How do I know if my fan relay is blown?
If your cooling fan or blower doesn’t run when it should, a blown relay is a common culprit. In many cases you can confirm this by listening for a relay click, checking fuses, and testing the relay’s coil and contacts or swapping in a known-good unit.
Signs your fan relay may be blown
These indicators can point to a relay problem, though other issues are possible. Inspect these symptoms carefully and in context with your vehicle or HVAC system behavior.
- The cooling fan fails to start when the engine is hot or when the air conditioner is engaged.
- You don’t hear the expected clicking sound from the relay when the engine reaches operating temperature or when AC is turned on.
- The fan runs intermittently or only at certain speeds (in some systems this can happen if the control bypasses the relay at high speed).
- A blown fuse or a burnt smell near the fuse/relay box, or visible damage to the relay itself.
- Engine overheating in traffic or idle conditions due to the fan not operating as required.
Conclusion: These signs raise the likelihood of a faulty relay, but it’s important to rule out the fan motor, wiring, resistor packs, and control modules as well.
Preliminary checks you can perform safely
Before diving into detailed electrical testing, run these quick checks to rule out easier causes and to avoid unnecessary work.
- Inspect and, if needed, replace the relevant fuse for the fan circuit.
- Check electrical connectors for corrosion, looseness, or damaged insulation in the fan harness and relay socket.
- Verify battery voltage and ground integrity to the fan circuit; a weak ground can mimic relay problems.
- Test whether the fan operates on the high-speed setting (if applicable) or when the AC is on, to see if the circuit is receiving power at the times it should.
Conclusion: These basic checks often reveal non-relay problems and help pinpoint the true cause before more complex tests.
How to test a relay with a multimeter
Two core tests can confirm whether the relay coil is functional and whether the contacts are making or breaking connection as intended.
Coil resistance test
Remove the relay from its socket and measure the coil’s resistance with a multimeter set to ohms. Compare the reading to the vehicle’s service specification or the relay’s datasheet. If the coil shows an open circuit (infinite resistance) or a short (very low resistance), the coil is damaged.
- Typical automotive coil resistance ranges vary by part, but many are in the tens to low hundreds of ohms. Always refer to the exact spec for your relay.
- Make sure the coil is disconnected from power during measurement to avoid false readings or circuit interference.
Conclusion: An out-of-spec coil resistance generally indicates a bad relay coil that should be replaced.
Contact function test (bench or in-circuit)
Test whether energizing the coil actually closes the contacts and allows current to pass to the fan. A bench test can be done with a 12V supply, or you can perform a live test in the vehicle with proper safety precautions.
- With the coil energized (12V applied), measure continuity between the common terminal and normally open (NO) terminal. You should see a closed circuit when energized.
- At the same time, verify that the output path to the fan harness receives voltage when the coil is energized. If there is no voltage at the fan despite a closed contact, the relay is not passing power properly or there is a wiring fault.
Conclusion: If the coil energizes but the contacts don’t close or fail to pass current, replace the relay or inspect the wiring for issues.
Swapping in a known-good relay to confirm
When in doubt, a quick swap can definitively identify a faulty relay without individual component testing.
- Turn off the vehicle and locate the suspected relay in the fuse/relay box. If you have a spare relay of the same type, remove the old unit and insert the known-good one.
- Power up and test the system by triggering the engine to operating temperature or turning on the AC.
- If the fan now operates normally with the spare relay, the original is likely defective.
Conclusion: Swapping in a known-good relay is a fast, practical way to confirm relay failure when diagnostic readings are inconclusive.
What to do if the relay is blown
Once you’ve confirmed a blown relay, follow these steps to replace it and restore function, while guarding against related issues.
- Obtain the correct replacement relay for your vehicle’s make/model or HVAC system; many relays are standard 4-pin 12V automotive types but verify part numbers.
- Disconnect power (turn off the ignition, and if possible disconnect the battery) and remove the old relay from its socket.
- Install the new relay, ensuring the pins match the socket orientation and seating is secure.
- Reconnect the harnesses, reinsert the fuse, and start the engine or run the HVAC system to verify normal fan operation.
- Carefully monitor for unusual signs (hot relays, buzzing, burning smell) and inspect related wiring if issues persist.
Conclusion: Replacing with the correct part typically resolves the issue if the fan motor and wiring are sound; if problems continue, consider additional diagnosis of the motor, resistor pack, or control module.
Other considerations
Note that some vehicles use a fan control module or electronic controller rather than a traditional relay. In those cases, a diagnostic scan and module testing/replacement may be required. A faulty fan motor, blown fuse, bad resistor, corroded connector, or damaged wiring can mimic relay problems and should be checked as part of a comprehensive diagnosis.
Summary
To determine if your fan relay is blown, start with listening for the relay click, verify that the fan runs when expected, and perform coil and contact tests with a multimeter or swap in a known-good relay. If tests show an open coil or non-closing contacts, replace the relay and recheck the system. Keep in mind some systems use electronic control modules instead of a classic relay, so consult your vehicle’s service manual or a professional if symptoms persist.
