Wondering on how to check a relay switch? There are multiple ways to keep your relay switch in check. One of the ways is to use a multimeter or test the voltage. Electrical components in a car are controlled by a relay so it’s best to check it.

If any of your car’s electrical components aren’t working properly, a reason could be a faulty relay. We’re here to help you out on how to check a faulty relay and how to fix the faults.

What’s A Relay?

Electrical components in a car are controlled by a relay, and a relay acts as a switch. It lets one circuit control another circuit even if the amperage is different. If a relay gets faulty, it’s best to know the cause of the issue to fix it without any damage to the car. Electrical damage to a car is worse than any external damage like dents.

They’re generally used so that a low amperage circuit can control a higher amperage level circuit. With the help of a relay switch, the low amperage circuits can switch on and off the high amperage circuits. A relay consists of an armature, a coil, and several contacts.

When a low voltage signal is given across the coil, it turns into an electromagnet, and the armature is pulled towards it. The armature proceeds to move towards one contact over the other. If voltage is removed, it de-energizes, and the electromagnet turns back into a coil. The contacts are called normally open NO and normally closed NC.

If you want to ensure electrical components don’t get an oversupply of current, then its best for you to know how to fit a voltage regulator.

We’re going to take your car’s headlights as an example. In the car’s dashboard, if the lights are connected directly to the on/off switch. The amperage levels could increase more than the capacity of the switch. This can melt wires and can also sometimes cause a fire. In case of an emergency, check out this guide explaining what should you do if your car catch fire.

You should know how to replace an old headlamp in this case as it can get damaged. A relay can control multiple systems simultaneously, like activating windshield wipers while your headlights are already on.

Safety Precautions For Relay Testing

Testing a relay is easy, but you should keep in mind all the safety precautions related to it, no matter how easy the task is. The following things should be kept in mind when testing a relay.

  • When you’ve figured out that the relay switch is faulty, you’ve to replace it to fix the issue. Avoid mixing and matching up relays from other parts of the car. An incorrect relay can cause a short circuit or cause a power surge that’ll damage the electrical system of your car. Therefore, choose the right relay and checking battery leads and its connections with other electrical components in your car helps as well.
  • Cautiously handle the relay without dropping it. If the relay damage is internal, it can cause your car’s wiring system to burn or melt. Avoid modifying the relay as well.
  • Avoid areas with flammable and explosive gases like gasoline or any other fuel when you’re working with electrical components such as a relay. Dispose of old gasoline if there is any.
  • Check your car’s manual to know how to work on the car wiring system and relays.

How To Check A Relay? Steps To Follow

  • Find where the relay you want to check is. The relay can be below the dashboard or the engine bay. It depends on what electrical component is it for. Check your car’s manual if you can’t find it.
  • The fastest and probably the easiest way to check a relay is to replace it with a good alternate relay. One drawback is that what cook the first relay could also cook the replacement relay. Clean and check the connectors while the relay has been taken out.
  • You can check the relay with a multimeter. You should know how to check electrics with a multimeter to successfully do this. First, set it to ohms. Connect the leads across the electromagnet coil pins and proceed to measure the resistance. If it’s somewhere around 50-120 ohms, it’s alright. If it either says it’s open or out of range, then it means it’s a bad electromagnet coil.
  • Next, leave the multimeter set in ohms. Connect the leads across the switch pins. As the electromagnet coil charges up and closes the switch, the relay will make an audible click sound. Polarity isn’t important on the 4-pin relay, but it matters on the diode relay.
  • Test the voltage of the relay. Bad contact points can lead to a drop in voltage. Remove the test light and set the multimeter to DC voltage. Then, connect the lead through the test light connectors or switch pin. Remember that the reading should be the same as the voltage of the battery.
  • Check the switch if the resistance is accurate. Take out the positive jumper wire. The electromagnetic coil should then be energized. Put the multimeter in ohms. Then, measure the resistance through the switch pins. A normally energized open relay should be somewhere near to 0 ohms. A normally energized closed relay is supposed to be OL or open.

What Does It Mean By Open or Closed

The words “open” and “closed” are commonly used to explain electrical circuits. It means that either current can flow or it cannot. Suppose there’s a fence gate. Now, if the gate is open, the fence isn’t complete. When contact points are open in a switch, the circuit is open, and like the gate, it’s not complete, so the current cannot flow through. When the contact points are together, the circuit is closed and is complete so that the current can flow through to its desired location from the battery.


Like any other electrical device, a relay switch can sometimes be damaged or faulty. It’s best to check if it’s faulty and replace it. Checking drive belts also help as they have a big role in electrical components of your car. 


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