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HomeTips & GuidesAutomotive relay boxes: How do they work with your auto wiring harness

Automotive relay boxes: How do they work with your auto wiring harness

About Automotive relay boxes, There are so many components under the hood that identifying the faulty one in case of vehicle breakdown is confusing. 

Whenever you face a situation like this, check your automotive relay box first. Ensure that relays and all the accessories are working fine.

If you do so, you save time, effort, and money to repair the relay in the future. 

Today, we will discuss automotive relay boxes and how they work with electrical systems and wiring harnesses of automobiles.

Table of Contents

What are automotive relays?

An automotive relay is a switch that you can find in almost all types of cars, trucks, and even go-karts. 

Every 12-volt electrical system uses this electrically operated switch. Relays come in various designs, but there are electro-mechanical relays in low-voltage marine and automobile applications.  

An electromagnet activates these relays, pulling contacts to break or make a circuit.

Automotive relays are either SPST (single pole single throw) or SPDT (single pole double throw). 

Automotive relays operate via very low currents mounted on control circuits, whereas the switch is on the load circuit. 

These relays are either normally closed (NC) or normally open (NO) contact types. These contacts can handle currents ranging from 30 to 40 amps.

Automotive relay wiring harness

Automotive relay wiring harness

Benefits of using automotive relays:

Relays allow a low amp circuit to switch ON/OFF a high amp circuit, like turning on the car’s headlights. 

It is required because if you connect your headlights directly to the headlight switch, you may exceed the switch’s amperage rating, thus melting wires and triggering a fire.

Further, wires and switches carrying high current are more expensive than low current carrying versions. 

Thus, with the use of relays, you reduce the possibility of buying all these costly components.

You can also use relays to turn on multiple components simultaneously with one output. 

This happens when you connect one output to various relays, allowing relays to open or close the circuits simultaneously. 

For instance, your radio turns ON when you extend the antenna.

Apart from these relays, there are electromagnetic relays you can use to perform more logical operations depending on the inputs. 

For instance, flashing left and right lights alternatively. Nowadays, electronic modules perform all these logical functions in OEM designs, but you can still use relays due to their use and cost-effectiveness in some after-market projects.

Plastic automotive relay box inside the engine compartment 

Plastic automotive relay box inside the engine compartment 

When do you require an automotive relay?

In a vehicle, auto relays are present in all wiring assemblies and box modules, including the engine area, passenger area, front of the car, powertrain, body control or roof, door, seat and fan modules.

Almost all auto circuits use relays to reduce the current that flows through the main control switch circuitry.

For example, if you want to change your headlights to halogen ones, you will need a relay for their operation as OEM headlights are designed for lower current value. 

Similarly, if you put a cooling fan (electrically powered) and twine it without a relay, the switch gets the extra strain. 

The result will be the same: untimely failure of the switch.

About automotive relay boxes:

A relay box or panel is a central unit where you can put multiple relays. All these relays control different systems and components of an automobile. 

Every relay comes with its fuse that helps in controlling the current flow, thus preventing sudden fires. 

Practically, relays and fuses are found in the same place, and you will sometimes find them in the same box.

Relay boxes are a safe and convenient place to put fuses and relays, ensuring complete protection and effective power distribution. 

Wiring becomes easier when all the relays and fuses are in one relay box. As a result, you can troubleshoot any issue in the electrical system quickly. 

Also, it helps minimize wire clutter. With relay boxes, you can find all the relays and fuses quickly and in an organized manner, allowing quick repair/replacement.

Organized wiring harness through an automotive relay box

Organized wiring harness through an automotive relay box

Wiring of automotive relay boxes

If you want to add any new electrical component to your car, you must know the car relay box wiring. 

Locate the box

In any vehicle, there are mainly two locations: under the bonnet and near the steering wheel.

Safety first

Cut off the power supply to the box. Be careful not to hurt yourself with sharp edges.

  • Clean the wires with a wire brash.
  • Remove the negative battery cable from the battery. 
  • Unclip the box and pull out the locking tab by depressing it. 
  • Now, remove the lid of the box.

Connecting wires in the box

You will see a diagram on the lid cover, or you can get help from the owner’s manual, as it will have all the electrical diagrams that can help you connect wires.

Mostly, the diagram has color-coded wires, which helps in easy identification. Follow the lead and connect the wires accordingly.

Put everything back together.

Put the lid back in its place and connect the box to the battery once you are done.

Custom 12-volt relay boxes, wiring harnesses

Relay box harnesses are the best for plug-and-play installations. You need to look for certain factors to design the correct relay box wiring harness.

  • Look for the number of relays you want in the relay box.
  • Identify all the electrical systems and components that relays will control.
  • Consider the mounting options and dimensions of the relay box to fit in the desired place. 
  • Make sure that the relay is compatible with the electrical setup.
  • Identify the relays’ electrical ratings and see if they are compatible with all your accessories.
  • Want pre-wired relay sockets or not?

You can trust Cloom for your relay box wiring harness options. 

  • We have a wide range of relay boxes available from various reputed brands, such as EDDIE Motorsports, Painless Performance, and American Autowire.
  • You can also opt for our custom relay panels for installing lights, fans, and pumps, from a few relay sets, a single relay block for heavy-duty purposes, to multi-relay panels for any other purpose.
  • Our auto relay boxes offer switching functions to support modern safety features and electrical convenience. Also, you can use them with DC voltages in infotainment systems.

Car fuse box wiring harness


It’s pretty easy to replace or upgrade your relay system. Suppose you are looking for a replacement relay wiring harness, contact Cloom

We have all the right components for your job. Whether it’s a starter, fuel pump, or horn relay, we have it all. 

So, don’t worry; we offer relay wiring harnesses for specific models, make, and year of the vehicle.

I am Lillian Yang, having been a sales manager for over 10 years.

I have received many positive reviews from customers. They have praised our excellent service, on-time delivery, and high-quality cable assemblies.

For your projects, please provide cable assembly files/images/smples, etc., so that I can send you a quotation within 24 hours.

Contact me now and let’s get started on building your wire harnesses!

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