HomeTips & GuidesUniversal relay diagram: How does the relay work on auto wiring harness

Universal relay diagram: How does the relay work on auto wiring harness

Universal relay diagram: One easy way to increase the efficiency and reliability of a vehicle is to use relays to switch components and accessories, such as fans, fuel pumps, lights, etc., ON and OFF.

But how does a relay work? Read our universal relay system guide to find out!

Table of Contents

What is a relay?

In simple terms, a relay refers to an electromagnetically operated switch.

You can find a relay in almost all automobiles, including boats, trailers, vans, trucks, and cars.

The relays use an electromagnetic device to mechanically move a switch to break or complete an electrical circuit.

The most common type of relay in the auto industry is the small cube-shaped unit called a mini relay or a standard relay.

A vehicle relay

A vehicle relay

What are the applications of relays in automobiles?

While manufacturers prefer doing these tasks using programmable logic, automotive relays provide a cheap and simple alternative.

Switching high current circuit

The most common auto relay application uses low-current circuits to switch a high-current circuit.

This relay application is ideal where an in-line switch does not have the required capacity to handle the current needed to switch a high-current electrical system.

You can observe this scenario while operating a set of high-powered work lamps. The work lamps can exceed the capacity of the existing loom set-up if they are wired to light when headlights are activated. An automotive relay can solve or fix this issue.

While low-current electrical circuit components are cheaper, high-current components are expensive. Using an automotive relay will allow you to limit the high cost of installing and repairing high-current circuit components at critical parts of the system.

Activating multiple circuits through a single switch

Using a relay activates multiple circuits through a single switch.

For instance, a single input in an electrical system can trigger or activate multiple relays installed in your electrical circuit. The relays can then break or complete circuits, thus executing multiple functions from a single input signal.

The vehicle’s central locking system is a simple example of this in real life. A single key press causes all the car doors to unlock or lock.

Executing logic functions

In recent times, OEM electrical module designs have largely replaced automotive relays.

However, you can still use car relays to perform more complex and simple logic tasks, such as time-controlled functions and momentary inputs. For example, the interior light time delay and the time-controlled operations of wiper blades.

Vehicle enthusiasts may find using automotive relays more appealing, cheaper, and easier to install a simple logic function.

A yellow 5-pin relay switch socket 

A yellow 5-pin relay switch socket 

How to wire a relay switch

Here is a detailed guide to wiring a relay switch.

Know about the relay switch wiring diagram

A 5-terminal automotive relay switch wiring diagram can be the following:

5-terminal relay wiring diagram

5-terminal relay wiring diagram

When power is applied to terminal 85, the relay connects the power from terminal 30 to the accessory connected to terminal 87. When no power is applied to terminal 85, terminal 30 is connected to terminal 87a.

Which relay should I choose?

We recommend avoiding cheaper relays at all costs and instead choosing OEM brands.

For instance, most late-model Gallagher Motorsports use the Bosch relays, an exclusively common PSI Conversion.

According to the PSI Conversion, you should snag them from your local junkyard before using a no-name relay.

You may want to consider the new stand-alone MSD solid-state relays, such as PN 7466. They come with LED indicator lights.

Besides simplifying your automotive wiring harness, an MSD solid-state relay will work on four relays with very few wiring. This makes it a perfect option if you require multiple relays.

A car starter relay

A car starter relay

Which is the proper wire size?

The proper wire size depends on two factors:

  • The length of a wire
  • The wire’s maximum amperage

Check the table below to learn more about the different wire sizes:

Amperage at 13.5 VoltsWire Length and gauge sizes
0 to 4 ft4 to 7 ft7 to 10 ft10 to 13 ft13 to 16 ft16 to 19 ft19 to 22 ft
0 to 10 amperes16-AWG (gauge)16 AWG (gauge)14 AWG (gauge)14 AWG (gauge)12 AWG (gauge)10 AWG (gauge)10 AWG (gauge)
10 to 15 amperes14 AWG (gauge)14 AWG (gauge)14 AWG (gauge)12 AWG (gauge)10 AWG (gauge)8 AWG (gauge)8 AWG (gauge)
15 to 20 amperes12 AWG (gauge)12 AWG (gauge)12 AWG (gauge)12 AWG (gauge)10 AWG (gauge)8 AWG (gauge)8 AWG (gauge)
20 to 35 amperes12 AWG (gauge)10-AWG (gauge)10 AWG (gauge)10 AWG (gauge)10 AWG (gauge)8 AWG (gauge)8 AWG (gauge)
35 to 50 amperes10-AWG (gauge)10-AWG (gauge)10 AWG (gauge)8 AWG (gauge)8 AWG (gauge)8 AWG (gauge)6/4 AWG (gauge)
50 to 65 amperes10 AWG (gauge)10-AWG (gauge)8-AWG (gauge)8-AWG (gauge) 6/4-AWG (gauge)6/4 AWG (gauge)4 AWG (gauge)
65 to 85 amperes10 AWG (gauge)8-AWG (gauge)8 AWG (gauge)6/4 AWG (gauge)6/4 AWG (gauge)4 AWG (gauge)4 AWG (gauge)
85 to 105 amperes8 AWG (gauge)8-AWG (gauge)6/4 AWG (gauge)4 AWG (gauge)4 AWG (gauge)4 AWG (gauge)4 AWG (gauge)
105 to 125 amperes8 AWG (gauge)8-AWG (gauge)6/4-AWG (gauge)4-AWG (gauge)4 AWG (gauge)4 AWG (gauge)2 AWG (gauge)
125 to 150 amperes8 AWG (gauge)6/4 AWG (gauge)4 AWG (gauge)4-AWG (gauge)2 AWG (gauge)2 AWG (gauge)2 AWG (gauge)
150 to 200 amperes6/4 AWG (gauge)4 AWG (gauge)4 AWG (gauge)2-AWG (gauge)2 AWG (gauge)1/0 AWG (gauge)1/0 AWG (gauge)
200 to 250 amperes4-AWG (gauge)4-AWG (gauge)2-AWG (gauge)2-AWG (gauge)1/0 AWG (gauge)1/0 AWG (gauge)1/0 AWG (gauge)
250 to 300 amperes4-AWG (gauge)2-AWG (gauge)2-AWG (gauge)1/0 AWG (gauge)1/0 AWG (gauge)1/0 AWG (gauge)2/0 AWG (gauge)

Step-by-step guide to wiring a relay switch

Step 1: Disconnect your car battery and install the relay switch in a location that is easily accessible and moisture-free.

Step 2: Use a ring connector to hook one end of the 12-14 AWG (gauge) wire to a +12-volt battery terminal. Then, connect the other end of the wire with a spade connector to the relay switch through terminal 30.

Remember to put an in-line fuse between these connections. You can also use a red wire as a good auto-wiring practice.

Step 3: Install a spade connector on the power cable (12-14 AWG) connected to the accessory.

Step 4: Using a 16-24 AWG black wire, crimp one end of a spade connector. Connect the crimped end to the relay switch through terminal 85.

Install a ring connector on the other end of your 16-24 AWG (gauge) black wire, and hook them to a good ground near your relay switch.

Step 5: Crimp one end of a spade connector using a longer wire and hook it to your relay switch through terminal 86. Connect the other end of the spade connector to a good power source for switched power.

If the accessory you are adding needs a switch, place your switch in the wire (16-24 AWG) going to the source of the switched power from terminal 86.

Step 6: When everything, except terminal 87 (the power to the accessory), is hooked up, reconnect your car battery. Switch on the ignition and use a multimeter to test terminal 87.

It should read +12 volts. When you power up your relay, you will hear a small clicking. You can now try to switch the ignition OFF and ON to check if you wired your relay switch correctly.

Step 7: Ground your accessory through the black wire.

Relay harness 

Relay harness 


Relays are some of the most vital components of an automotive wiring harness. Wiring them is easy if you understand the universal relay wiring diagram. 

However, we recommend a custom auto relay wiring harness assembly for a much easier play-and-plug installation.

Cloom Tech is a leading manufacturer of pre-wired auto relay harness assemblies.

The pre-wired relay has many output and input wires with specially determined wire lengths and terminals. Therefore, you can easily achieve a direct connection.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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