How to Tell If a Relay Fuse Is Blown


Vehicles have many components that help them to run. Some of those components are relay switches. Relay switches send messages to different parts of the car so the item they are assigned to will work. An ignition switch helps power the car to start and the starter switch makes sure power goes from the battery to the starter. 

Any relay switch, whether it’s a starter switch or a relay for the horn, can go bad. Some cause more problems than others. There are two ways to check to see if the fuse is bad. One is using a wire with the connections to see if that makes it work. Another way is with the multimeter to check for resistance or check it with a car battery.

One problem may be that overvoltage destroyed the coil’s insulating film. There can be a computer failure, a resistance problem, damage, or exposure to water.

Read on to figure out how to tell if a relay fuse is blown and what could be causing the problem. 

Related: Why Car Won’t Lock or Unlock

A Look at Symptoms

Different relay switches will have different symptoms. There are three to take a hard look at because they are big signs of replay fuse issues. They are:

  • A car that stalls.
  • A car that won’t start every time.
  • Several rapid clicks from the starter. 

One problem may be that overvoltage destroyed the coil’s insulating film. There can be a computer failure, a resistance problem, damage, or exposure to water.

Most of these relate to either the ignition relay switch or the starter relay. However, another relay could be bad if one of your accessories doesn’t work. Either way, you check the fuse for each of these items the same way.

Easy Way to Check Fuse

An easy way to check a possible bad fuse involves visually looking at it and using wire to check its connectivity. You will need a friend. 

Open the fuse compartment so you can get to the relay. The fuse compartment is usually either underneath the dashboard or under the hood. Have your friend turn the key to the accessory position and you touch the relay. It should have a click. It is a bad relay if they aren’t a click. 

To further check out the relay, turn off the car and pull it out of its socket. Put a short piece of wire into the two relay connections. You will need to look at a diagram for the right pins.

Related: Why Does My Car Turn Off While Driving?

Normally, they are called C (common) and NO (normally open). They can also be pins 30 ad 87 on a Bosch automotive relay. Turn the switch to the accessory again and see if it works. You have a bad relay if it works now. 

Second Easy Method

Another way to test a possible bad relay is to swap it out with one that you know is good. If it goes bad too, you will need to inspect and clean connectors to see if it will work. 

Using a Multimeter

Using a multimeter to test a relay fuse is pretty simple. Set the multimeter to Ohms and touch the leads on the electromagnet coil pins to measure resistance. It should be between 50 to 120. If it’s out of that range or says it’s open, you have a bad electromagnet coil. 

Now use the multimeter to touch the leas across the switch pins. It could read open or OL. 

Using a Car Battery

You can use the combination of a multimeter and your car battery to test a relay. You can use a 9-12 volt battery across the pins to energize the electromagnet coil. You will hear an audible click from the coil as it closes a switch. 

Now jump positive to one terminal switch. Attach a test light between the ground and the other switch terminal. The test light should light up. Remove the positive jump cable and it will go off. 

You can now test the relay’s voltage at the switch. Remove the test light and set the multimeter to DC. Touch the leads across the switch pins and see if the reading matches the battery voltage. 

Test the switch for resistance. To do that, disconnect the positive jumper cable. Energize the coil. Set the multimeter back to ohms and measure the resistance across switch pins. It will measure near zero if it’s open and OL if it’s closed. 


What Will Happen If I Ignore The Problem?

You can’t ignore a relay problem or just throw in any old relay that fits. Both can result in burning wires and maybe even starting a fire under your hood. It’s much simpler just to fix a relay problem. 

What If I Can’t Find My Relays?

Most can be found easily but you can have a mechanic look at it if you are uncertain. You don’t want to be messing around under your hood if you don’t know what you’re doing. That can damage your good relays. 

How Much Does It Cost To Test a Relay?

It doesn’t cost hardly anything to test it as many mechanics will test a relay for free. Replacing one is pretty cheap as some can cost as little as $5. However, some can cost up to $100 and you will still need to pay for labor to replace it. That is inexpensive as well with the average cost between $35 to $75. 

How Much Do The Tools To Test It Yourself Cost?

Tools to test your relays are inexpensive with most costing less than $20. Test lights typically cost between $20 and $40. Jumper wires range from $2 to $50, depending on the brand and length of the wires. 

What Causes a Relay To Fail?

Many things can contribute to a bad relay but the most common issue is corroded connectors. When they get clogged by corrosion, they can’t send the full amount of current to start the car. 

Dirt and debris will collect over time and can affect how well a relay works. This type of problem will be signaled by several clicking or buzzing sounds from the relay. 

Bad connectors can also cause a relay to fail. Defective or broken connectors will mean the correct amount of current won’t pass through. 

Other problems that could cause problems with your relays are too much heat that melts contact and an old relay that just wears out. 

How Do Relays Work?

It is an electromagnetic switch that uses a low dose of electric current to turn a larger electric current on or off. The electromagnet is a coil of wire that turns into a magnet when current goes through it. 

Related: Symptoms of a Bad Ignition Coil & How to Fix It

Does a Relay Need To Be Grounded?

A relay that has metal housing may have only one pin. The ground is the second pin. It is the housing that is mounted directly to the chassis. A relay in a plastic housing will have two pins and they are typically reversible. 

What Sound Does a Bad Relay Make?

The electrical signal won’t get to the battery with a bad relay so your engine won’t turn over and may not make a sound. Sometimes, there is an audible clicking sound when you turn on your car but that can indicate other problems as well. 

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