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Summary
  • Relays are electronically operated switches. They rely on small currents from a remote switch or computer to cut off or complete a circuit.
  • Relays are used in vehicles to handle high currents. Relays also allow the driver to control multiple electronics from a single input device.
  • Relays also allow the vehicle’s Powertrain Control Module (PCM) to control critical vehicle systems.

Electric relays are electronically operated switches. They rely on small electrical currents to trigger bigger currents. Basically, relays allow you to control high-powered devices using just small currents. They are also used to reverse polarity when used in pairs and can be used to switch the same circuit from power to ground.

Why Are Relays Used in Cars?

What does a relay do in a car? Relays are used extensively in vehicle electrical systems for a wide variety of reasons.

To Handle High Currents

Relays allow drivers or passengers to control high-amperage electronics with a simple switch inside a cabin. A relay makes controlling electronics less complicated since they allow high-amperage electronics to connect to a nearby relay box located under the hood instead of connecting to a switch inside the cabin. The relay box contains the relays for various vehicle systems like air-conditioning, lights, fan motors, and other vehicle equipment.

One clear example of this is the starter solenoid, which is basically a contactor-type relay that delivers very high current to the starter motor but is triggered by a relatively weak circuit from the ignition switch.

illustration of small relay triggering larger relay
The above circuit illustrates how a small relay can be used to trigger a larger relay. | Image Source: Richard McCuistian

The proximity of the relay box allows vehicles to have less high-amperage wires and switches, which tend to be bulkier and more expensive. As a result, automakers can have more space to fit more features. If the high-amperage devices like the starter motor, blower fan, or headlights are wired directly onto the dashboard buttons, then thick wires would need to lead into the cabin. Directing and hiding these bulky wires would be a challenge.

Relays Make Controlling Electronics More Convenient

The dashboard and steering column usually have the controls to basic electronics such as lights, windshield wipers, and air conditioning. Relays allow drivers to operate those high-amperage electronics with small buttons or controls on the dashboard. If it weren’t for relays, the buttons or controls inside the cabin would need to be designed to handle high-amperage currents.

, What Is a Relay In a Car?

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Relays used in pairs can be used to control DC circuits that provide reversed polarity to function one way and another, which is simpler than doing the same job with only switches.

illustration of circuit showing relays
Example circuit showing relays | Image Source: Richard McCuistian

Without a relay, you also wouldn’t be able to control these electronics from a single input. For example, the two steering-mounted stalks control the vehicle’s turn signal lights, headlights, high beams, and windshield wipers. The stalks can control several key vehicle features because they outsource the responsibility of handling large currents to the relays.

Relays Allow the Computer to Control Vehicle Features

Relays also allow the vehicle’s Powertrain Control Module (PCM) to control critical vehicle systems. Some examples of components that the PCM controls include the fuel pump, radiator blower fan, and ignition, among several critical parts. Modern vehicles have advanced ignition, fuel, and cooling systems that rely heavily on the PCM to make the vehicle function optimally and efficiently.

Types of Vehicle Relays

Relays come in different types, and each type plays a crucial role in the operation of various components in vehicle electrical systems. There can be different types of relays depending on their design and construction. Distinguishing one from another involves looking at their markings that can indicate their circuit schematic, voltage rating, current rating, and terminal numbers.

Make and Break Relay

Make and break relays are the most simple. As their name suggests, they simply make the connection when they’re energized and break it when they’re de-energized. These are four-terminal relays, but five terminal relays have a normally open terminal and a normally closed terminal so that whatever is coming in the common terminal will switch from one of those to the other.

standard iso relay illustration
The relay shown in the illustration is a standard ISO relay, but there are many other relay configurations too numerous to illustrate here. | Image Source: Richard McCuistian
relay diagram
Relay diagram | Image Source: Richard McCuistian

Many other types of relays work similarly to a make and break relay. However, they can have integrated fuses, two output paths, diodes, and resistors, depending on the application.

Time-Delay Relays

Time-delay relays can cut off power to components after a set amount of time and have internal solid state circuits in addition to their electromagnetic switch. They can also add a delay before a component receives power. Time-delay relays are further subdivided into two types.

On-delay timers start timing when the input voltage is applied and power the second circuit after a predetermined amount of time. This can be used to delay when power is transferred to multiple other components to prevent power surges.

On the other hand, off-delay timers only send power after a certain delay from when the input signal is removed. The delay is reset by reapplying the input signal. Relays like these are used to power features like flashing lights, glow plugs, and the fuel injector.

Overall, relays might be small components but they play a huge role in your vehicle’s operation. If you need to have a relay replaced in your vehicle, then you should do so immediately because a device in your vehicle won’t work without it.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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