What Is A Relay Fuse In A Car?-[Detailed Description!!] 

Relay fuses are a type of electromagnetic switch entirely different from a fuse. Relays can provide a connection and operate a circuit rather than just breaking it. It uses electromagnetism to control a high-power circuit using a low-power signal. Thus, it also reduces power consumption.

But, this is not all. Thus, I have elaborated further on this throughout this article. Be sure to read all of it.  

Relay Fuse Structure

A relay fuse has a box-like structure. It’s quite bigger than a fuse. On top of the relay, you might see its number and manufacturer name. The relay also has its wiring diagram drawn on its body. 

The pins of the relay are also numbered using 5 numbers. The numbers 85 and 86 represent the coil pins. The numbers 87, 87a, and 30 represent switch pins. Although, different relays have different numbers of pins. 

Different Types of Relay Fuses and How They Work

Relays can be of different categories. Mainly, relays are of three types:

  1. Electromagnetic
  2. Reed
  3. Solid-state

Based on these types, relays can be further categorized. In the automotive industry, these relays are divided into eight sub-categories. First, let’s see what they are:

Relay FuseSubcategoryPins
Change Over Relays5 pins
Dual Open Contact Relays5 pins
Flasher RelaysElectromechanical Flasher2 or 3 pins
Thermal Flasher
LED Flasher
Overload Protection Relay3 pins
Static Relay (Solid State Relay)Electronic Static RelayNo pins
Transductor Static Relay
Rectifier Bridge Static Relay
Potted Relay3 or 5 pins
Skirted Relay3  or 4 pins
Time Delay Relay4, 5, 8, 11 pins

Now, let’s see what these fuses are.

Change Over Relay

A changeover relay is a type of electromagnetic relay. They are generally made to switch one feed line. This line has two active functions in a circuit. It switches between normally closed (NC) and normally open (NO) to achieve these functions. 

This switch is achieved by a push/toggle switch. 

Here, the low voltage current activates the coil and initiates magnetism. This pulls the armature down and switches the current flow from NC to NO.

An example of the function of Change Over relays is car headlights. By switching from NC to NO, it can switch from low beam to high beam. 

There are mainly four types of changeover relays:

  • Standard Line Changeover Relay: Here, the input changes from 30 to 87a and 87. 
  • Changeover Relay with Diode: It’s used to suppress coil voltage spikes. Here, the 85 and 86 terminals are polarity sensitive, and 86 remains positive.
  • Changeover Relay With Resistor: It does the same thing but is more reliable than a diode. Here, the 85 and 86 terminals can be swapped. 
  • Mechanical Latching Relay: Here, the relay can switch between circuits while the positions remain the same. That is until the coil is energized again.

Dual Open Contact Relays

Dual Open Contact relays are also like Change Over relays with five pins. But, the main difference is they have a dual set of contacts, unlike Change Over relays.

Thus, 87 and 87a terminals are used as NO in these relays. Whereas, 87a is used by NC in others. 

Flasher Relay

Flasher relays are those which control the flash rate of your vehicle’s indicators. There are mainly three types of flasher relays: 

  1. Electro-mechanical Flasher
  2. Thermal Flasher
  3. LED Flasher

There are also solid state-type flasher relays. But, mostly electromechanical thermal relays are used in the automotive industry. As they’re most commonly used in directional indicators, let’s use them to describe the process.

                                                 Source: www.thirdgear.com

Normally, the flasher relay has three pins. One pin connects to the earth which resembles a negative connection. One is connected to the battery for the positive charge and the other is for the load. 

When you activate the right indicators, the switch shifts right. Thus, the load connects to the right indicators. A positive charge flows through and lights the indicator. This connection is continuously interrupted and reconnected. Thus, we get the blinking light.

The same process applies to the left indicators. Although, the hazard switch is also connected to this relay. It’s directly connected to the indicators through diodes and the load of the relay.

When you activate it, the load is connected. And, the charge flows through both side  indicators at the same time. Thus, all of them light up. 


The flasher relay works by establishing a connection with the desired device. It’s mainly used in indicators to make them blink by continuously making and breaking the connection.

Overload Protection Relay

In the automotive industry, the overload protection relay is usually used to protect the alternator from damage. When there is too much current, the relay can sense it through the heat. Then, it breaks the connection and prevents damage. 

Usually, eutectic alloys are used in these types of relays. They are placed in a tube that is connected to a ratchet wheel. This tube is wrapped with a heater coil. The ratchet wheel is connected to a spring with a pall.

                                                Source: youtube.com Electrical Training

When excessive current flows through the coil, it heats up. This heat instantly melts the eutectic alloy in the tube. This enables the wheel to turn. Thus, the pall also turns and extends the spring.

                                                Source: youtube.com Electrical Training

When the spring extends, the contact is broken. Thus, electricity can’t flow anymore. In due time, the alloy cools down. The driver can then press a reset switch to re-establish the connection. 

Summary: By melting and solidifying eutectic alloys, this relay breaks and establishes a connection. 

By melting and solidifying eutectic alloys, this relay breaks and establishes a connection. 

Static Relay

Static relays are also called Solid-state relays. They don’t have any moving parts in them. They can operate more quickly and accurately. Also, they have a higher life expectancy. But its overloading capacity is less and costs more. 

Let’s try to understand how it works using this block diagram. The output from the current transformer (CT) is connected to the rectifier. This output is always electrical. Even if the input is something else, the transducer converts it to an electrical signal. 

The current that the rectifier receives is AC. This is turned into DC current and sent to the relay measuring circuit. This circuit already has a predefined current value set. 

It only generates output if the input current is greater than this predefined value.

This output is sent to the amplifier which amplifies this signal. Then, it’s sent to an output device which activates a circuit breaker. Thus, the circuit gets cut off.  


The static relay works by breaking a connection based on its predefined value.

Potted Relay

Potted relay is also used in the automotive industry and works like any other relay. There’s only one key difference. The bottom part of the relay is sealed using epoxy so water can’t get inside.

Although it has one demerit. If the relay is damaged, it can’t be repaired again. You’ll just have to replace it.

Skirted Relay

Skirted relays are different from the rest because of their extended skirt around the bottom. It’s used to seal the relay around a socket. It serves as protection from water and contaminants. 

These relays are mostly used in the open areas of automotive.

Time Delay Relays

Time Delay Relays can control and maintain the electricity flow through their contacts. Its specialty is that it can delay the current flow for 2-180 seconds. 

Here, a solid-state timing circuit is used with a variable resistor that controls the time delay effect. Generally, it’s used in parking lights to light them for extended periods.

How to Check A Relay Fuse In A Car?

If a relay is bad, the item connected to it won’t work. Thus, you’ll have to see if the relay is working correctly. So, how to tell if a relay is blown? Just, follow these steps: 

Step 1: First, you’ll have to check which relay is responsible for the malfunctioning part. Check the cover of the relay box. It’ll have a diagram of the relays and the associated parts for it.

Step 2: Now, touch the top of the relay. Ask someone to activate that part of the vehicle. If you can’t feel a clicking from the relay then it’s blown.

Step 3: It might be that you can feel the clicking but the part isn’t working. Check for the number on top of the relay. Match it with another relay in the relay box. Then, replace the relay with one having the same number. Now, check to see if the part works. 

Step 4: If the part still doesn’t work, the problem isn’t in your relay. But, if it does, the relay is faulty. 

Step 4.1: If you can’t find a relay with the same number, use a relay tester. First, connect the tester to the battery terminals. Then, connect the relay to the designated spot of the tester. Press the test button.

Step 4.2: If the relay is bad, you’ll see a red light. And, if the relay is good, you’ll see a green light. 

Now that you know your relay is blown, you can either clean or replace it.

Solution: Replace The Relay Fuse

If the relay still doesn’t work, you’ll have to replace it. Take out the faulty relay and purchase a new relay of the same specification. Place the new relay and check if it works. 

Summary: If you think that your relay is bad, verify it using a relay tester. If it’s bad, try cleaning it first. If it still doesn’t work, replace the relay. 

Are Relays Interchangeable?

Relays are replaceable but only with the correct one. Both the amperage and the voltage of the relays should be the same. Also, check if they have the same location and number of pins. Lastly, match their wiring diagrams and functionality. 

Suppose the original relay was a 6V 10A relay. You might take a relay of the same model and wiring diagram. But, if these measurements are different, it won’t work properly. 

However, if you think it requires testing then you can test any type of fuse using a multimeter. 


How Much Does A Starter Relay Fuse Cost?

On average, a starter relay fuse costs approximately $65-$72. The labor cost and the price of the actual part are about the same. The labor cost will be approximately $30-$37. And, the price for the part will be approximately $35. Although, these fees don’t include taxes and other fees. 

How Long Do Fuse Relays Last?

The electrical life expectancy of a relay is about 100000 cycles. Although, the mechanical life expectancy of a relay can exceed 10 or 100 times more than that. Thus, it should last through your car’s life expectancy. Although solid state relays usually last longer than the others.

Can I Start A Car Without A Relay?

No, you can’t start a car without a starter relay. The relay is connected to the fuel pump and the ignition system. Thus, if the relay is bad or doesn’t work, the power will be cut off. The fuel pump and the ignition won’t receive any power. Thus, you won’t be able to start unless the relay is replaced.

Bottom Line!

We hope that now you know what is a relay fuse in a car. Still, there’s an additional tip for being with us till the end. While working with a relay, If you don’t properly understand the diagram, always consult a professional. Thanks for going through the whole article.

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William Baldwin

This Is William Baldwin, The Founder & Managing Editor of this website. Me and my team share automotive tips, tricks, and news

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Talha Atta

Talha Atta, a Mechanical Engineer and experienced technical content writer and editor at Autoglobes.com with a passion for the automotive industry.