How to Test a Relay with Multimeter (Step-by-Step Guide)

Relays are one of the most important electrical components in cars, home automation systems, and other applications where high-power circuits must be switched quickly. However, like electromechanical devices, relays are subjected to wear and tear and will eventually malfunction at any time. Thus, it’s crucial to test your relays regularly to ensure better performance.

In general, a successful relay testing with a multimeter involves the following steps:  

  • Locate the relay 
  • Check and clean the connectors
  • Grab your multimeter
  • Energize the electromagnet coil
  • Connect a test light
  • Test the relay’s voltage
  • Test the switch

One of the various methods to test a relay is through a digital multimeter. Let me walk you through the steps to start relay testing with a multimeter.

About Relays

relay

A relay is an electrical control device with a control system (input loop) and a controlled system (output loop) which is frequently seen in control circuits. It functions as a circuit regulator, a safety circuit, and a converter. The relay features a fast reaction, steady operation, extended service life, and small size. (1)

Relays are commonly utilized to control a higher-current circuit from a low-current circuit. They are present in almost any vehicle. Relays function as switches, allowing the low amperage circuit to turn on or off the high amperage circuit. Moreover, relays can also regulate numerous systems simultaneously, such as turning on lamps while the wipers are on or extending an antenna when the radio is on.

What You Need when Testing a Relay

relay and multimeter

Checking your vehicle’s relay is a straightforward procedure that does not necessitate the use of a complete kit. To start relay testing, you’ll need the following items:

Tools: 

  • A test light with a high impedance
  • An ohmmeter, often known as a digital multimeter (DMM)
  • Service handbook for vehicles (not required but highly recommended)

Parts:            

  • Correct replacement relay
  • Jumper wires

Steps to Relay Test

technician testing relay with multimeter

Step 1: Locate the Relay 

Depending on what it’s controlling, you could find the relay under the dash or in the engine bay. Check your service manual’s electrical chapter and wiring diagram if you’re confused about the placement.

Step 2: Check and Clean the Connectors

After finding the relay, take it out. Then, clean and examine the connectors while that relay is out. Swapping the main relay out for a suitable replacement is the quickest and easiest way to test it.

Step 3: Grab your Multimeter

Set your multimeter to ohms mode. Then, measure the resistance by touching the leads across the coil pins. A standard coil is anywhere between 40Ω to 120Ω. A poor electromagnet coil winding suggests the relay is out of range or open, and it’s time to replace it. Then, keep the multimeter in ohms or continuity mode. After that, connect the switch pins with the leads. It should display open or OL if it’s a typical open relay.

Step 4: Energize the Electromagnet Coil 

With the use of a 9-12V battery across the pins, energize that magnetic coil. As the coil energizes and closes the switch, the relay should make an audible “click.” On a 4-pin relay, polarity is unimportant, but it is critical on diode relays.

Step 5: Connect a Test Light 

Jump the battery’s positive to one of the switch terminals while the coil is still active. Then, connect a test light between the ground and the switch terminal. The test light should draw electricity and illuminate. Then, remove the positive jumper from the battery. The test light should turn off after a few moments.

Step 6: Voltage’s Relay Test

At the switch, check the relay’s voltage. Bad contact points can cause a voltage loss. Remove the test light and change your multimeter to DC volts. Then, connect the leads to the test light connectors or switch pins. The reading should correspond to the voltage of the battery.

Step 7: Test the Switch

Check for proper resistance in the switch. The positive jumper wire must be disconnected, and the electromagnet coil should be energized. Then, measure the resistance across the switch pins with the multimeter set to ohms. A typically open relay should measure close to zero ohms when energized, while a normally closed relay should measure open or OL when energized.

Relay Testing Professional Tips

When working with relays, it’s a good idea to keep the following in mind:

Avoid Mixing-and-Matching 

When you have a faulty relay that needs replacement, it’s not a good idea to mix and match relays from other car components or a random garbage bin in your garage. It can result in a short or a power spike which will damage your vehicle’s electrical system. (2)

man tagging box with fragile label

Handle with Care

It’s crucial to take good care not to drop the relay. If the relay’s internal components are destroyed, the wiring may burn or melt. Also, refrain from tampering with the relay.

Stay Away from Flammable Gases 

Do not work with relays or anything requiring electricity in areas where explosive or flammable gases, such as gasoline or other fuels, are present.

Read the Repair Manuals

technician holding a manual

Check your vehicle’s service handbook (not the owner’s manual) to identify and understand the wiring system and relays, even if you’re an experienced garage tinker already.

Organize Your Tools 

Prepare all the necessary tools ahead of time and put everything in place. It can save you valuable time and allow you to stay focused on the task at hand without looking for instruments during the process.

FAQs 

What does it cost to replace a relay?

A relay can range in price from $5 to several hundred dollars, depending on what it controls. Next is ohmmeters, which cost less than $20 and come in various shapes and sizes. Next, high-impedance test lamps are slightly more expensive, costing between $20 and $40 on average. Finally, jumper wires are inexpensive, ranging from $2 to more than $50 depending on the wire’s length.

What will happen if I ignore the potential issue?

Ignoring a faulty relay or slamming in any old relay that fits might cause severe issues. If a relay fails or is fitted incorrectly, it could fry your wires and possibly ignite a fire.

I don’t have an ohmmeter or a test light. Can I still test relays?

No. You only have two options if you’re confident that there’s a problem with your relay, and both require the use of an ohmmeter, test light, etc. First, be cautious and replace your main relay with the necessary tools. Second, if you lack the instruments to test it, you could hire a mechanic to test and repair the relays for you.

You may also check other multimeter test learning guides below;






References
(1) control system – https://www.britannica.com/technology/control-system
(2) garbage – https://www.learner.org/series/essential-lens-analyzing-photographs-across-the-curriculum/garbage-the-science-and-problem-of-what-we-throw-away/

About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gI realized early on carpentry was a huge passion for me and I’ve stayed in the industry for over 20 years now. This gives me a unique ability to really be able to tell you what the best tools and recommendations are. I’m not only a carpenter but I also like machinery and anything to do with electrics. One of my career paths starting off was as an apprentice electrician so I also have a lot of experience with electrical products and anything related.