How To Check Car Relays

When an electrical component fails, a faulty relay is often the silent culprit. Learning how to check car relays 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 test results.

Relays are electromagnetic switches. They allow a small current from a switch or computer to control a much larger current for a device like a fuel pump or cooling fan. When they malfunction, entire systems stop working.

Checking a car relay involves a few standard methods. You can perform a basic swap test, a visual inspection, or use a multimeter for a definitive diagnosis. The best approach often depends on your tools and the relay’s location.

Understanding Relay Basics And Common Symptoms

Before you start testing, it helps to know what you’re looking at. A standard automotive relay has four or five terminals, numbered on the bottom or the socket. These terminals correspond to the control circuit and the load circuit.

Common symptoms of a failing relay include:

A component that doesn’t work at all (e.g., no start, no fuel pump hum).
Intermittent operation, where a component works sometimes and not others.
Clicking sounds from the relay box with no component activation.
A component that stays on when the car is off, draining the battery.

Tools You Will Need For Testing

Gathering the right tools before you start makes the job smoother. You don’t need a professional mechanic’s kit for basic checks.

A known-good, identical relay for swap testing.
A digital multimeter (DMM).
A test light or a fused jumper wire.
Your vehicle’s service manual for wiring diagrams (optional but helpful).

Choosing The Right Multimeter

Any basic digital multimeter will work. Ensure it can measure resistance (Ohms) and DC voltage. The continuity test function, which beeps when a circuit is complete, is extremely useful for checking relay coils and contacts.

Locating The Faulty Relay

The first practical step is finding the relay. Most modern cars have multiple relay boxes. Common locations include under the hood in the engine bay fuse box, under the dashboard, or in the trunk near the battery.

Your owner’s manual will have a diagram showing which relay controls which component. The lid of the fuse box often has a similar diagram. If not, a quick online search for your car’s model and “relay layout” can provide the answer.

Method 1: The Simple Swap Test

This is the fastest and easiest way to check for a bad relay, provided you can find an identical one. Many relays in your car are the same type, controlling non-critical systems like the horn or high beams.

Identify a relay that is the same shape and has the same terminal numbers as the suspect one.
With the car off, gently pull out the suspect relay and the known-good donor relay.
Insert the known-good relay into the socket of the component that isn’t working.
Turn the car’s ignition to the “on” position or try to operate the component.
If the component now works, your original relay is faulty. If it doesn’t, the problem lies elsewhere in the wiring or the component itself.

Method 2: Visual And Auditory Inspection

Sometimes, a relay reveals its failure visibly or through sound. This check requires careful observation.

Listen for a Click: With the car’s ignition on and an assistant operating the switch (like the A/C), listen closely to the relay. You should hear a distinct, solid click. A faint click or no click at all suggests a problem with the coil or the control signal.
Smell and Look: Remove the relay. Look for signs of melting, cracking, or corrosion on the plastic case and terminals. A burnt smell is a clear indicator of internal damage from overheating.

These checks are not conclusive on their own, but they provide strong clues. A relay that looks and smells fine could still have failed electrically.

Method 3: Testing With A Multimeter

For a definitive diagnosis, a multimeter is your best tool. There are two main parts to test: the coil and the switch contacts.

Step-By-Step Coil Resistance Test

This test checks the electromagnet inside the relay. You’ll need the relay’s terminal diagram, usually marked as 85, 86, 30, 87, and sometimes 87a.

Set your multimeter to measure resistance (Ohms, Ω).
Identify the coil terminals. These are usually 85 and 86.
Touch the multimeter probes to terminals 85 and 86. The polarity doesn’t matter for this test.
Read the resistance. A typical relay coil will show a resistance between 50 and 120 ohms. A reading of infinite resistance (OL or “open loop”) means the coil is broken and the relay is bad. A reading of zero ohms indicates a shorted coil.

Step-By-Step Contact Continuity Test

This test checks the physical switch inside the relay that the coil operates. In a normal state, one set of contacts is closed.

Set your multimeter to the continuity setting (the one that beeps).
For a standard 5-pin relay (SPDT): Check for continuity between terminals 30 and 87a. There should be continuity (a beep). There should be no continuity between terminals 30 and 87.
For a standard 4-pin relay (SPST): There should be no continuity between terminals 30 and 87 when the relay is at rest.
To test the switched state, you need to energize the coil. You can use a 9V battery or a car battery. Apply power to terminals 85 and 86 (connect positive to one, negative to the other). You should hear a click.
With power applied, the continuity should reverse. For a 5-pin, 30-to-87a should open (no beep) and 30-to-87 should close (beep). For a 4-pin, 30-to-87 should close (beep).

If the contacts do not change state when the coil is powered, the relay’s internal mechanism is faulty.

Method 4: Testing The Relay In The Vehicle Socket

This method tests both the relay and the power supply to its socket. You’ll use your multimeter in DC voltage mode.

Locate the relay socket with the relay removed and the ignition key in the “on” position.
Set your multimeter to DC voltage (20V range).
Ground the black probe to the car’s chassis or battery negative.
Touch the red probe to the socket terminal that should have constant battery power (usually terminal 30). You should read battery voltage (around 12.6V).
Next, probe the socket terminal for the control circuit (terminal 86 or 85). Have an assistant activate the component’s switch (like turning on the headlights). You should see battery voltage appear at this terminal when the switch is on.

This test confirms that the car’s wiring is sending the correct signals to the relay socket. If power is missing, the problem is upstream in a fuse, switch, or computer.

How To Identify Relay Terminal Numbers

Terminal identification is crucial for accurate testing. The numbers are often tiny but are almost always marked on the relay itself or the socket.

85 & 86: Coil control terminals. These receive the switch signal.
30: Common contact. Receives constant battery power from the fuse.
87: Normally Open (NO) contact. Connects to terminal 30 when the coil is energized, sending power to the device.
87a: Normally Closed (NC) contact. Connected to terminal 30 when the coil is *not* energized, and disconnects when it is. Only on 5-pin relays.

Common Relay Problems And What They Mean

Understanding the failure helps prevent future issues.

Burnt Contacts: Caused by a high-current draw from a failing component (like a fuel pump on its last legs). The contacts weld shut or burn open.
Open Coil: The thin wire of the electromagnet breaks, often from vibration or age. The relay becomes completely dead.
Intermittent Failure: Often due to worn internal contacts or a weak coil connection. This is the most frustrating type, as the relay may test fine on the bench.

Safety Precautions When Checking Relays

Working with car electrical systems requires caution to avoid damage or injury.

Always disconnect the battery before removing or installing relays if you are unsure of the circuit. For simple testing in the socket, this is usually not necessary.
Use a fused jumper wire if you are manually applying power during tests. This prevents a short circuit.
Never force a relay into a socket. If it doesn’t fit, you likely have the wrong type or it’s oriented incorrectly.
Replace a relay with one of the exact same specification. The amperage rating (e.g., 30A) and terminal layout must match.

When To Call A Professional

While checking relays is generally straightforward, there are times to seek help. If you have performed all the tests and confirmed the relay and its socket have correct power and ground, but the component still doesn’t work, the issue is deeper. The problem could be a broken wire, a bad ground connection, or a faulty control module. Diagnosing these requires more advanced tools and knowledge.

FAQ Section

What Are The Signs Of A Bad Relay In A Car?

The most common signs are a component that doesn’t activate at all (like a dead fuel pump or A/C compressor), intermittent operation, audible clicking from the relay box with no action, or a device that stays on and drains the battery.

Can You Test A Relay Without Removing It?

You can perform some tests, like listening for the click or checking for socket voltage, without removing it. However, for a complete coil and contact test with a multimeter, you need to remove the relay from its socket to isolate it from the car’s circuit.

How Much Does It Cost To Replace A Car Relay?

Relays are inexpensive. A standard automotive relay typically costs between $10 and $25 from an auto parts store. Labor costs at a shop are minimal, often a half-hour of diagostic time plus the part.

What Causes A Relay To Fail?

Common causes include normal wear and tear on the internal contacts, excessive current draw from a failing component it controls, heat exposure in the engine bay, vibration, and moisture corrosion on the terminals.

Is It Safe To Drive With A Bad Relay?

It depends entirely on which relay has failed. A bad headlight or horn relay is a safety issue but may not immediately strand you. A bad fuel pump relay or engine control relay will cause the car to not start or stall, making it undrivable. It’s best to adress any suspected relay failure promptly.