How To Test A Car Relay

When your car’s electrical component suddenly stops working, a faulty relay could be the silent culprit. Learning how to test a car relay is a straightforward skill that can save you time and money. This guide will walk you through the entire process, from identification to diagnosis, using simple tools you likely already have.

A relay is a remote-controlled switch. It uses a small electrical signal from, say, your headlight switch, to activate an electromagnet. That magnet then closes a much heavier-duty circuit that delivers full power to the component, like the headlights themselves. This protects delicate switches from high current.

Before you begin testing, you need to gather a few basic tools and locate the relay in question. Having everything ready makes the process smooth and safe.

Tools You Will Need For Testing

You do not need expensive equipment. Here is what you should collect:

A multimeter (digital or analog).
A set of jumper wires or a piece of wire with alligator clips.
A 9-volt or 12-volt battery (a car battery is perfect).
The vehicle’s service manual or a wiring diagram (helpful but not always essential).
Your car’s key.

Locating The Faulty Relay

Most modern cars have multiple relays grouped together in a box, often called a fuse box or power distribution center. Common locations include under the hood near the battery and under the dashboard on the driver’s side.

Consult your owner’s manual: It has a diagram labeling each relay’s function.
Use the lid map: The plastic cover of the fuse box usually has a printed guide.
Listen for the click: Have a helper turn the problematic component on and off while you listen near the boxes for a distinct clicking sound from the relay. The one that doesn’t click might be faulty.
Swap with an identical relay: Find a relay with the same part number that controls a non-essential system (like the horn) and swap them. If the problem moves to the other component, you’ve found the bad relay.

Understanding Relay Terminals And Diagrams

Most standard automotive relays have four or five terminals. The terminals are usually labeled with tiny numbers on the relay casing or the socket.

Terminals 85 and 86: This is the coil circuit. This is where the control signal from the switch connects. It is a low-current circuit. Polarity usually doesn’t matter for the coil.
Terminal 30: This is the common connection to the power source (battery positive). It carries the high current.
Terminal 87: This is the normally open (NO) contact. When the relay coil is energized, terminal 30 connects to 87, sending power to the component.
Terminal 87a: This is present only on 5-pin relays. It is the normally closed (NC) contact. When the relay coil is *not* energized, terminal 30 is connected to 87a. It breaks this connection when energized.

A clear diagram is often on the relay’s side. If you understand this layout, testing becomes logical.

Common Automotive Relay Pin Configurations

While layouts can vary, the most common setup for a 4-pin relay is: 85 and 86 for the coil, 30 for battery power, and 87 for the output to the device. For a 5-pin relay, you have the added 87a terminal for a second circuit path.

Preliminary Visual And Auditory Checks

Before breaking out the multimeter, perform these simple checks. Sometimes the problem is obvious.

Remove the relay from its socket carefully.
Inspect the terminals: Look for corrosion, melting, or bent pins.
Shake the relay: Listen for any loose internal parts rattling, which indicates physical failure.
Reinstall the relay: With the car’s key in the “on” position (engine off), activate the component (e.g., turn on headlights). You should feel and hear a definitive click from the relay. No click suggests a problem with the coil, the control signal, or the relay itself.

Step-By-Step Testing Methods

Now we move into the hands-on testing procedures. We will cover two reliable methods: using a multimeter and using a power source.

Method 1: Testing A Car Relay With A Multimeter

A multimeter is the most precise tool for the job. We’ll test both the coil and the switch contacts.

Testing The Relay Coil (Terminals 85 & 86)

The coil should have continuity, meaning it allows electricity to flow through it, creating resistance.

Set your multimeter to the resistance (Ohms Ω) setting.
Touch the meter probes to terminals 85 and 86.
Read the resistance: A typical automotive relay coil will show a resistance between 50 and 120 ohms. A reading of infinite resistance (OL or “1” on digital meters) means the coil is open and the relay is bad. A reading of zero ohms indicates a shorted coil.

Testing The Switch Contacts (Terminals 30, 87, 87a)

This checks the high-current switch inside the relay.

Set the multimeter to continuity or resistance.
Test the Normally Closed circuit (if present): On a 5-pin relay, with the coil *not* powered, you should have continuity between terminals 30 and 87a. There should be no continuity between 30 and 87.
Test the Normally Open circuit: Now, apply 12 volts to terminals 85 and 86 (you can use a car battery and jumper wires). You should hear a click. While power is applied, test for continuity between terminals 30 and 87. Now, there should be continuity. On a 5-pin relay, the connection between 30 and 87a should now be open (no continuity).

If the contacts do not change state when the coil is powered, the internal switch is faulty.

Method 2: Testing A Car Relay With A Power Source

This is a practical, real-world test that confirms the relay can actually switch a load. For this, you’ll need a 12-volt battery, some wire, and a test light or the actual component (like a headlight bulb).

Build a control circuit: Connect a wire from the battery’s positive terminal to relay terminal 86. Connect another wire from relay terminal 85 to the battery’s negative terminal. You should hear the relay click immediately upon making the last connection.
Build the load circuit: Connect the battery positive to terminal 30. Connect a test light or bulb to terminal 87.
Complete the load circuit: Attach the other end of the test light to battery negative.
Activate the relay: With the control circuit connected (step 1), the test light should illuminate. When you disconnect the control circuit, the light should turn off. This proves the relay can successfully switch power.

If the relay clicks but the test light does not illuminate, the contact terminals (30/87) are burned or damaged. If it doesn’t click at all, the coil is dead.

Testing The Relay Directly In The Vehicle Socket

Sometimes you want to test the relay without removing it, or you want to check if the car’s wiring is sending the correct signals.

Locate the relay socket diagram to identify which socket pin does what.
Test for control signal: Set your multimeter to DC Volts. With the relay removed and the car key on, probe socket terminals 85 and 86 (refer to your diagram). Activate the component’s switch. You should see 12 volts (or very close to it) appear. This confirms the switch and wiring to the relay are good.
Test for power at terminal 30: With the meter still on DC Volts, probe the socket for terminal 30 (carefully!). It should have constant 12V battery power at all times, key on or off.

If you have power at terminal 30 and a control signal at 85/86 when the switch is activated, but the component doesn’t work, then the relay itself is almost certainly the problem.

Interpreting Test Results And Next Steps

Once testing is complete, you need to understand what the results mean and what to do next.

Common Relay Failure Symptoms

No click when activated: Points to a failed coil or no control signal.
Clicks but no power output: Internal contacts are corroded, burned, or welded open.
Intermittent operation: Contacts or coil connections are failing internally, often due to heat or vibration.
Component stuck on: Contacts are welded closed, so the circuit cannot be turned off even when the relay clicks.

What To Do With A Bad Relay

If your tests confirm the relay is faulty, replacement is the only option. Relays are inexpensive and not repairable.

Purchase an identical replacement: Match the part number, terminal layout, and current rating (usually in amps, printed on the relay).
Install the new relay: Ensure it is seated firmly in the socket.
Test the system: Turn on the component to verify operation.

If a new relay fails immediately or quickly, there may be an underlying issue, like a short circuit in the component it controls, causing excessive current draw that damages the relay contacts.

When The Relay Tests Good

If the relay passes all your tests, the problem lies elsewhere. The next steps in your diagnosis should include:

Checking the fuse: It might be blown.
Inspecting the component: The headlight bulb, fuel pump, or fan motor itself could be dead.
Examining the wiring: Look for damaged, corroded, or loose wires between the relay, the battery, the component, and ground connections.
Verifying ground connections: A poor ground can prevent any circuit from working.

FAQ Section

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

Symptoms include a component not working at all (like headlights or fuel pump), a component working intermittently, a relay that clicks repeatedly without the component activating, or a component that is stuck on and cannot be turned off.

Can You Test A Relay Without A Multimeter?

Yes, you can. The power source and test light method described above is an effective way to test a relay without a multimeter. The simple swap test with an identical relay is also a very practical diagnostic method.

What Causes A Car Relay To Go Bad?

Common causes include normal wear and tear from repeated cycling, overheating due to high current draw or poor connections, internal corrosion, vibration leading to broken connections, and voltage spikes in the electrical system.

Is It Safe To Drive With A Faulty Relay?

It depends on the relay’s function. A faulty relay for a non-critical item like a horn or interior light is usually safe. However, a failed relay for the fuel pump, engine cooling fan, or headlights can strand you or create a dangerous driving condition. It is best to adress the issue promptly.

How Much Does It Cost To Replace A Car Relay?

Relays are generally inexpensive. Most standard automotive relays cost between $10 and $25. The cost is primarily in the part, as the labor to replace one is minimal if you do it yourself, often taking just a few minutes.