How to Test Automotive Relay (Car Relay)?


In modern cars, the use of relays to control electrical components is essential for proper vehicle operation. Without these small devices, you wouldn't be able to start your car or turn on the headlights. But when something goes wrong with a relay, it can cause major problems that are difficult to diagnose and repair.

That's why knowing how to test automotive relays is so important. In this article, we will explain the basics of how these devices work and provide step-by-step instructions on how to test them in order to identify any potential issues. With this knowledge in hand, you'll be better equipped to troubleshoot and fix any electrical problems that may arise in your vehicle quickly and easily.

Understand Car Relay and Its Working

A Relay is an electronic switch. It contains a coil, armature, and two contacts. When we apply a low voltage across the coil, it magnetizes and draws in the armature to bridge between both of its designated contacts. After switching off this power supply , the electromagnet deactivates, causing the armature to return back onto its original contact point.

We refer to the contacts as Normally Open (NO) and Normally Closed (NC). In its de-energized position, the armature connects with NC. When energized, it makes contact with NO instead. As the link between both types of contacts is an armature – known as COM for "Common" - these settings will alternate depending on whether or not power flows through them.

The COM, NO, and NC terminals are completely independent of the coil's energization - when it is activated or deactivated, the switch between these three terminals will change accordingly.

Materials Needed to Test a Relay

The best thing about testing relays is that you don't need much to get started. Here are the materials you will need:
  • Test light.
  • Jumper wires.
  • Vehicle service manual.
  • Ohmmeter, or digital multimeter.
  • Replacement relay.

To ensure success, have all the necessary items at hand before you begin your project; there are better solutions than walking away from what you are working on time and time again. Keeping your supplies within arm's reach will help keep distractions to a minimum and will make your job much easier.

How to Test an Automotive Relay

Once you have the necessary materials, you can begin testing automotive relays. Follow these steps to get started:

Locate the Relay
Before you can assess the functionality of your relay, you must first identify its exact location. Some relays are concealed beneath the dashboard, yet others may be situated in the engine bay – whereabouts depend on what it regulations. If you're unsure where to Locate it specifically, consult with a vehicle service manual for an accurate wiring diagram.
Note: If you're dealing with multiple relays, label each one clearly and distinctly – this will make it easier to determine which relay is causing a problem.

Swap the Relay
Although swapping a known good relay with the suspected bad one may be the simplest way to test, this method carries a risk of ruining both relays due to potential electrical shorts or issues. Not only that, but it could also cause you to incur unnecessary costs. If you do decide on this approach, ensure that all connectors are carefully cleaned and inspected before and after fitting them in their sockets.

Use a Multimeter
To ensure accuracy, you'll need to employ your multimeter for testing the relay. Change the setting on your multimeter to Ohms and clip one of its leads onto each input and output pin in order to measure resistance - a working relay should show between 50 -120 ohms. If all readings display Open or Out of Range, it indicates that there has been damage done to some internal components; thus necessitating, a replacement device is necessary. However, if everything checks out fine, connect the remaining two leads across the switch pins and observe any signs reading OL (Open Load).

Add Power
Now, we can activate the relay with a 12-volt battery. When the coil closes, you should be able to hear a clicking sound; only 4-pin relays don't require polarity here. To test if everything's running efficiently, attach your test light between the ground and switch terminal - when on, this will signify that it is all set up correctly. The light should turn off once you remove the jumper cable.

Test Switch
After verifying that everything is running properly, you should then proceed to the switch. A poor contact point can result in a voltage drop or no voltage at all. Remove your test light and set your multimeter to read DC volts. Connect the leads on the pins of the switch; this reading ought to reflect your battery's standard 12V output.
Disconnect the jumper, energize the relay, and set your multimeter to ohms. Measure the resistance of the switch in order to test its strength. The open relay will measure close to 0 ohms when it is powered on, whereas as a closed one reads as " OL" or Open.
After attempting these steps, if you are still uncertain about the relay's condition, it may be advisable to seek help from an expert.

Important Relay Safety

Although testing automotive relays is relatively simple, please note that they should only be done with due caution. Failing to do so could result in dangerous consequences and/or harm yourself or others. If you have any doubts about what you are doing, it would Be best to seek the assistance of an experienced professional.
Here are some important safety tips to keep in mind:

  • Never attempt to utilize a relay from another car or any other random object, as this could lead to faulty power surge, potentially causing harm and damage to your vehicle's electronics.
  • It is critical to handle it with the ultimate care. Dropping or subjecting your relay to damage may result in melted wires, charred insulation, and even a potential fire hazard – making modification an absolute no-no!
  • When dealing with electrical relays, it is essential to ensure you avoid any flammable gases. By finding a secure spot in your garage or workshop, you can work safely and efficiently with automotive relays.
  • Ultimately, ensure that you consult your service manual prior to starting any work involving a relay. You might be well-versed in the fundamentals of its operation, but it is critical to replace components according to factory specifications.


Testing a relay can be a relatively simple task if you are equipped with the right knowledge and safety measures in place. By following these above steps, you will be able to troubleshoot any faulty relays and get your car up and running again in no time. With that said, if you ever feel you are in over your head, it is best to seek help from a qualified technician.

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