OMG! | Super Guide About Car Relay, Working, Types, Tests, Purposes | A Magical Explanation |

Car Relay

17 Minutes Read

Do you know, sometimes a relay warms up due to constant usage, causes electrical short circuits inside the relay, and stops the electrical flow.

When the relay cools, it starts the electrical flow again. A relay is a simple switch used to connect and disconnect the electrical circuit.

If you want to gather information about car relay. Or If you want to troubleshoot a car relay.

You came to the right place.

If you are an automotive technician or a non-technical guy.

This page is for you.

In this powerful guide, you will find everything about an automotive relay. Relay types, construction, working, testing, and many more things are explained very easily and also step by step tutorial to troubleshoot relay problems.

You need to know that automotive relays have a big market. The demand for safety, luxury, and comfort in a car makes the automotive relay market grow bigger and it is estimated that by 2027 its market will grow up to 21.8 billion USD from 14.2 billion USD in 2019.

Automotive Relay Market
Automotive Relay Market

What Is A Car Relay


In most simple words we can say that a relay is a switch. It is the same switch we people have in our home walls turning the fan and bulb ON and OFF. Both switches are the same, they both make and break the circuits.

But, listen there is one difference and that is, the home switch is operated manually by hands while in automobiles, it is operated electrically. So, a car relay is also called an electric switch or electromagnetic switch.

Who Invented The Relay

An American Scientist Joseph Henry is considered to be the inventor of the relay in 1835.

Construction Of Car Relay


The basic theme and construction of all electro-mechanical relays are the same. But, the design, pin arrangement, and size differ from one another.

A relay is made up of a small coil of wires wrapped around the central iron core. The coil is fixed on a small metal piece frame called Yoke. And a relay also has a movable contact which is supported by spring. This spring brings back the movable contact to its original rest position when the relay is de-energized.

Moreover, there are two separate circuits in a relay. One circuit is for a coil circuit, which has two wires positive and negative power sources. And the second circuit of a relay is for a high amperage circuit.

When a relay’s coil circuit is supplied with voltage, it generates a magnetic field.

How A 12V Relay Works


Car relay works on the principle of electromagnetism. When the current flows to the relay, it produces an electromagnetic field to attract the movable rod to make the connections.
Now here I am explaining the science behind how an electromagnetic field is produced in the relay.

Consider a relay as two halves. One circuit is called the Coil circuit (also called a low voltage circuit, a low amperage circuit, controlling circuit. The second half circuit is called a load circuit or high amperage circuit.

Remember that the load circuit is controlled by the coil circuit. When a current is applied to the relay’s coil circuit, a magnetic field is produced which attracts the movable rod and makes a physical connection to the high amperage circuit.


On the other hand, when the current is removed from the coil circuit, the movable rod moves back to its original rest position, causing it to disconnect the high amperage circuit.

If you were to look at a coil circuit, you would see an electromagnetic coil. Now here is the interesting point that why there is a magnetic coil in a relay.

Wait? I will come to it later, now I want to help you understand electromagnetism. Look, there is a rule when you bring a magnet closer to the conductor, the electricity is produced.


This happens with electricity when you apply electricity to a conductor, the magnetic field is produced.


You might be wondering why a magnetic field is produced when a current goes in a conductor. The answer is, it is only known to Mother Nature.

However, scientist Michael Faraday has discovered the relationship between the magnetic field and the electric field.

According to Faraday’s law, any change in the magnetic field will produce an electromotive force (Electricity), and any change inflow of electric current in a conductor, will produce a magnetic field around that conductor. When the current flows to the relay’s coil circuit, an electromagnetic field is produced, which attracts the movable rod to connect the high amperage circuit of the relay.

To sum it up, the purpose of the coil in a relay is to bring the movable rod closer to the load wire and make a high amperage circuit connection.

This is an Info
The Coil Circuit controls the high Amperage Circuit of the relay.

This is an Info
The purpose of the coil in a relay is to bring the movable rod closer to the load wire and make a high amperage circuit connection.

Relay Types


Relay is a wide subject. It comes in many different types, designs, and sizes. Each relay has its application.

Some types of relays are Electro-thermal Relay, Solid State Relay, Hybrid Relay, Reed Relay, and Electro-mechanical (Electromagnetic) Relay. Here our focus will be on the electro-mechanical type relay because it is widely used in automotive applications.

Electro-Mechanical (Electromagnetic) Relay


One of the relay’s functions is to perform logic functions. Relays are used for momentary input, time base function, and doing alternative work (like flushing alternative left or right lights).

Some examples of logic functions in a car, interior lights, time delay operation of wiper blades. Since a mechanical contact (A movable contact that makes and breaks the connection) and an electrical coil (which produces magnetism to attract movable contact) is used to do their job that is why a car relay is called in, either way, Electromechanical or electromagnetic Relay.

An electromechanical relay comes in many shapes and sizes. All are used for various demands of various applications. Here are some most common types of the electromechanical relay.

1. Timer Relay

A Timer relay is operated for a while before turning off. A good example is the heated rear window, where the relay is activated for a while then is turned off.

2. Fuel Injection Relay

A Fuel injection relay electronically operates fuel injectors in the petrol engine.

3. Flasher Relay


Almost all cars have a flasher relay. They are used for indicators and hazard warning lights.

4) Glow Plug Relay

The Diesel engine uses a relay to heat the glow plug for some time.

Types Of Automotive Relays

Relays come in different voltage ratings 5 volts, 6 volts, 12 volts, and 24 volts. It also comes in different sizes and designs.

5 volts relays are found especially for automotive sensors. These kinds of relays are also very common in the car.

Electromagnetic relays are a traditional type of relays used in an automotive vehicle. Most of the relays are 12-volt and some cars have 24-volt relays. Here are the most common types of relays used in cars.

  1. 4 Pin Relay (Make And Break Relay)
  2. 5 Pin Relay (Changeover Relay)
  3. 3 Pin Relay

4 Pin Relay (Make And Break Relay)


Four-pin relays are the most common type of car relays. It is the most widely used relay in an automotive vehicle.

Most automotive relays you see in a car are 4 pin relays. In four pin relay, there are two circuits, primary and secondary circuits. Each circuit has two wires.

Sounds good, doesn’t it? Let me further elaborate.


The primary circuit is also called a coil circuit consisting of terminals 85 and 86. Similarly, the secondary circuit is called high amperage or load circuit consisting of terminals 30 and 87. In the coil circuit terminal, 85 is usually connected to a negative power, and terminal 86 is connected to a positive power source.

But be cautious, here is the risky point. If you connect it the other way, you are more likely to harm the device, especially when it is connected to a sensor or transistor. It is because, the coil circuit produces a voltage spike when the current is removed, which can harm the electronic devices. Therefore a diode is fitted across the coil circuit to dump the electrical spikes.

In the secondary circuit, one wire which comes out of the relay is called terminal 30. It is also called “The Power In” terminal, and the second one is called the load terminal or power out terminal. It means when the relay is activated, the power goes in through terminal 30 and exits through terminal 87.


Whenever a relay is energized, the secondary circuit’s common wire contact comes closer to make a connection with the load wire (30 and 87 Terminal).

On the other hand, when the relay is de-energized, the secondary circuit breaks and disconnects the current flow. That is why a four-pin relay is also called “Make and Break Relay”. Usually, a car horn system uses a 4 pin relay.

5 Pin Relay (Changeover Relay)

5 Pin Relay
5 Pin Relay

A five-pin relay is a type of car relay, which has five pins. It is mainly used for circuits that need power when the relay is not activated.

In other words, a five-pin relay is mainly used for circuits that do not need power when the relay is active. It comes in various design and pin arrangements but, the main function is the same.

5 Pin Relay Diagram
5 Pin Relay Diagram

By energizing and de-energizing the relay, the movable contact of the high amperage circuit shifts back and forth to 87 to 87a terminals. Due to these changing features, the five-pin relay is also called Changeover Relay.

When there is no voltage applied to the coil circuit, one circuit of high amperage (terminal 30 to 87a terminal) is in touch with each other and can receive current, but the other terminal (Terminal 30 and 87) does not receive current. While the coil circuit gets energized, the second circuit of high amperage (Terminal 30 and 87) receives current.

A five-pin relay is usually used in headlight operation where the relay can switch between two circuits to activate full beam and low beam as required.

Five Pin Relay Terminals
Five Pin Relay Terminals

3 Pin Relay

3 Pin Relay
3 Pin Relay

Here, I explain the three-pin relay in the last, because when you have a better understanding of the four and five-pin relay, then it gets easy for you to understand a three-pin relay.

A three-pin relay is a type of car relay, which has three pins. It works on the same principle as a four and five-pin relay. Its basic construction and operations are the same as of four and five-pin relay.

In a four-pin relay, four wires come out of the relay. Two wires for the coil circuit (85 and 86 Terminals) and the remaining two wires are for the high amperage circuit (30 and 87 Terminals). But, here is a twist.

3 Pin Relay Wiring Diagram
3 Pin Relay Wiring Diagram

In the three-pin or three-prong relay coil circuit’s 86 terminal (which is usually connected to a positive power source) is connected internally to the high amperage circuit of terminal 30. Both wires (Terminal 86 and 30) are joined together and come out as a single wire from the relay.

And other two wires (85 terminal of coil circuit and 87 terminal of high amperage circuit) also come out separately.

3 Pin Relay Construction
3 Pin Relay Construction

Here you should remember one thing. Always connect the hot power source to the joint terminal (Terminal 86 and 30). And the other two terminals, terminal 85 should be given the ground power source, and the 87 will go-to accessory. So, the total numbers of pins are three “85, 30, and 87”.

These relays were used in older cars especially for horns. Nowadays these three-pin relays rarely come in a car for horn.

Three Pin Relay Terminals
Three Pin Relay Terminals

Car Relay Case Labeling


Some relays are marked on the case for identification purposes.

1. Voltage

A relay is marked up with 6, 12, and 24 volts.

2. Current

A relay is labeled with a current rating of 30/40 amp. Remember this current rating is for a high amperage circuit.

3. Relay Terminals

The numbers 85, 86, 30, 87, and 87a are marked on the plastic case of the relay along with circuit schematics.

What Is The Purpose Of A Relay

The automotive relay has many different purposes depending on the work need. Here are the purposes and functions of an automotive relay.

1. Reliability Of A Component

The purpose of a relay is to increase the efficiency of a component by providing the proper amount of current. It also allows the component to function at a peak performance level.

2. Reduces Voltage Drop

Some circuits take a long distance to operate like a car headlamp or air conditioning system, which takes a few meter wire distances to operate from the dashboard to the engine compartment. And also we know that the longer the wire distance, the more resistance it has.

Hence, causes a high voltage drop and the component will not function in a fully excellent way. So, by using a relay we reduce the voltage drop.

3. Handles High Amperage Circuit

One great use of a relay is to handle the high current load by using a low current load. Some accessories need too much current which exceeds the capacity of the wire, causing burned to the wire. A relay’s coil circuit uses almost 0.5 amp current. In this case, if you use a relay, you will protect the circuit from overloading or overstressing the wire.

Also, a benefit of using a relay is that you will not need high current-rated cables. Hence, reduces cost and weight.

For example, high output headlamp requires a high amount of current which overloads the wire capacity. By using a relay we switch high current circuits using a lower current circuit. A normal relay can handle current up to 40 amp.

4. Controls Many Circuits With Single Relay

Central Lock Door Actuator
Central Lock Door Actuator

The function of the relay is also to control multiple circuits with one relay. A single relay’s trigger will activate one or several higher current circuits.

These circuits do multiple functions at the same time without using a bunch of relays and wires. A good example is a car control locking system. A single trigger result in all door locks locking or unlocking simultaneously.

5. Allows Circuit For Proper Fuse

Another good function of using a relay is allowing you to use a separate and proper fuse for each circuit.

6. Does Logic Functions

One of the relay’s functions is to perform logic functions. Relays are used for momentary input, time base function, and doing alternative work (like flushing alternative left or right lights). Some examples of logic functions in a car, interior lights, time delay operation of wiper blades.

12V Relay WIth Diode

Diode In Relay
Diode In Relay

In an automotive relay, a diode has a special purpose that prevents high voltage spikes produced by the relay’s coil circuit to protect the sensitive electronic circuits.

When an energized relay is switched OFF, the magnetic field collapses, which produces high voltage spikes in opposite polarity across the coil circuit’s windings. This voltage is several hundred volts, but with a very low amp current, and will cause arching on switch contacts or can damage sensitive electric devices.

A diode is installed across the coil in a reverse-biased position and prevents the damage by absorbing the high voltage spikes and dissipating them. Thus, no current will flow through the diode, and electrical components are protected from damages.

Resistor In Automotive Relay Circuit

Resister In Relay
Resister In Relay

Sometimes a resistor is used in place of a diode. The function of the resistor in a relay is the same as in a diode to dissipate high voltage spikes. It absorbs high voltage spikes produced by the relay’s coil circuit due to the magnetic field collapse.

But, here is a problem, this configuration is not as effective as a diode in stopping high voltage spikes. A little bit of voltage can leak through a resistor and can cause electrical damage.

But, here is also a piece of good news for you. The resistor has one advantage over the diode. It is not sensitive to polarity and you can connect any power source (Ground or Hot) to any terminal of the coil circuit (Terminal 85 or 86).

How To Test A Relay

Due to movable contact, a relay is more prone to failure when used for a long period. Too much current can also cause relay failure. Diagnosing a faulty relay is not a hard task. In this tutorial, I will show you step by step procedure of how to test an automotive relay.

One thing you need to know about testing a relay is that the basic procedure of testing every relay in a car is the same. Whether that is a fuel pump relay, Starter motor relay, Car Air Conditioner relay, Headlight relay, radiator fan relay, or horn relay. Don’t be afraid.

I will take the horn relay as a sample to help you understand it in a better way. Suppose your car horn does not work.

Car Horn Does Not Work

Horn Beep
Horn Beep

Here I will show you in seven steps how to figure out the fault when the horn does not work.

Step 1. Check The Fuse First

Fuse Testing
Fuse Testing

Before checking the relay, it is wise to check the horn fuse. The fuse schematic is usually printed on the inside of the fuse box cover or sometimes outside of the cover.

Step 2. Locate And Listen To The Clicking Noise Of The Relay

Testing Relay
Testing Relay

First of all, check the relay location. Fuse box comes in many different locations. It comes under the hood, under the dashboard, or side kick panel.

But usually, a car horn relay comes under the hood fuse box. Once a relay is figured out, have a helper beep the horn and touch the relay if it makes a clicking sound.

If so the relay is working fine. In this case, the horn might be faulty or have a wiring issue. If the relay does not make a clicking sound, then proceed to the next step.

This is an Info
Sometimes a relay makes a clicking sound but still can be faulty. This is because too much carbon is built upon the relay’s high amperage circuit’s contacts (Terminal 30 and 87). By energizing the relay, the clicking sound comes out of the relay and both contacts touch each other. But due to too much carbon buildup, the current does not flow.

Step 3. Remove Relay From The Fuse Box

Extracting Relay
Extracting Relay

Now, it is time to remove the relay from the fuse box for inspection purposes. Smoothly grasp and pull the relay upward by hand while slightly wiggling the relay for gentle extraction.

Step 4. Visually Inspect

Corroded Relay
Corroded Relay

Before you start making any test, first visually inspect the relay, and look for corrosion and carbon deposits. Check relay pin connector and fuse box female connector of the related relay.

Make sure the connectors are clean. Sometimes overheating prevents proper current flow in a circuit.

Step 5. Identify Relay Terminals

Now you have to identify the relay’s terminals. Find out which terminal belongs coil and load circuit. Some relays have a diagram printed on the case itself to identify pin terminals.

Step 6. First Test The Horn

Horn Checking
Horn Checking

Before you test the relay It is good to check the horn. Make sure you have a good working horn.

To test the horn, take a jumper wire, strip both ends of wire insulation exposing copper. Connect one side of the jumper wire with a 12-volt positive battery source and put the other side of the wire in 87 female connectors of horn relay in the fuse box.

Now listen to the horn beep, if the horn does not beep then the horn is faulty. If the horn does beep sound then precede further to test the relay.

Step 7. Check The Relay

Now, it is time to test a relay. First of all, have two jumper wires (if possible fused wires). Connect one side of both jumper wires with battery +ive and -ive terminals and connect the other side of both jumper wires with the relay’s coil circuit. Check for the relay clicking sound.

If the relay does not produce a clicking sound, then the relay is faulty. If it does produce a clicking sound then check for high amperage circuit.

Put a load (Light Bulb) in the relay’s high amperage circuit, the bulb should illuminate. If the light bulb illuminates, then the relay is correct. If it doesn’t, then the relay is faulty.

Step 8. Replace The Relay

While replacing a bad relay make sure the new relay is in good condition and also terminals match with the old relay.

Final Note:

If all tests prove that the relay is correct and still horn does not work. Then check the wires, corroded terminals, loose connections, disconnected wires or the accessory itself be faulty. All of those conditions can cause your circuit to fail.

The Fastest Way To Test A Relay

One of the fastest ways to test a relay is to remove the suspected relay, put the new relay of the same size, shape, and pin arrangement.

Check if the accessory works, if yes. Then the original relay is bad.

How To Test A Relay With Multimeter

There are two ways to test a relay using a multimeter.

  1. Testing A Relay Using Voltmeter
  2. Testing a Relay Using Ohmmeter

1. How To Test A Relay With Voltmeter

A voltmeter can be used to test the relay. First, energize the relay and hear a clicking sound.

Now connect the red lead of the voltmeter to relay’s terminal 87 and the black lead of the voltmeter to the ground power source. At this time voltmeter will show a reading of 12-volt while by de-energizing the relay, the voltmeter should read zero volts.

Apart from this you can set the multimeter to the “Continuity” scale and listen to the beeping sound.

2. How To Test Relay With Ohmmeter

Testing A Relay Using Multi Meter
Testing A Relay Using Multi Meter

Take the voltmeter and place it in the ohm (Ω) reading and set the lowest range of the ohm scale.

Before energizing the relay, connect the ohmmeter leads to a high amperage circuit of the relay. Your ohmmeter should read infinity (0L, Open Circuit).

Now energize the relay, your ohmmeter instead of infinity should detect zero ohms or any resistance value in the number of hundredth or thousandth. Or you can also check the continuity of the beep sound.

This is an Info
Be cautious do not use an ohmmeter on a live wire. It can damage the ohmmeter while testing the resistor when the current is flowing. Here I am talking about the high amperage circuit of the relay. Do not use an ohmmeter when the high amperage circuit is connected to the battery.

This is an Info
The high amperage circuits of some relays are Normally Closed (NC) and should open by energizing the relay. So, be careful to check the relay diagram before testing. Usually, these kinds of relays are marked 30 and 87a terminals.

Testing Relay’s Coil Circuit

The resistance of the coil circuit is approximately 50 to a 120-ohm value between coil circuit pins. If it is less than 50 ohm, it could be faulty.

Some Helpful Tips

Openable and Waterproof Relay
Openable and Waterproof Relay
  1. Mostly moisture can get inside the relay making it prone to failure.
  2. Sometimes testing a relay, power is touched accidentally with ground forcing to blow the fuse. Be careful.
  3. Sometimes, a relay in the fuse box vibrates out due to loosely fitted, which does not work properly, when the car goes on a bump the accessory loses work.
  4. Most relays come with a holder grip. Make sure to have clean and tight holder terminals.
  5. Sometimes too much current passing through a relay causes electrical connectors to “Stick” with each other due to heat. Power still goes on while the ignition is turned off.
  6. Some relays are openable while some are fully waterproof packed.

Frequently Asked Questions (FAQs)

What Does A Relay Do In A Car

A car relay connects and disconnects the electrical circuit. An automotive relay is used to unite and separate the circuit. It uses a low current circuit to make and break a high current circuit.
The magnetic field attracts a movable rod of high amperage circuits to connect with each other. When the relay is de-energized, the spring pulls back the movable rod to its original rest position making the circuit disconnect.

When To Use A Relay In Car

This is a very common question clients often ask me when to use a relay. Car users should know when to use a relay as it saves the wire from burning.
Some components need a high amount of current such as a headlamp, air conditioning, fans, wiper, etc. Such kinds of components can overload the circuits causes overheating or sometimes burning out the whole circuit. So, we usually put a relay in-line to handle a high amount of current by using a low amount of current.
For example, turning the headlamp ON and OFF from the dash. The current will flow in direction of the battery to the dashboard switch to the headlamp. In this way, the length of a wire increases, and also voltage drops.
Therefore, by putting a relay in in-line, we reduce the length of the wire as well as its resistance. So, we use a relay when the component needs a high amount of current or a circuit of lengthy wire to reduce the voltage drop.

Choosing A Relay For A Car

Whenever you choose a relay make sure its pin number, pin arrangement, and relay size exactly match with the old one. Also, make sure you have the same voltage rating and current rating.

Where To Install A Relay

Whenever a load exceeds the limit of wire capacity, a relay should be installed. It will be better to install the relay near the component.
Also, be careful not to install the relay in an open area because rainwater or a body wash can damage the relay, the water can get in and begin to form rust. Install the relay in a safe and closed area, so that it does not shake on a bumpy road.

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