When an electrical component in your car stops working, a faulty relay is a common and often simple culprit to test. Learning how to check a car relay is a fundamental skill that can save you time and money. This guide will walk you through the entire process, from locating the relay to interpreting your test results.
Relays are essentially remote-controlled switches. They allow a small current from a switch, like your headlight knob, to control a much larger current needed for the component itself. When a relay fails, it can leave you with a dead fuel pump, silent horn, or inoperative cooling fan.
Testing one is straightforward with basic tools. You will gain the confidence to diagnose many electrical gremlins yourself.
How To Check A Car Relay
This section provides the core, step-by-step methodology for testing a standard automotive relay. We will cover the visual inspection, the bench test, and the in-vehicle electrical test. Having a basic multimeter is highly recommended for accurate diagnosis.
Understanding Relay Basics And Terminology
Before you start testing, knowing how a relay works and what its pins do is crucial. Most standard automotive relays have four or five pins arranged in a specific pattern.
A typical four-pin relay has two circuits: the control circuit and the load circuit. The control circuit connects to the coil inside the relay. When you apply power, the coil becomes an electromagnet.
This magnet pulls a metal armature, which physically closes a set of contacts. These contacts are the load circuit, which completes the path for high current to flow to your car component.
Here is the common pin layout for a standard 4-pin relay:
- 85 and 86: These are the coil terminals. They connect to the control switch. Polarity usually doesn’t matter for a standard relay; one gets power, the other goes to ground.
- 30: This is the common terminal. It connects to the power source (usually battery power) for the device you’re controlling.
- 87: This is the normally open (NO) terminal. When the relay coil is NOT energized, there is no connection between 30 and 87. When the coil IS energized, this terminal connects to pin 30, sending power to your device.
A five-pin relay adds an extra terminal:
- 87a: This is the normally closed (NC) terminal. It is connected to pin 30 when the relay coil is OFF. When the coil is energized, it breaks contact with 30 and connects to 87 instead. This is used for applications like a fuel pump primer or changeover circuits.
Tools You Will Need For Testing
Gathering the right tools before you start makes the job smoother and safer. You likely have most of these already.
- A Multimeter: This is your most important tool. A digital multimeter that can measure resistance (ohms) and DC voltage is perfect. An analog one will also work.
- A 9V or 12V Power Source: A small 9V battery is excellent for bench testing. You can also use your car’s battery with a couple of jumper wires, but be cautious of sparks.
- Jumper Wires with Alligator Clips: These are invaluable for making temporary connections to the relay pins and your power source.
- The Vehicle’s Service Manual (Optional but Helpful): This provides the relay location diagram and the specific pinout for your car’s relays, which can vary slightly.
- A Test Light: A simple 12V test light can be used for basic power and ground checks if you don’t have a multimeter.
Step 1: Locating The Faulty Relay
First, you need to find which relay is causing the problem. Modern cars have multiple relay boxes, often under the hood and inside the cabin.
Consult your owner’s manual or a service manual for a layout diagram. The cover of the relay box usually has a printed guide showing which relay controls which component.
If the guide is missing, you can often identify a suspect relay by listening for a click. Have a helper activate the component (like turning on the headlights) while you listen near the relay box. A silent relay when it should click is a prime candidate.
Another method is to swap the suspect relay with an identical one from another circuit that you know works, like the horn or rear window defogger. If the problem moves to the other component, you’ve found your bad relay.
Step 2: Performing A Visual And Physical Inspection
Before any electrical testing, always do a thorough visual check. Remove the relay from its socket carefully.
Look for signs of damage such as:
- Melted, cracked, or discolored plastic casing.
- Corrosion or greenish deposits on the metal pins.
- A burnt smell emanating from the relay.
- Any signs of overheating, like bubbling or charring.
Shake the relay gently near your ear. If you hear anything rattling loose inside, the internal mechanism is likely damaged. Any of these visual cues mean the relay should be replaced, and further electrical testing may be unnecessary.
Step 3: The Bench Test (Checking The Relay Coil)
This test checks the health of the relay’s electromagnetic coil using a multimeter. It’s best performed with the relay removed from the vehicle.
- Set your multimeter to the resistance (ohms, Ω) setting.
- Touch the multimeter probes to pins 85 and 86. You are measuring the resistance of the coil.
- Read the value. A typical automotive relay coil will show a resistance between 50 and 120 ohms. Consult a spec sheet if possible, but any reading within this general range usually indicates a good coil.
- Interpret the results: A reading of infinite resistance (OL or “1” on the display) means the coil is open and broken. A reading of zero ohms means the coil is shorted. In both cases, the relay is faulty.
Step 4: Bench Testing The Switch Contacts
Now, test the the main switching contacts. For this, you’ll need your multimeter and a small battery to activate the relay.
- Set your multimeter to the continuity or resistance setting. Continuity mode beeps when a circuit is complete.
- For a 4-pin relay, place the probes on pins 30 and 87. With the relay de-energized (coil off), you should have NO continuity (infinite resistance, no beep).
- Now, use jumper wires to connect a 9V or 12V battery to pins 85 and 86 to energize the coil. You should hear and feel a distinct click.
- With the coil energized, check pins 30 and 87 again. Now, you should have continuity (low resistance, a beep). This confirms the contacts are closing properly.
- For a 5-pin relay, also check between pins 30 and 87a. The behavior should be opposite: continuity when the coil is off, and no continuity when the coil is on.
If the contacts do not switch states correctly, the relay’s internal contacts are worn, burnt, or stuck. The relay needs replacement.
Step 5: In-Vehicle Electrical Testing
Sometimes a relay tests fine on the bench but fails under load in the car. This test checks if the relay is getting the correct signals from the car’s wiring.
Reinsert the relay into its socket in the vehicle. You will need to back-probe the connector with your multimeter probes or use a test light.
- Check for Power at the Load Terminal (Pin 30): Set the multimeter to DC voltage. With the ignition on, back-probe the wire at pin 30. It should show constant battery voltage (around 12.6V). If not, there’s a wiring problem from the battery.
- Check the Control Circuit Power (Pin 86 or 85): Activate the component (e.g., turn the headlights to “on”). Back-probe one of the coil pins. You should see battery voltage here when the switch is commanded on. If not, the switch or its wiring is faulty.
- Check the Control Circuit Ground (The Other Coil Pin): With the switch still on, check the other coil pin. Set your multimeter to DC voltage and place the red probe on this pin and the black probe on a known good ground (like the battery negative). You should see battery voltage. If you see 0V, the ground path for the coil is broken.
- Check for Output Power (Pin 87): Finally, with the component activated, check pin 87. It should now show battery voltage. If pin 30 has power, the coil is being energized (you heard a click), but pin 87 has no power, then the relay’s internal contacts are failing under load.
Step 6: Interpreting Results And Next Steps
After completing your tests, you can make a clear diagnosis.
- Relay Fails Bench Test: The relay itself is definitively bad. Replace it with a new one of the same part number.
- Relay Passes Bench Test but Fails In-Vehicle Test: The relay is likely good. The problem lies in the vehicle’s wiring. You may have a bad switch, a blown fuse, a broken wire, or a poor ground connection. Further circuit tracing is needed.
- No Click Heard In-Vehicle but Coil Tests Good: The relay is not being told to switch. Focus on the control circuit: check fuses, the switch, and wiring to pins 85 and 86.
- Click Heard but No Output Power: The relay coil is working, but the contacts are not passing current. The relay is faulty and needs replacement.
Common Car Relay Failure Symptoms
Recognizing the signs of a failing relay can help you diagnose problems faster. Symptoms are often intermittent at first.
- Component Works Intermittently: A headlight that flickers or a fuel pump that sometimes doesn’t prime can point to a relay with failing internal contacts.
- Component Does Not Work At All: A complete failure of a single electrical device, like the horn or radiator fan, is a classic sign.
- Clicking Sound From The Relay Box: Rapid, repeated clicking can indicate a weak coil or a problem in the control circuit causing the relay to chatter on and off.
- Related Systems Fail: Sometimes, one relay controls multiple functions. A failure could affect several seemingly unrelated components that share a circuit.
Safety Precautions When Working With Relays
Always prioritize safety when working on your car’s electrical system. A small mistake can cause damage or injury.
- Disconnect the Battery: Before removing or installing relays, it’s a good practice to disconnect the negative battery terminal. This prevents accidental short circuits.
- Use the Correct Replacement: Always replace a relay with an identical part number or one with matching specifications (coil resistance, current rating). Using an incorrect relay can cause a fire.
- Mind the Pins: Do not force a relay into a socket. If it doesn’t seat easily, check that you have the correct orientation and that the pins are straight.
- Avoid Water and Moisture: Perform tests in a dry environment. Water can cause immediate shorts and corosion on electrical connections.
Maintenance Tips To Prevent Relay Issues
While relays are generally maintenance-free, a few practices can extend their life and prevent future problems.
- Keep relay boxes clean and dry. Check seals on under-hood boxes periodically.
- During routine maintenance, you can gently remove and reseat relays to prevent corrosion buildup on the pins. This is known as “exercising” the connections.
- Address electrical gremlins promptly. A short circuit elsewhere in the system can send a surge that damages a relay.
- When replacing any high-current component (like a headlight bulb or fan motor), consider the age of its controlling relay. If the old component failed due to age, the relay has endured the same cycles.
Frequently Asked Questions (FAQ)
What Does A Car Relay Do?
A car relay is an electromechanical switch. It uses a small electrical current from a control switch (like your dashboard button) to activate an electromagnet. This magnet then closes a separate set of contacts that can carry a much larger current required by a device like the starter motor, fuel pump, or headlights. This protects delicate switches from high current.
Can You Bypass A Relay To Test A Component?
Yes, but with extreme caution. Bypassing a relay involves directly connecting the power source (pin 30) to the output (pin 87) with a heavy-gauge jumper wire. This will send full power to the component. Only do this momentarily for diagnostic purposes to see if the component works. Never leave a bypass in place, as it removes all circuit protection and can overheat wires or cause a fire.
How Much Does It Cost To Replace A Car Relay?
Car relays are generally inexpensive. Most standard automotive relays cost between $10 and $25 for the part itself. The cost of replacement is minimal if you do it yourself, as it simply plugs in. A mechanic might charge a small diagnostic fee plus the part cost, typically totaling $50 to $100 depending on labor rates.
Why Does My Relay Make A Clicking Sound?
A single, solid click is normal and indicates the relay is activating. However, rapid, repeated clicking is abnormal. This chatter can be caused by a weak battery (low voltage to the coil), a poor ground connection for the coil, or a failing relay with a weak internal magnet. It can also indicate a problem in the control switch itself.
Is It Safe To Drive With A Faulty Relay?
It depends entirely on which relay has failed. A faulty headlight or horn relay is a safety issue but may not immediately strand you. However, a failed fuel pump relay, ignition relay, or cooling fan relay can cause the car to stall or lead to severe engine overheating, resulting in catastrophic damage. It is best to adress any suspected relay issue as soon as possible.