How Do You Check Relays In A Car

When an electrical component fails, a faulty relay is often the simple, inexpensive culprit. Learning how do you check relays in a car is a fundamental skill that can save you time and money. This guide provides clear, step-by-step instructions to diagnose a bad relay with basic tools.

Relays are electromagnetic switches. They allow a small current from a switch or computer to control a much larger current for a high-power device like a fuel pump or headlights. When they fail, the component they control stops working.

There are several reliable methods to test a car relay. The best approach often depends on your tools and the relay’s location. We will cover the most common techniques, from a simple swap test to precise electrical measurements.

Understanding Relay Basics And Common Symptoms

Before testing, know what you’re looking at. A standard automotive relay has four or five terminals in a plastic housing. These terminals are labeled on the casing, usually with standard numbers: 85, 86, 30, 87, and sometimes 87a.

Here is what each terminal does:

85 and 86: The coil terminals. These receive the low-current control signal.
30: The common terminal. This connects to the power source (battery positive).
87: The normally open terminal. This connects to the device (like a fan motor).
87a: The normally closed terminal (only on 5-pin relays). It’s connected to terminal 30 when the relay is off.

Recognizing a failing relay is the first step. Common symptoms include:

A component that doesn’t work at all (no sound, no movement).
Intermittent operation, where a device works sometimes and not others.
Hearing a rapid clicking sound from the relay or the component.
A component that works only after tapping on the relay.

Gather Your Tools And Locate The Relay

You don’t need many tools for basic relay testing. For most methods, you will need:

A known-good relay of the same type (for the swap test).
A multimeter (digital or analog).
A 9-volt or 12-volt battery with wires for bench testing.
Your car’s owner’s manual or a service manual to find the relay box.

Relays are typically housed in one or more fuse boxes. Common locations include under the dashboard near the driver’s knees, under the hood in the engine bay, or in the trunk. The owner’s manual will have a diagram showing which relay controls which circuit.

Safety Precautions Before You Start

Always disconnect the negative battery cable before removing or installing relays if you are working near live circuits. This prevents accidental shorts. Be aware that some circuits, like the fuel pump, may have residual pressure or require special steps to disable.

The Simple Swap Test: Your First Diagnostic Step

This is the fastest and most practical way to check a suspect relay. It requires an identical, working relay. Many car use the same relay for multiple non-critical circuits, like the horn and the blower fan.

Follow these steps:

Identify the relay for the faulty component (e.g., the fuel pump relay).
Find another relay in the box with the same part number. Choose one for a system that is currently working, like the high-beam headlights.
Carefully swap the two relays. If the problem moves to the other component, you have confirmed the original relay is bad. If the problem stays with the same component, the issue lies elsewhere in the wiring or the device itself.

Testing A Relay With A Multimeter

A multimeter allows for a more thorough check without swapping parts. You can perform tests both in the vehicle and on your workbench.

Checking The Relay Coil Resistance

The coil inside the relay has a specific resistance. A reading that is too high (open circuit) or too low (short circuit) indicates a bad coil.

Set your multimeter to the ohms (Ω) setting.
Touch the probes to terminals 85 and 86.
A good relay coil typically shows a resistance between 50 and 120 ohms. Consult a service manual for the exact spec. A reading of “OL” (open loop) means the coil is broken and the relay is faulty.

Testing The Switch Contacts

This test checks the high-current switch inside the relay. For a 4-pin relay, terminals 30 and 87 should be open (no connection) when the relay is at rest.

Set the multimeter to the continuity or resistance setting.
With the relay de-energized, place probes on terminals 30 and 87. You should get no continuity (“OL” on resistance).
Now, you need to energize the coil. Connect a 12-volt source to terminals 85 and 86. You should hear a distinct click.
With the coil energized, check terminals 30 and 87 again. The meter should now show continuity (near 0 ohms). If it doesn’t, the internal switch contacts are worn out or burned.

Bench Testing A Relay With A Battery

This is a definitive test that simulates the relay’s real operation. You’ll need a 12-volt battery (a car battery is fine) and two sets of jumper wires.

Connect one wire from the battery’s positive terminal to relay terminal 86.
Connect a second wire from the battery’s negative terminal to relay terminal 85. You should hear and feel a solid click.
Now, use your multimeter in continuity mode. Check for a connection between terminals 30 and 87. There should be continuity only when the coil is powered.
For a 5-pin relay, also check terminal 87a. It should have continuity with terminal 30 when the coil is *not* powered, and lose it when the coil is energized.

Listening For The Click And Feeling The Vibration

A simple but not foolproof test is the “click” test. With the relay plugged in and the circuit turned on (e.g., turn the key to “run” for the fuel pump relay), you should hear a soft but audible click from the relay. Have a helper operate the switch while you listen and gently place a finger on the relay to feel the internal mechanism move.

No click could mean a bad coil, but it could also mean the control circuit isn’t sending power. A click without the component working points to burned contacts inside the relay. This test is best combined with another method for a solid diagnosis.

Inspecting The Relay Visually

Sometimes, physical inspection reveals the problem. Carefully pry off the plastic cover of the relay if it is removable. Look for signs of damage:

Burnt, melted, or discolored plastic.
Corroded or oxidized metal contacts.
A cracked housing or loose internal components.

If you see any of these signs, the relay needs to be replaced. Not all relays have removable covers, so don’t force it.

What To Do After Identifying A Bad Relay

Once you’ve confirmed a relay is faulty, replacement is straightforward. Purchase a new relay with the same part number. They are generally inexpensive and available at auto parts stores.

When installing the new relay:

Ensure the ignition is off.
Align the relay terminals with the socket correctly. It will only fit one way.
Press it firmly into place until it seats fully.
Test the system to confirm the repair worked.

If a new relay fails immediately or quickly, there may be an underlying issue, such as a short circuit in the device it controls or a problem with the control signal. Further electrical diagnosis may be needed.

Advanced Troubleshooting: When The Relay Tests Good

If your relay passes all tests but the component still doesn’t work, the problem lies elsewhere. Your next steps should involve checking for power and ground.

Checking For Power At The Relay Socket

Use your multimeter on the DC voltage setting. With the relay removed and the ignition or appropriate switch turned on:

Check for constant battery voltage at terminal 30 (this should always be hot).
Check for switched power at terminal 86 (or 85, depending on the circuit design) when the control switch is activated.
Check for a good ground at the other coil terminal. You can test this by setting the meter to continuity and checking between the socket terminal and a known good ground point on the chassis.

Checking The Load Circuit

If power and ground to the coil are good, the issue might be in the high-current circuit. You can test for voltage at terminal 87 in the socket when the relay should be engaged. Alternatively, check for continuity to ground from the device itself, which could indicate a motor seizure or internal short.

FAQ Section

How can I tell if a relay is bad in my car?
The most common signs are a component not working at all, intermittent operation, a clicking sound from the relay, or the component working only when you tap on the relay. The definitive test is swapping it with a known-good relay or using a multimeter.

What is the easiest way to test a car relay?
The easiest method is the swap test. Find an identical relay in your car’s fuse box for a system that works, swap it with the suspect relay, and see if the problem moves. This requires no tools and gives a quick answer.

Can you test a relay without removing it?
You can perform the “click” test by listening and feeling for operation, and you can check for voltage at the socket terminals with a multimeter. However, for a complete test of the coil resistance and internal contacts, you usually need to remove the relay.

What causes a car relay to go bad?
Common causes include age, heat cycling, moisture and corrosion, vibration, and electrical overload from a failing component it controls (like a motor drawing too much current). The internal contacts can wear out or weld shut over time.

Is it safe to drive with a faulty relay?
It depends on the relay. A failed A/C compressor relay is an inconvenience. A failed fuel pump or ignition relay will cause the car to stall and not restart. A failed cooling fan relay can lead to engine overheating. It’s best to replace a faulty relay as soon as possible.