How To Test A Relay With A Multimeter? - ALION

Introduction

A multimeter is your best tool for testing relays in cars, factories, and homes. Think a relay might be causing trouble? A few quick tests will give you a clear answer.

A relay is basically an electric switch. It uses a tiny bit of power to control a much bigger electrical load.

This guide covers everything you need to know. You’ll learn safety basics, required tools, and how to test both parts of a relay step by step.

We’ll show you how to read your multimeter correctly. You’ll know for sure if your relay works or not. We’ll also cover professional testing methods for deeper understanding.

Protection Relay – Critical Components of Electrical Power Grid

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Safety First: Essential Precautions

Why Safety Matters

Safety comes before everything else. Many relays use low voltage, but electric shock is always possible. This is especially true when working on systems connected to main power.

Wrong connections can cause short circuits. This might damage your relay, multimeter, or the entire electrical system.

Safety Checklist

Following safety rules isn’t optional. It’s the foundation of all electrical work. Safe relay testing means accurate results.

Precaution	Reason
Disconnect All Power	Always remove the relay from its socket or disconnect the vehicle’s battery. Never test a relay while it’s in a live circuit.
Wear Protective Gear	Safety glasses are a must to protect your eyes from potential sparks. Gloves can provide an extra layer of insulation.
Work in a Dry Area	Moisture and electricity are a dangerous combination. Ensure your hands, tools, and work surface are completely dry.
Inspect Your Tools	Check your multimeter leads for any cracks or exposed wiring. A damaged probe is a safety hazard.
Know Your Terminals	Never guess which terminal is which. Accidentally sending power to the wrong pins can destroy the relay instantly.

Tools and Basic Knowledge

What You Need

Get everything ready before you start. This makes relay testing smooth and efficient. Using the right multimeter for relay testing is key.

• A Digital Multimeter: You need resistance (Ohms, Ω) and continuity functions. These come standard on almost all digital multimeters.
• Correct Power Source: The relay coil needs specific voltage to work. This is usually printed on the relay (like “12VDC”). A 9V battery works for some tests. But a 12V car battery or DC power supply is better for car relays.
• Jumper Wires: Wires with clips work best. They make secure connections to relay terminals and power sources. This is critical for powered tests.
• Relay Diagram: This shows what each pin does. Most car relays have diagrams printed on them. If not, search online for the part number.

Testing Steps

We’ll do three tests. Together, they give you a complete picture of relay health.

Step 1: Test Coil Resistance

This checks if the control circuit coil is good.

Set your multimeter to resistance (Ω) mode. Use 200Ω or 2kΩ range.

Connect your probes to terminals 85 and 86. Direction doesn’t matter for resistance tests.

You should get a reading. Most 12V car relays show 50 to 120 Ohms when healthy.

“OL,” “1,” or infinity means the coil wire is broken. The relay is bad. Near-zero Ohms means the coil is shorted. This relay is bad and might blow fuses.

Step 2: Check Contact Position

This test checks the switch contacts when the relay isn’t powered.

Set your multimeter to continuity mode. This beeps when probes touch, showing a complete circuit.

First, test the Normally Closed (NC) contact if your relay has one (5-pin relays). Connect probes to terminal 30 (Common) and terminal 87a (NC).

The multimeter should beep continuously. This confirms the default path is closed. No beep means the contact is bad.

Next, test the Normally Open (NO) contact. Move one probe from 87a to 87. Keep the other on terminal 30.

The multimeter should be quiet. No continuity means the switch is open by default. If it beeps, the contacts are stuck closed. The relay is bad.

Step 3: Powered Test

This is the most important test. It checks if the relay actually works. Be very careful here.

Keep your multimeter on continuity mode. Keep probes on terminals 30 (Common) and 87 (Normally Open). The meter should be silent.

Now carefully connect power to the control circuit. Connect negative from your 12V source to terminal 85. Connect positive to terminal 86.

Two things should happen at once. You should hear a sharp “click” from the relay. At the same moment, your multimeter should start beeping.

The click is the internal switch moving. The beep confirms the NO contact closed, completing the circuit.

If your relay has an NC contact (87a), do one more check. Keep power on 85 and 86. Move the multimeter probe from 87 to 87a. The beeping should stop. This confirms the NC contact opened when energized.

No click when powered means the coil isn’t activating the switch, even with correct resistance. Click but no beep means the contacts are burnt or corroded.

Reading Your Results

Let’s put all results together for a final diagnosis.

Good Relay Checklist

A working relay passes every test predictably. Use this table to check what “passing” looks like.

Test Step	Terminals Tested	Expected Multimeter Reading	What it Means
1. Coil Resistance	85 & 86	50-120 Ω (or per spec)	The coil is intact.
2. NC Contact (De-Energized)	30 & 87a	Continuity (Beep)	The default path is closed.
3. NO Contact (De-Energized)	30 & 87	No Continuity (No Beep)	The default path is open.
4. NO Contact (Energized)	30 & 87	Continuity (Beep) + Audible Click	The switch activates correctly.
5. NC Contact (Energized)	30 & 87a	No Continuity (No Beep)	The switch deactivates correctly.

Common Problems

If your relay failed any test, here’s what it means.

**Problem 1: Infinite Resistance (OL) on Coil Test.**

The wire inside the coil is broken. The electromagnet can’t form. Replace the relay.

**Problem 2: Near-Zero Resistance on Coil Test.**

The coil wire is short-circuited. It will draw too much current and blow fuses. Replace the relay.

**Problem 3: No “Click” When Powered.**

This shows mechanical failure. Either the coil is broken or the switching mechanism is stuck. Replace the relay.

**Problem 4: “Click” But No Continuity on NO Contact.**

This is very common. The coil and mechanism work, but the contacts are damaged. They can’t pass current properly. Replace the relay.

**Problem 5: Continuity on NO Contact Without Power.**

The contacts are welded together from past electrical damage. The switch is stuck on. This is dangerous. Replace the relay immediately.

How Relays Work

Think of a relay as a remote-controlled switch. A small signal controls a much larger circuit.

Relays have two separate circuits. Understanding both is key to testing.

• Control Circuit (The Coil): Uses terminals 85 and 86. When you apply correct voltage here, an electromagnet creates a magnetic field inside the relay.
• Power Circuit (The Switch): This involves the contacts. Terminal 30 is “Common” input. Terminal 87 is “Normally Open” (NO). Terminal 87a, if present, is “Normally Closed” (NC).

Look for the diagram on the relay’s side first. This is your roadmap. No diagram? Search for the part number online. Don’t guess which pin does what!

The main relay parameters include rated operating voltage, rated operating current, coil resistance, contact load, etc.

• When a relay is operating normally, it must have a voltage that the coil requires to operate properly. For DC relays, it refers to DC voltage (Figure a), for AC relays, it refers to AC voltage (Figure b). The same type of relay usually has multiple evaluated working voltages to meet the circuit requirements, and the specification number is added to the end of the component to distinguish.

• Rated working current refers to the current required by the coil when the relay is working normally.

Relays are selected based on their rated working voltage, rated working current, and coil resistance. When selecting a relay, be sure it has the appropriate rated working voltage and current.

• Contact load refers to the load capacity of relay contacts, also known as contact capacity. JZX-10mrelay contact load, for example, is 28v*When in use, the voltage and current passing through the relay contacts cannot exceed the rated value, otherwise the contacts will be burned out and the relay will be damaged. The load of multiple sets of contacts of a relay is usually the same.

Recommended reading: Video tutorial on the basics of relay electronics The role and working principle of relays

Relay Components and Functionality

Relays are important electrical components found in many devices, such as: cars, bikes, home automation systems, household appliances.

Key features of a relay:

• Electrically Operated Switch: Relays function as switches controlled by electricity.
• Components: They consist of a coil, an armature, and contacts.
• Operation:
  • When the coil receives a low voltage signal, it becomes an electromagnet.
  • This electromagnet pulls the armature towards it.
  • The armature then switches between Normally Open (NO) and Normally Closed (NC) contacts based on the coil’s energization.

Relay Coil

The relay coil is the heart of the relay, responsible for converting the low voltage signal into an electromagnet. The coil is typically made of copper wire wrapped around a core, and its resistance can vary depending on the relay type and application.

When the coil is energized, it creates a magnetic field that pulls the armature towards itself, switching the contacts.

The coil pins or coil terminals are usually marked on the relay body, and it’s essential to consult the data sheet or wiring diagram to determine the correct voltage and polarity for the coil.

Relay Contacts

The relay contacts are the switching elements of the relay, responsible for connecting and disconnecting the high-power circuit.

The contacts are typically made of metal and are designed to withstand high currents and voltages.

The Normally Open (NO) contact is usually connected to the load, while the Normally Closed (NC) contact is connected to the power source. When the relay is energized, the armature switches the contacts, connecting the load to the power source.

How to test A relay?

Relays are widely used in power protection, automation, sports, remote control, measurement and communication equipment, so it is very important to check and maintain the normal operation of the relay. There are many kinds of relays. Therefore, the detection of the relay cannot be judged only by measuring the resistance value of the coil. According to the different types of relays, multiple detection methods are required.

Overall test idea

1) Measuring contact resistance

Apply the specified operating voltage to the relay coil, and use a multimeter to detect the on-off condition of the contacts at the “R×1k” gear. When no power is applied, the normally open contact does not work, and the normally closed contact conducts. When the power is turned on, you should hear the sound picked up by the relay. At this time, the normally open contact is conductive, and the normally closed contact is reversed, and the switch contact should switch accordingly. Otherwise, the relay will be damaged. The remaining contacts of a multi-group relay can still function if some of their contacts are damaged.

2) Measuring coil resistance

You can use a multimeter to measure the resistance of the relay coil at the R×10Ω position to determine whether the coil is disconnected.

3) Measure the pull-in voltage and current

Use an adjustable regulated power supply to input a set of voltages to the relay, and connect an ammeter to the power supply circuit for monitoring. Slowly increase the power supply voltage. Take note of the voltage and current when the relay starts pulling in.To be more accurate, you can try several times to get the average value.

4) Measure the release voltage and current

Same as the test connection above. When the relay is closed, gradually reduce the power supply voltage. Try to get the average release current and voltage several times when you hear the release sound. Jot down the voltage as well as the current each time.Under normal circumstances, the release voltage of the relay is about 10-50% of the pull-in voltage. If the release voltage is too small (less than 1/10 of the pull-in voltage), it cannot be used normally, affecting the stability of the circuit and causing abnormal operation.

Types of relay test

• Electromagnetic relay test

Place the multimeter on the “R×100” or “R×1k” gear, and connect the two test leads (regardless of positive or negative) to the two pins of the relay coil (as shown in Figure 5). The instructions of the multimeter should be It basically matches the resistance of the relay coil. If the resistance value is obviously too small, it means that the coil is partially short-circuited; if the resistance value is 0, In the event of an infinite resistance value, it means that the pin or coil is disconnected; a short circuit occurs between the two coil pins.

 Reed relay test

Reed relays are also one of the most commonly used relays. The reed switch is made up of a coil and reed switch, as illustrated in Figure 6.The reed switch is made by sealing two non-connected ferromagnetic metal bands in a glass tube, and the reed switch is placed in a coil. When the current passes through the coil, the magnetic field generated by the coil magnetizes the metal pieces in the reed tube. The two metal pieces attract each other due to opposite polarities and connect to the controlled circuit. Several reed tubes can be placed in the coil, and they act simultaneously under the action of the coil’s magnetic field.

Relays are equipped with coil pins and relay switches, and most shells have corresponding marks for identification.

Reed relays can also use a multimeter to detect their coils and contacts. The detection method is the same as that of electromagnetic relays.

Solid State Relay (SSR) testing

The input can be tested with a multimeter. The multimeter is placed in the “R×10 k” equipment, the black test lead (the positive meter of the battery) is connected to the positive SSR input terminal, and the red test lead (that is, the negative meter of the battery) is connected to the negative input terminal of the Soviet Socialist republic. The hand should deflect more than halfway (Figure 9). Retest after exchanging the two test leads. The hands should not be moved. If the pointer deflects to the top or does not move regardless of the forward or reverse voltage access, the solid state relay has been damaged.

The test circuit can also be made according to Figure 10. When the control voltage at the SSR input terminal is turned on, the light emitting diode VD is turned on; when the control voltage at the SSR input terminal is cut off, the light emitting diode VD is turned off.

 Thermal relay test

1) Heating element detection

The heating element is composed of an electric heating wire or an electric heating sheet, and its resistance is very small (close to 0Ω). The detection is shown in Figure 11. The normal resistance of the three groups of heating elements should be close to 0Ω. If the resistance is infinite (the digital multimeter displays “1” or “OL” symbol to indicate that the range is exceeded), the heating element is turned on.

Figure 11.

①Choose 200Ω gear.

②The red probe and the black probe are respectively connected to the two ends of the heating element.

③The resistance is close to 0Ω, indicating that the resistor is normal as a heating element.

2) Contact detection

Thermal relays usually have a normally closed contact and a normally open contact. The test includes working and non-working conditions. The first picture is the detection when the normally closed contact resistance is not working. Under normal circumstances, it should be close to 0Ω. Then test under the opposite conditions. Move the test rod, as shown in Figure 2, to simulate over-current heating and bending of the heating element to make the contacts act. Normal closed contacts become open circuits, and the resistance is infinite.

Figure 12.

①Choose 200Ω gear.

②The red and black probes are respectively connected to the two ends of the normally closed contact.

③The resistance is close to 0 Ω, which means the normally closed contact is closed.

④ Move the test rod by hand.

⑤ The over-range symbol “1” is displayed, indicating that the normally closed contact is open.

 Intermediate relay test

A multimeter’s resistance gear is used for both coils and contacts of the intermediate relay.

1) The contact is detected when the control coil is not energized. Contactors include normally open contactors and normally closed contactors. When the control coil is de-energized, the normally open contact is disconnected and the resistance is infinite. At this time, the normally closed contact is closed and the resistance is close to 0Ω. The detection of the above-mentioned normally open contact is shown in the figure below.

Figure 13.

①Choose 200Ω gear.

②The red and black probes are connected to both ends of the normally open contact.

③The over-range symbol “1” is displayed, indicating that the normally open contact is not closed.

2) The detection of the intermediate relay control coil is shown in Figure 14. Generally speaking, the larger the rated current of the contact, the smaller the resistance of the control coil. This is because the larger the rated current of the contact, the larger the volume of the contact. Only a smaller control coil resistance (a thicker wire diameter) can flow a larger current and generate a stronger magnetic field to attract the contacts.

Figure 14.

①The gear switch selects 200Ω gear.

②Connect the red and black wires to the two pins of the control coil.

③“6.60” shows that the control coil resistance is 6.6kΩ.

3) Turn on the control coil and detect the contact. Apply rated voltage to the control coil, and then use a multimeter to detect the resistance of the normally open and normally closed contacts. The normally open contact should be closed and the resistance should be close to 0Ω; the normally closed contact should be open and the resistance should be infinite.

How to Test a Time Relay

The detection of the time relay mainly includes the normal state detection of the contact, the coil detection and the coil energization detection.

1) Detection of the normal state of the contact. It refers to the detection of contact resistance when the control coil is not energized. Normal open contacts have infinite resistance, while normally closed contacts have close to zero resistance.The normal detection process is shown in the figure below.

Figure 15.

①The gear switch selects 200Ω gear.

② The red and black leads are connected to the two pins of the normally closed contact.

③The resistance is close to 0Ω, indicating that the normally closed contact is closed.

2) Control coil detection. As shown in Figure 16.

Figure 16.

①The gear switch uses 20kΩ gears.

②Connect the red and black wires to the two pins of the control coil.

③ “4.93” means a 4.93k* resistance for the control coil.

3) Turn on the control coil and detect the contact. Test the contact status of the time relay by applying the rated voltage to the control coil.Take the delay time relay as an example. After a period of delay, check whether the delay contact is closed (resistance is close to 0Ω) and whether the delay contact is disconnected (resistance is infinite).

Advanced Information

Professional Testing Standards

Simple continuity tests aren’t enough for critical industrial, utility, or medical uses. Professional groups like IEC and ANSI set strict standards for relay reliability and safety. IEC relay test standards become critical here.

This section explains the “why” behind thorough testing, not just the “how.”

The IEC 60255 series, ‘Measuring relays and protection equipment,’ is a key standard set. It outlines dozens of specific performance tests. Professional tests check things a basic multimeter can’t measure.

• Pickup and Dropout Voltage: This finds the exact voltage where the relay activates and deactivates. A relay might click at 12V but fail at 11.5V in real conditions with voltage drop.
• Contact Resistance: Continuity tests just show closed circuits. Specialized equipment measures tiny contact resistance (in milliohms). High contact resistance causes heat and voltage loss.
• Operating Time: This measures how quickly the switch moves after coil activation (in milliseconds). For high-speed systems, slow relays are faulty relays.

You don’t need these tests for your car’s headlight relay. But understanding them shows why relays become “slow” or “weak” over time, even when they still “click.”

Frequently Asked Questions about Relay Test

1）How to check if the relay is broken?

Multimeters are the only tools required to test the relay.When the relay is removed from the fuse box, the multimeter is set to measure the DC voltage, and the switch in the cab is activated. First, check whether the 85 positions (or the position where the relay is located) where the relay is inserted in the fuse box have 12 volts.

2）How to test a 12 volt relay?

To test a 12-volt relay, first remove it from the fuse box and set your multimeter to measure DC voltage. Connect the multimeter probes to the relay coil terminals and activate the switch in the cab to check for the correct voltage. If the relay clicks when energized and the multimeter shows the expected voltage, the relay is likely functioning properly.

3）How to check the overload relay with a multimeter?

CEP7 overload relay test program

Measure the normal operating current of the motor (i motor).

Turn off the motor and let cool for about 10 minutes.

Calculate the following ratio: i(motor)/i(overload minimum FLA).

Set the overload to the minimum FLA, and then turn on the motor.

Wait for the overload situation to occur.

4）How to test solid state relays?

If a load is connected, the SSR can be tested as described below. Connect the load and the power supply, and check the load terminal voltage with the input ON and OFF. When the SSR is off, the output voltage will be close to the load supply voltage.

5）Will a bad relay drain your battery?

Dead battery or dead battery

A failed ECM power relay can also cause battery drain or battery drain. If the relay is short-circuited, it can keep the computer’s power on even when the vehicle is turned off. This will form parasitic leakage on the battery and eventually lead to battery failure.

6）What should I do if the main relay is broken?

The engine won’t start

It is impossible for the engine to crank and run correctly if the main relay isn’t providing the computer with the power it needs. If the main relay cannot be replaced, it usually renders the car unusable.

7）How to test the battery relay?

To test a battery relay, first remove it from the fuse box. Set your multimeter to measure resistance or continuity, and connect the probes to the relay’s coil terminals. If the multimeter shows infinite resistance or no continuity, the relay coil may be faulty, indicating a need for replacement.

8）How to test the protection relay?

Protection relay self-check program

This usually includes checking the relay watch circuit, performing all digital inputs and outputs, and checking whether the relay analog input is within the calibration range by applying a test current or voltage.

9）How to check if the relay is working?

Only a multimeter is required to test a relay.

A multimeter is all that’s required for testing the re

When the relay is removed from the fuse box, the multimeter is set to measure DC voltage, and the switch in the cab is activated. First, check whether the 85 positions (or the position where the relay is located) where the relay is inserted in the fuse box have 12 volts.

10）How to test electromagnetic relay?

Take a multimeter and set it to ohms. Touch the wire on the solenoid coil pin and measure the resistance. Any place between 50-120 ohms is fine. Out of range or opening means a bad solenoid coil winding and time for a new relay.