How To Test A Relay

Think of your car’s wiring as electrical synapses in the brain. Each wire branches off from the central nervous system … Continued

Think of your car’s wiring as electrical synapses in the brain. Each wire branches off from the central nervous system (aka the ECU) and sends signals to its extremities (radio, instrument cluster, windows, wipers, etc). What interprets those signals and completes the actions are your car’s relays, i.e. the muscles of your automobile—though not as gooey.

Relays go bad for a variety of reasons, including age, and can impede your car’s momentum and functionality when busted. Without functioning relays, your commutes to work, trips to the grocery store, or tattoo appointments could be in danger. Thankfully, testing a relay and replacing it isn’t rocket science. With a little help from your friends at Car Bibles, even you (yes, you!), can do it from the comfort of your own garage.

Let’s get started.

Common Bad Relay Symptoms

Car electric fuses and relays
Car electric fuses and relays

As with any sickness, a bad relay will tell you if it’s unwell with a set of specific symptoms. Here are four of the most common symptoms of a bad relay.

  • Dead Battery

Relays regulate electrical currents, and if one is going bad, that flow could become unstable and deplete the battery’s juice.

  • Ignition Failure

*Click-click-click is a sound everyone knows and dreads. It’s the harbinger of a car that won’t start. An ignition failure could also mean you have a bad ignition relay.

  • Burnt Relay

Overloading a relay with too much electrical current can, in certain circumstances, cause the mechanical parts of a relay to burn or melt. This can produce visible symptoms such as smoke and, though seldomly, actual flames. Yikes.

  • Stalling

As a relay regulates the flow of electricity to a number of systems, it can be interrupted and the car may stall as that flow isn’t maintained or steady enough to keep the combustion cycle running.

Is My Car Battery the Problem?

This is a common question with a complicated answer. TLDR; maybe. Technically, because a relay regulates the flow of electricity and the battery supplies that current, a bad battery could affect the flow and share similar symptoms as a bad relay. You can, however, more easily test a battery than a relay. All you need to do is follow Car Bibles’ How To Test a Car Battery guide.

What You’ll Need To Test A Relay

Car Bibles HQ has the type of garage most would sell their left arms for, with all the tools necessary for a variety of jobs. But we understand that DIYers may not have the stocked, locked, and ready to rock garages of tools we enjoy. So here’s a handy guide of what you’ll need to test a relay.

Relays with five outputs
Relays with five outputs

Time

It’ll take anywhere between 30 minutes to an hour and a half.

Space

You’ll also need a flat workspace, such as a garage floor, driveway, or street parking.

Tools

  • An ohmmeter, also called a digital multimeter (DMM)
  • A high-impedance light.
  • Vehicle service manual (strongly recommended but not required)
  • Jumper wires
  • A 9-12V battery

Parts

  • A replacement relay.

How To Test A Relay

Now that you’re ready to rock, and feel confident that your car’s relay is the evildoer, let’s test this relay!

  1. Locate the relay, either under the dash or in the engine bay.
  2. Turn or press your vehicle’s ignition to Accessory.
  3. Grab your high-impedance test light and attach the ground wire to a ground spot on your vehicle.
  4. Use the test light to probe the wire coming out of the relay. If the light comes on, the relay has voltage and is working as intended. Probe the rest of the wires that lead into the relay with your test light.
  5. Turn your vehicle off.
  6. Locate the power and control terminals for the relay.
  7. Use your ohmmeter to check for continuity between power terminals. If there is continuity, you’ll need to replace the relay.

FAQs About Testing a Relay

Do you have questions? Do you need answers? Don’t worry, Car Bibles has your back.

Q: What Happens If I Ignore the Problem?

A: Ignoring a malfunctioning relay can become a big problem underneath your hood. In extreme cases, you could fry your car’s wires and possibly start a fire.

Q: Can I Test a Relay Without an Ohmmeter?

A: No, but you can replace the suspected bad relay, which is the cheapest option. That said, it might not be the culprit, and you’ve just spent money you may not have had to. Just pony up for a cheap ohmmeter and get the reassurance you need. Every garage has to have one.

Q: Where Are My Vehicle’s Relays?

A: Most of your car’s relays should be located in easily accessed locales. Thankfully, your car’s manufacturer gave you the greatest tool in finding them: that dusty manual you’ve stashed in your glove box. Flip through its pages to find the exact locations or hit up the good ole interwebs.

Q: Do All Vehicles Have Relays?

A: Given the number of electronic devices and systems in modern vehicles, all new vehicles have relays.

Q: How Much Does It Cost To Test A Relay?

A: If you already have an ohmmeter, it won’t cost you a thing. Just time. If you don’t have an ohmmeter, it could cost you between $20-$50 for a quality tool. Seriously, buy one.

Sources:

  1. How Relays Work– HowStuff Works
  2. How Do Relay’s Work? –Explain That Stuff
Jonathon Klein
Jonathon Klein

Jonathon has jumped Aston Martins for Automobile Magazine, clocked 200 mph in a McLaren 720S for Playboy, and sampled his best life behind the wheel of a Ferrari Dino Evo for Road & Track. He’s hopelessly addicted to the strongest coffee he can brew. Please send him more. Contact the author here.