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How to Test an Automotive Relay at Home (5 Easy Steps)

Your vehicle includes a multitude of electrical components, all of which are necessary to work together in harmony for a seamless experience. These components are responsible for all systems, from headlights to air conditioning. When something goes wrong with electronics, you may be looking for a faulty relay and wondering how to test a relay.

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

Important safety relay

Although testing car relays is simple, there are some important safety precautions you should take. If you are not sure what you are doing, you could easily create big problems or cause injuries.

First of all, never replace relays with parts from another car or something lying around. Using the wrong relay can cause a surge or short circuit, damaging the vehicle’s electronics.

Second, you should always handle your relay with care. If dropped or damaged, the cables may melt or burn. This is another reason why you should never modify a relay either.

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

Finally, consult your service manual before working on a relay. You may understand the basics, but it’s important that everything is replaced to factory specifications.

RELATED: 3 Symptoms of a Bad Main Relay

Equipment needed to test a relay

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

  • Pilot lamp
  • Ohmmeter or digital multimeter
  • vehicle service manual
  • Replacement relay
  • Bonding wires

For best results, organize all your supplies before you begin. You want to have everything at your fingertips, so you don’t have to deviate from what you’re working on.

RELATED: 5 Symptoms of a Bad Fuel Pump Relay

How to test an automotive relay

Here are some steps to test an automotive relay at home:

1. Locate the relay

Starter Relay Location

Before you can test the relay, you must find it. Some relays are under the dash, while others may be in the engine compartment.
The location will depend on what the relay controls. If you don’t know where it is, check your vehicle’s service manual for the wiring diagram.

2. Change the relay

The easiest way to test the relay is to replace it with a working one. However, when you do this, you risk ruining a good relay due to an electrical short or an existing problem.
Additionally, this procedure could cause you to spend more money than necessary. If you decide to go this route, be sure to clean the connectors and inspect them while removing the relay.

3. Use a multimeter

multimeter relay

With your multimeter, you can test the relay. Set the multimeter to ohms and connect the wires between the input and output pins to measure resistance. You want to see 50 to 120 ohms.
If the multimeter indicates open or Out of range, it may mean a defective coil winding. In this case, you need a replacement relay.
If everything looks good, you can connect the wires between the switch pins. An open relay will display OL or open.

4. Add power

Next, you want to power the relay with a 12-volt battery. You should hear a click as the relay coil closes the connection. Polarity is vital with diode relays, but not with 4-pin ones.
Connect your pilot light to the ground and change the terminal. Its test light will read the current, causing it to turn on if everything is working as it should. When you remove the jumper, the light should go out.

5. Test switch

If everything works as it should, you want to move on to the switch. A poor contact point can cause a voltage drop or no voltage at all. Remove the test light and set your multimeter to read DC. Connect the wires to the switch pins. This reading should equal the battery voltage, usually 12 volts.
Next, test the resistance of the switch. You must disconnect the jumper and activate the relay. The multimeter should be set to ohms, measuring the resistance of the switch. The energized open relay will measure near 0 ohms, while the closed relay will be OL or open.
If you are unable to diagnose the relay after these steps, you may need professional help.

What is an automotive relay?

Relays are found in all modern vehicles. They are also part of other applications, such as aviation and ships. Typically, the relay is used to control a low-amperage circuit over a higher-amperage circuit. The relay acts as a switch would, allowing low-amperage circuits to be powered from high-amperage circuits.

If you take the car headlights as an illustration, you will see that a relay is used. Otherwise, you would be plugging the lights into your vehicle’s switch and the amperage would exceed the capacity. This problem would cause melted wires or a car fire. Instead, a relay is used to control the system. You can also activate multiple systems simultaneously, such as turning on the headlights while the wipers are working or extending a radio antenna when you turn on the stereo.


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