How to Test an Automotive Relay at Home

Car Relay

Your vehicle includes a multitude of electrical components, all required to work together in harmony for a seamless experience. These components are responsible for every system, from the headlights to the air conditioning. When something fails with the electronics, you might be looking at a faulty relay, leaving you wondering how to test a relay.

Because there are numerous relays in the vehicle, you want to know these simple steps. Relays operate just like a switch, allowing one circuit to control another one even though there might be differences in the amperage. With our simple steps, you can find the problem and replace it quickly, even if you are an amateur mechanic. 

Important Relay Safety

While it is simple to test automotive relays, there are some important safety precautions you should take. If you aren’t sure what you are doing, you could easily create large problems or cause injury.

First, never replace relays with parts from another car or something you have lying around. Using the incorrect relay can lead to power surging or a short, damaging the vehicle’s electronics.

Secondly, you should always handle your relay with care. If you drop it or it becomes damaged, it might cause wires to melt or burn. This is another reason you should never modify a relay either. 

Because the relays involve electricity, you don’t want to use them anywhere that there are flammable gases. Find a safe location in your garage or workshop to work with the automotive relays.

Finally, check your service manual before working with any relay. You might understand the basics, but it’s important that everything is replaced according to factory specifications.  

RELATED: 3 Symptoms of a Bad Main Relay

Materials Needed to Test a Relay

You don’t need a lot of tools to test an automotive relay. In fact, you might have many of these supplies in your home garage right now.

  • Test light
  • Ohmmeter, or digital multimeter
  • Vehicle service manual
  • Replacement relay
  • Jumper wires

For the best results, organize all of your supplies before you get started. You want everything within an arm’s reach, so you don’t have to walk away from what you are working on.

RELATED: 5 Symptoms of a Bad Fuel Pump Relay

How to Test a Relay

Here is a few steps of how you can test an automotive relay at home.

  1. Locate the Relay

    Starter Relay Location

    Before you can test the relay, you need to find it. Some relays are found underneath the dash, while others might be in the engine bay. 
    The location will depend on what the relay controls. If you aren’t sure where it is found, look in your vehicle service manual for the wiring diagram.

  2. Swap the Relay

    The simplest way to test the relay is by swapping it with one that is working. However, when you do this, you risk ruining a good relay because of an electrical short or problem that exists.
    Additionally, this procedure could cause you to spend more money than needed. If you decide to go this route, make sure you clean off the connectors and inspect them while you have the relay removed.

  3. Use a Multimeter

    Multimeter Relay

    With your multimeter, you can test the relay. Set the multimeter to Ohms and connect the leads across the input and output pins to measure resistance. You want to see 50 to 120 ohms. 
    If the multimeter reads Open or Out of Range, it could mean a defective coil winding. In this case, you need a replacement relay.
    If everything looks good, you can connect the leads between the switch pins. An open relay will show OL or Open. 

  4. Add Power

    Power Relay

    Next, you want to energize the relay with a 12-volt battery. You should hear a clicking sound when the relay coil closes the connection. Polarity is vital with diode relays, but not 4-pin relays.
    Connect your test light across the ground and the switch terminal. Your test light will read the current, causing it to light up if everything is working as it should. When you remove the jumper, the light should go off.

  5. Test Switch

    If everything is operating as it should, you want to move onto the switch. A bad contact point can lead to voltage dropping or no voltage at all. Take off the test light and set your multimeter to read DC. Connect the leads on the switch pins. This reading should show the same as your battery’s voltage, typically 12V.
    Next, test the resistance of the switch. You need to disconnect the jumper and energize the relay. The multimeter should be set for ohms, measuring resistance of the switch. The energized open relay will measure nearly 0 ohms, while the closed relay will be OL or Open.
    If you are unable to diagnose the relay after these steps, you might need further help from a professional.

What is an Automotive Relay?

Relays are found in every modern vehicle. They are also part of other applications, such as aviation and marine vessels. Typically, the relay allows a lower-amperage circuit control over another with higher amperage. The relay acts just like a switch would, allowing lower amperage circuits power over high amperage circuits.

If you take the car headlights as an illustration, you see that a relay is used. If it wasn’t, you would connect the lights to the switch in your vehicle and the amperage would exceed capacity. This problem would lead to melted wires or a car fire. Instead, a relay is used to control the system. It can also simultaneously activate multiple systems, such as turning on the headlights while the windshield wipers run or extending a radio antenna when you turn on the stereo. 

 

Written by: Magnus Sellén

Founder, owner & main author of Mechanic Base. I have been repairing cars for more than 10 years, specialized in advanced diagnostics & troubleshooting. I have also been a drifting driver and mechanic for over 7 years.