How To Wire Electric Fan With Relay

Is your electric cooling fan acting up? Perhaps it's running constantly, not running at all, or cutting in and out erratically? A failing or improperly wired relay is a common culprit behind these issues. Getting your electric fan wired correctly with a relay is crucial for maintaining optimal engine temperature, preventing overheating, and extending the life of your vehicle. This guide will walk you through understanding the problem, identifying the symptoms, and implementing the correct solution.

Identifying the Problem: Symptoms of a Faulty Electric Fan Relay Wiring

Diagnosing an electric fan issue related to the relay wiring requires careful observation. Here's a detailed list of symptoms to watch out for:

Overheating Engine: This is perhaps the most critical symptom. If your electric fan isn't kicking in when it should, your engine temperature will rise, potentially leading to severe damage.
Fan Running Constantly: If the relay is stuck in the "on" position, the fan will run continuously, even when the engine is cold. This drains your battery and shortens the lifespan of the fan motor.
Fan Not Running at All: Conversely, a faulty relay or wiring connection might prevent the fan from running altogether, even when the engine is hot.
Erratic Fan Operation: The fan might cut in and out intermittently, or operate at incorrect speeds. This suggests a loose connection, a failing relay, or a problem with the wiring harness.
Blown Fuses: A short circuit in the relay wiring can cause the fuse protecting the fan circuit to blow repeatedly.
Check Engine Light (CEL): In some vehicles, a malfunctioning fan circuit can trigger the check engine light. Codes related to engine temperature or fan control might be stored.
Unusual Noises: Clicking sounds from the relay itself could indicate a failing relay.

Understanding the Root Cause: The Relay and Wiring Connection

The electric fan relay acts as an electrical switch, controlling the flow of power to the fan motor. It receives a signal from the engine control unit (ECU) or a temperature sensor, and when the engine reaches a certain temperature, the ECU or sensor signals the relay to close, completing the circuit and activating the fan. Here's a breakdown of potential root causes related to wiring:

Corroded or Loose Wiring: This is a very common issue. Over time, exposure to the elements can cause corrosion on the wiring connectors and terminals, increasing resistance and preventing proper current flow. Loose connections can also cause intermittent operation.
Damaged Wiring: The wiring itself can be damaged by heat, abrasion, or physical impact. This can lead to short circuits, open circuits, or high resistance in the circuit.
Incorrect Wiring: If the relay was replaced or the wiring was modified, it's possible that the wiring is incorrect. This can cause the fan to operate improperly or not at all. It is essential to ensure the correct wiring diagram is followed.
Faulty Relay: While this guide focuses on wiring, the relay itself can fail. A faulty relay might not close properly, or it might get stuck in the "on" or "off" position.
Poor Ground Connection: The relay and the fan motor both require a good ground connection to function properly. A corroded or loose ground connection can cause intermittent operation or prevent the fan from running.
Incorrect Fuse Size: Installing a fuse with the wrong amperage rating can lead to blown fuses or, in severe cases, damage to the electrical system.

How the Relay Works: A Simplified Explanation

Think of the relay as a gatekeeper. It has two main circuits: a control circuit and a power circuit. The control circuit is a low-current circuit that activates the relay. When the ECU or temperature sensor sends a signal (a small voltage), it energizes a small electromagnet inside the relay. This electromagnet pulls a switch closed, completing the power circuit. The power circuit is a high-current circuit that provides power to the fan motor. When the switch closes, power flows from the battery, through the relay, and to the fan motor, causing the fan to spin.

The Consequences of Ignoring the Problem

Ignoring an electric fan issue can have serious consequences for your vehicle. The most obvious is engine overheating, which can lead to:

Blown Head Gasket: This is a common and expensive repair.
Warped Cylinder Head: Another costly repair requiring machining or replacement of the cylinder head.
Cracked Engine Block: In severe cases, the engine block itself can crack, rendering the engine unusable.
Damaged Pistons and Rings: Overheating can cause excessive wear and damage to these critical engine components.

Beyond engine damage, a constantly running fan can drain your battery, leaving you stranded. A malfunctioning fan can also affect your air conditioning performance, as the AC condenser relies on the fan to dissipate heat. Finally, constantly blowing fuses can lead to electrical system damage and potentially even a fire hazard.

Recommended Fixes: Rewiring the Electric Fan with a Relay

Here's a step-by-step guide to properly wiring your electric fan with a relay. Always disconnect the negative battery cable before working on any electrical components.

Gather Your Tools and Materials:
- New electric fan relay (check your vehicle's specifications for the correct type and amperage rating).
- Wiring harness (or individual wires of the correct gauge – typically 12 or 14 gauge for the power circuit).
- Wire strippers and crimpers.
- Electrical tape or heat shrink tubing.
- Volt meter or multimeter.
- Wiring diagram for your vehicle (crucial for identifying the correct connections).
- Terminal connectors (spade connectors, ring terminals, etc.).
- Fuse holder and appropriately sized fuse (if needed).
Locate the Relay Mounting Point:
Find a suitable location to mount the relay. It should be protected from the elements and easily accessible. Many vehicles have a designated relay box in the engine compartment. If not, choose a spot with a solid mounting surface.
Wiring the Relay (Generic Wiring Diagram):
This is a generic wiring diagram. Always consult your vehicle's specific wiring diagram.
- Pin 30: Connect this pin to the positive (+) terminal of the battery, through a fuse (typically 20-30 amps). This provides the main power to the fan.
- Pin 87: Connect this pin to the positive (+) wire of the electric fan motor.
- Pin 85: Connect this pin to a switched 12V source. This source is activated when the engine is running. This can be a wire that is powered when the ignition is switched to the "on" position or the ECU signal wire. (Refer to your car's wiring diagram)
- Pin 86: Connect this pin to a good ground point on the vehicle's chassis.
Making the Connections:
Carefully strip the insulation from the ends of the wires and crimp the appropriate terminal connectors onto them. Securely attach the connectors to the relay pins and to the designated points on the battery, fan motor, and chassis. Ensure all connections are tight and properly insulated with electrical tape or heat shrink tubing.
Verify the Wiring:
Double-check all your connections against the wiring diagram. Use a multimeter to verify that you have 12V at pin 30 when the ignition is off (battery voltage) and at pin 85 when the ignition is on (switched 12V). Verify continuity between pin 86 and a known good ground point.
Test the Fan:
Reconnect the negative battery cable. Start the engine and allow it to warm up. Monitor the engine temperature gauge. When the engine reaches the normal operating temperature, the ECU or temperature sensor should signal the relay, and the fan should turn on. Verify that the fan turns off when the engine cools down.
Troubleshooting (If the Fan Doesn't Work):
- Check the Fuse: Make sure the fuse in the fan circuit is not blown.
- Check the Relay: Try swapping the relay with a known good relay. If the fan now works, the original relay is faulty.
- Check the Wiring: Use a multimeter to check for voltage at each of the relay pins and at the fan motor. If you're not getting voltage where you expect it, there's a problem with the wiring.
- Check the Ground: Ensure the ground connection is clean and secure.
- Consult a Professional: If you're still having trouble, it's best to consult a qualified mechanic.

Cost Estimates and Shop Advice

The cost of rewiring an electric fan with a relay can vary depending on the extent of the damage and whether you choose to do it yourself or hire a professional.

DIY: The cost of materials (relay, wiring, connectors, fuse) is typically between $20 and $50.
Professional Mechanic: Labor costs can range from $100 to $300 or more, depending on the complexity of the job and the hourly rate of the shop. This includes diagnosis, parts, and labor.

Shop Advice: When choosing a mechanic, look for a shop with experience in automotive electrical systems. Ask for a detailed estimate before authorizing any work. If you're comfortable working on your car's electrical system, you can save money by doing the job yourself. However, if you're not confident in your abilities, it's best to leave it to the professionals. Always ensure the shop uses a proper wiring diagram for your vehicle. Improper wiring can damage other components.

Credibility and Common Failure Points

Electric fan relay issues are quite common, especially in older vehicles. While there aren't specific widespread Technical Service Bulletins (TSBs) covering all instances of electric fan relay wiring failures (TSBs tend to be very specific to make, model, and year), online automotive forums and communities are filled with discussions about these problems. Many owners report failures around the 100,000-mile mark, but this can vary greatly depending on environmental factors, the quality of the original components, and driving habits. Regular inspection of the wiring and relay can help prevent major issues down the road.