itself isn’t the fault; if it runs when bypassed, the relay/controller path is the likely culprit.
Concluding the PC/server section: in most consumer builds, you won’t find a discrete relay for daily operation; relays appear mainly in specialized high-power or high-availability setups. When in doubt, reference the system’s manual or the controller’s documentation to identify whether a relay exists and where it sits inside the hardware.
Safety and maintenance tips
Best practices for identification and testing
Always disconnect power before inspecting fuses, relays, or wiring to avoid electric shock or shorts. When testing, use the vehicle’s ignition run position or power supply off-state methods recommended by the manual. If you hear a relay click but the fan doesn’t run, it’s often a failing fan motor, a bad ground, or a controller issue rather than a faulty relay alone. If the relay shows signs of burning, melting, or corrosion, replace it promptly and re-test the circuit.
For complex systems or if you’re unsure about handling high-current circuits, seek professional help. A faulty relay can lead to engine overheating or unsafe electrical conditions, so accurate diagnosis matters.
Summary
Across most cars, the radiator cooling fan relay sits in the under-hood fuse/relay box and is labeled accordingly, though some models use a separate relay pack or a temperature-controlled switch instead. In computer and server setups, relays are far less common for routine fan control, surfacing mainly in specialized or high-power configurations within a controller or relay box. To locate and test a cooling fan relay, consult your manual, identify the fuse/relay diagram, and use safe swap or voltage-check methods to confirm functionality. When in doubt, a trained technician can help prevent overheating and electrical damage.