How to Test a Radiator Fan Relay with a Multimeter

If your radiator fan isn’t coming on when it should—or runs constantly—one of the first things to check is the fan relay. The fan relay is an electronic switch that allows the low-current signal from your car’s control system to activate the high-current fan motor. If it fails, your engine can quickly overheat, especially in traffic or when the AC is on. Testing the relay with a multimeter is a straightforward process. Here’s how to do it safely and accurately.

What You Need

• Basic digital multimeter (settable to Ohms and Volts DC)
• Owner’s manual or repair manual (for relay location and pinout)
• Needle-nose pliers (to remove relay if needed)
• Alligator clip leads (helpful but optional)
• 12V power source (car battery or bench supply, only for advanced testing)

Understanding Radiator Fan Relays

Most radiator fan relays are simple “cube” style devices with four or five pins. They typically include:

• Control Circuit: Low-current side, usually pins 85 and 86. One pin receives 12V “trigger” (from ECU or sensor), the other is ground.
• Load Circuit: High-current side, usually pins 30 (power from battery/fuse) and 87 (to fan motor). Some relays have a fifth pin, 87a (normally closed contact, not always used).

When the control circuit is energized, the relay “clicks” and connects 30 to 87, sending full power to the fan.

Step 1: Locate the Radiator Fan Relay

• Check your owner’s manual, fuse box lid diagram, or repair manual for the relay’s location (usually in the engine bay fuse/relay box).
• The relay may be labeled “RAD FAN,” “COOLING FAN,” “FAN RELAY,” or something similar.
• Remove the relay by gently pulling it straight up. Some relays may be tightly fitted—use pliers if needed.

Step 2: Visual Inspection

• Check relay pins for corrosion, burns, or melting—any visible damage means replace the relay.
• Check the socket for damage or loose connections.

Step 3: Test for Continuity on the Coil (Control Circuit)

• Set your multimeter to “Ohms” (Ω).
• Identify the coil pins (usually 85 and 86—check the relay diagram on the side).
• Touch one multimeter lead to each pin.
• You should see a resistance reading between 40 and 120 Ohms (typical for most relays; check specs for your car).
• If the meter reads “OL” (open loop) or zero, the coil is broken—replace the relay.

Step 4: Test for Continuity on the Switch (Load Circuit)

• Identify the “common” (30) and “normally open” (87) pins.
• With no power applied, there should be NO continuity between 30 and 87 (multimeter reads “OL”).
• If there’s continuity, the relay is stuck closed—replace it.

Step 5: Bench Test the Relay (Optional but Best)

• Connect 12V power to pins 85 and 86 (polarity usually does not matter for the coil).
• You should hear or feel a “click”—the relay is engaging.
• While power is applied, use your multimeter (in continuity or Ohms mode) to check between pins 30 and 87.
• There should now be continuity (close to 0 Ohms, or multimeter beeps)—the switch is closed and the relay works.
• If no click or no continuity, the relay is bad.

Step 6: Voltage Test in the Vehicle (Advanced)

• Reinstall the relay and back-probe the control pins while the car is running and the engine is hot (or AC is on, if that triggers the fan).
• The trigger pin should show ~12V when the fan is supposed to run. If there’s no voltage, the relay may not be the problem—check wiring, fuses, or sensors.

Additional Tips

• If your fan still doesn’t run after confirming the relay is good, check the fan fuse, temperature sensor, wiring, and the fan motor itself.
• If you’re unsure, try swapping the relay with another identical one from the fuse box (for example, the horn or AC relay)—if the fan works, the relay was bad.
• Always reinstall relays securely to prevent poor connections and overheating.

Safety Precautions

• Always disconnect the battery before probing high-current circuits directly.
• Never touch relay or fan wires with bare hands when the system is powered—fans can start unexpectedly and cause injury.
• Be careful around hot engine parts and moving components.

Summary

Testing a radiator fan relay with a multimeter is quick and easy—and can save you from expensive engine damage due to overheating. Just remember to test the coil for resistance, check the switching contacts, and do a bench test with 12V if possible. If the relay passes all tests but your fan still won’t run, the problem lies elsewhere in the circuit. Replacing a bad relay is cheap and simple, making this a great first step in troubleshooting a non-working radiator fan.