How to Tell if a Relay is Bad in a Car, Truck, or SUV

How to Tell if a Relay is Bad in a Car, Truck, or SUV

What Does a Relay Do?

Before we discuss how to tell if a relay is bad in a car, let’s cover the purpose of a relay. Relay switches are used to reduce the amount of electric current that’s flowing through a particular circuit inside your car. It takes a huge amount of amperage to operate some of the vehicle’s functions. As such, allowing all of it to go through a switch without a relay could cause the switch to burn a lot faster. 

Relays are used with headlights, the car’s horn, automatic windows, air conditioning, and other functions that require a circuit. They are designed to take on the excessive amperage but can also burn out after some time. 

How do Relays Work? 

Schematic of a car horn circuit including relay switch

A car relay works like a switch that’s controlled by electrical power. When you turn on something in your car that requires a lot of current, say your wipers, horn, or lights; the current is passed through the relay where two pints of contacts close to power the accessory. 

A typical car relay has two circuits – an energizing circuit that has a coil and a contact circuit that has a lever. To better understand how these work, we will take the example of a vehicle’s horn.

A car’s horn comes with a four-pin relay. Two of the pins sit opposite each other and are connected to the coil. The other two are connected to the horn with an open contact between them. Each pin will have its own number to make it easy to identify the power source. 

When you press the horn button on your steering wheel, current flows through the two pins that are connected to the coil. This coil then generates an electromagnetic field that attracts a lever found along the contact circuit. The lever closes the point of contact, allowing the current to reach and activate the horn. 

How to Tell If a Relay Is Bad in a Car

Step 1: Check for Clicking Sounds 

Mechanic pointing to a relay switch inside the fuse box.

If any one of your vehicle’s accessories isn’t working and you suspect that the relay is bad, you can do a simple test to confirm your suspicion. Your relays will most likely be found inside a fuse box that’s under the hood. Consult your user manual, if you can’t tell how to locate the relay in your car, truck, or SUV.

Taking our horn example, if you have one that doesn’t produce sound when you press the button on your steering wheel, have someone press it as you place your finger on the relay. You want to feel and listen for any clicks. 

If you feel and hear clicks, the relay works on one side – the side with the energizing circuit and coil. However, if there was no sound from the horn, the relay has a problem along the contact circuit. It could be that there’s no power getting to that side or you have a bad contact, a bad fuse relay, a bad horn, or a faulty ground connection. 

On the other hand, if you didn’t feel or hear any clicks as the horn button was being pressed, the relay has a problem along its energizing circuit. It could be that there’s no power getting to that circuit or you have broken internals in the relay, a bad horn switch, a broken connection, or bad ground. 

Step 2: Swap Relays

Swapping relay switches in a fuse box to test for bad relay.

Another easy way of finding out whether your relay is bad is to swap it with another one that’s identical to it. You’ll need to first ensure that the numbers indicated on both relays match. 

Secondly, the swap should be between the bad relay and another working relay. You can check the circuit of the second relay to ensure it works. If it’s for the headlights, ensure the headlights work. If your horn or any other vehicle accessory works with the second relay, you’ll be now sure that there’s a problem with the first one. 

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Test to Tell if a Relay is Bad in a Car

The first two tests described above help to tell if a relay is bad without using any tools. Below are two alternative tests that you can perform using reliable tools from 1A Auto to confirm that your relay is indeed bad. 

Step 1. Relay Tester

Relay Testing Tool - How to Tell if a Relay is Bad in a Car, Truck, or SUV

A relay tester is a tool that helps to test a relay’s electronics for any electrical problems. It applies a signal to the coil along the energizing circuit and checks whether the lever along the contact circuit responds appropriately. 

All you’ll need to do is connect the tester to your car’s battery, plug the relay into one of the tester’s configurations, and press the TEST button. Remember to set whether you’re testing a 4-pin relay or a 5-pin relay. The tester will produce a green light if all is well and a red light if the relay is bad. 

Step 2: Relay Kit

Using a test light to test for bad relays

With the relay kit, you can test both relay circuits for power using 4-pin and 5-pin adapters that come inside the kit. Each adapter has numbers to match each pin on the relay itself. All you need to do is connect your relay to the appropriate adapter, insert the adapter where the relay sits in the fuse box, and test each of the pins for power and ground using a test light. When testing for power, you should see the test light turn on. When testing for ground, you should hear the sound of the horn go off. 

Step 3: Fused Jumper Harness

Fused jumper harness

When using a fused jumper harness, you’ll need to connect it to where the relay pins connect inside the fuse box. You can start by testing the contact circuit. Remember that pins that form part of the same circuit sit opposite each other. This is mostly in a diagonal fashion.  

If there’s no break in the contact circuit, you should hear the sound of the horn or see the vehicle accessory that’s not working turn on. This could be your wipers, air conditioning, or headlights. 

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How to Tell if a Relay is Bad in a Car, Truck, or SUV
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How to Tell if a Relay is Bad in a Car, Truck, or SUV
Description
Before we discuss how to tell if a relay is bad in a car, let's cover the purpose of a relay.
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1A Auto
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