What Does a Relay Do in a Car?

The relay is an electromechanically operated switch that turns the electrical circuits on and off in a car. Learn more here.
Written by Talullah Blanco
Reviewed by Melanie Reiff
The relay in a car automatically switches the electrical circuits on and off, including circuits for the headlights, automatic windows, air conditioning, and more. If your car relay isn’t working, you’ll need to replace it with a new one.   
Your car relay is a small but mighty component in your vehicle used to operate different components in your car’s electrical system like the horn, headlights, fuel pump, air conditioning, windshield wipers, and more. If one of your relays burns out, you won’t be able to use a vital electrical component in your vehicle.
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What is a relay?

An automotive relay is an electronically controlled switch in your car’s electrical system used to control the amperage. It enables a low-current circuit to turn a high-current circuit on and off. 
When you turn something on in your car that requires a lot of electrical current, like your air conditioning or headlights, the current passes through the relay to power the accessory.
Relay switches that are correctly wired can control multiple circuits (think master ignition switch) with one output and will minimize the voltage drop from the power source and the device, allowing it to function at peak performance. 

How does a relay work?

Relays use an electromagnetic coil to move a switch and turn a circuit on and off. Now let’s break down what that actually means with an example. 
The relay has two circuits—an energizing circuit and a control circuit. When you press the horn button on your steering wheel, current flows through the relay coil in the energizing circuit, which then generates a magnetic field that moves the switch closed in the control circuit. Once this switch closes, the current can reach and power the horn, and you’ll hear the noise. 

Types of automotive relays

While automotive relays look similar, there are actually many different types of relays used for different purposes. Here are some common automotive relays and their function:
  • Normally open: These normally open relays have four pins and can only be wired one way since they default in the open position.
  • Normally closed: This closed relay is similar to the normally open relay but is defaulted in the closed position.
  • Change-over: These common automotive relays have five pins and can be wired three ways—normally open, normally closed, or as a changeover (both open and closed) switching current from one circuit to another and back again. 
  • Double throw: You can control two different circuits with this relay by wiring one circuit to the normally open contact and another to the normally closed contact.
  • Thermal flashers: Heat from electrical current flowing through the coil activates this relay’s contacts, which close when they cool.
  • Skirted relays: These relays are used on exposed parts of the vehicle. They have a skirt that seals the socket to protect the relay contact from water and contaminants. 
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How do you replace a relay that isn’t working?

Let’s say your ignition system isn’t working and you’ve used a multimeter to confirm you have a bad ignition starter relay. You’ll need to replace it to get it up and running again. 
Luckily, replacing a relay that isn’t working is fairly straightforward. All you need is a replacement relay and to follow these steps:
  • Make sure your vehicle is off
  • Locate your car’s fuse box and remove the cover. Your car may have multiple fuse boxes located under the hood, passenger compartment, and trunk, so make sure you’ve got the right one
  • Identify the relay corresponding to the malfunctioning component
  • Unplug the faulty relay to remove it
  • Put the cover back onto the relay fuse box
  • Insert the new relay, making sure it is in the correct position
Finally, start your vehicle and check the electrical components to see if the relay is functioning properly. If everything is turning off and on as it should be, congratulations are in order. You’ve successfully replaced an automotive relay!

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When a relay fails, it won’t power the accessory it is connected to. For example, if the relay connected to your car’s headlights has blown, your headlights won’t turn on.
Relays are remote switches that use small voltages and currents to control larger volts and make or break an electrical circuit, while a fuse protects the circuit.
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